Colloids and Surfaces A: Physicochemical and Engineering Aspects, 74, 169-215 (1993)
Review
Solubilization-emulsification mechanisms of detergency
Clarence A. Millera,* Kirk H. Raneyb
a
b
Department of Chemical Engineering, Rice University, P.O. Box 1892, Houston, TX 77251-1892, USA
Shell Development Co., Westhollow Research Center, P.O. Box 1380, Houston, TX 77251-1380, USA
(Received 14 November 1992; accepted 23 January 1993)
Abstract
The removal of oily soils from fabrics having high contents of polyester or other synthetic materials
occurs largely by a solubilization-emulsification mechanism. A systematic investigation of this mechanism
has been conducted during the past several years and is reviewed here. The research has utilized a variety of
oily soils containing hydrocarbons, triglycerides, and long-chain alcohols and fatty acids and has included the
determination of equilibrium phase behavior, the observation of dynamic behavior which occurs when
surfactant-water mixtures contact oily soils, and measurement of soil removal from polyester-cotton fabrics.
In most cases, pure surfactants and oils have been used for simplicity, but data showing the applicability of
major conclusions to systems containing commercial surfactants are presented. Because typical anionic
surfactants are too hydrophilic to achieve the desired phase behavior, the work has employed non-ionic
surfactants and mixtures of non-ionics and anionics, One major conclusion is that maximum soil removal
usually does not occur when the soil is solubilized into an ordinary micellar solution, but instead when it is
incorporated into an intermediate phase such as a microemulsion or liquid crystal that develops during the
washing process at the interface between the soil and washing bath. Indeed, for hydrocarbon and triglyceride
soils, the washing bath is itself a dispersion of a surfactant-rich liquid or liquid crystalline phase in water for
conditions of optimum detergency, i.e. the temperature of the surfactant solution is above - sometimes far
above - its cloud point temperature.
Key words: Detergency; Emulsification; Solubilization
1. General remarks on detergency
Fabric detergency is a surprisingly complex
process involving interactions among aqueous
detergent solutions, soils, and fabric surfaces.
This Process may occur in an industrial setting
in which large volumes of similarly soiled
fabrics are washed, or in a household setting in
which small amounts of fabrics containing
differing amounts of a wide variety of soils are
washed. Because of their unique ability to
adsorb at both fabric-water and soil-water
interfaces, surfactants play an essential role in
soil removal processes. To achieve the desired
levels of surfactants in the washing solution, the
* Corresponding author.
concentration of surfactants (typically nonionic,
anionic, or both) in most liquid and powder
detergent formulations is in the range 10-40%
by weight [1].
Several factors influence the effectiveness of
surfactants in laundry detergents. The
composition of a fabric is important in
determining the mechanism by which soils are
lifted from it. Cotton fabric contains rough and
irregularly shaped hydrophilic fibers [1,2]. In
contrast, synthetic polyester fabric contains
uniform cylindrical fibers which, being of a
more hydrophobic nature than cotton, are more
tenaciously covered by oil. The differences in
surface properties of the two materials are
demonstrated by the much smaller contact angle
in air formed by water on a cellulose film (32º)
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than that by water on a polyester film (79º) [3].
In addition to fiber surface composition and
morphology, the weave of the fabric can
influence detergency, more loosely woven
fabric typically being easier to clean.
Not surprisingly, the amounts and types of
soils present on fabric are key factors in
determining the effectiveness of a detergent
solution. Oily soils include such common soils
as skin sebum, dirty motor oil, and vegetable oil.
Clay is classified as a particulate soil. Scanning
electron microscopy, both conventional and
environmental, has proven quite useful for
viewing the distribution of both oily and
particulate soils within woven fabric samples
[2,4]. While surfactants play the key role in
removing oily and particulate soil, protease
enzymes are commonly present in both powder
and liquid laundry detergents to chemically
break down polymeric protein soil stains such as
blood, egg, and cocoa. Lipase enzymes are also
now being used in detergents to hydrolyze
triglyceride soils and thereby aid the surfactant
in soil removal [5]. Bleaches, both peroxygen
and chlorine-based, decolorize stains such as
those from tea and wine by destroying the
chromophores in the organic molecules
adsorbed to the fiber surfaces [6].
Water hardness and temperature can
profoundly influence detergent effectiveness. In
hard water, e.g. 300 ppm hardness, calcium and
magnesium ions may precipitate certain
surfactants prior to their being able to act on the
soil. The divalent ions also form complexes
between soils and fabric which increase their
attraction, making the soil more difficult to
remove [1]. Builders such as zeolite, sodium
tripolyphosphate (STPP) and sodium carbonate
are used in powders to negate the effect of water
hardness by either precipitating the divalent
ions, as in the case of sodium carbonate, or
sequestering the ions from the water, as in the
cases of zeolite and STPP. The latter is
particularly effective in this regard. Surfactant
precipitation may occur in cold water for
surfactants with high Krafft points [I]. Also,
wash water temperature, in addition to changing
the performance characteristics of the dissolved
surfactants, determines the physical properties
of oily soils left on the fabric. As the washing
temperature is reduced, the viscosity of the soils
increases or the soils may even solidify, making
them more difficult to remove with the same
level of agitation.
Recent trends in washing habits around the
world have made the proper choice of a
surfactant system for a detergent formulation
more critical than ever. Greater use of
temperature-sensitive synthetic fabrics such as
polyester or polyester-cotton blends as well as
energy conservation have led to a world-wide
trend to lower washing temperatures [7].
Phosphate limits or bans have resulted in the use
of less effective builders or of no builders, so
that surfactants are less protected from the
negative impact of water hardness. Also, as a
result of the effort to reduce the total amount of
chemicals released to the environment as well as
the volume of packaging materials, detergent
manufacturers are formulating products with
lower dosage requirements. As a result of these
trends, the cleaning efficiency required from
surfactant systems is steadily increasing.
2. Mechanisms for removal of oily soils
The trends described above can have a
particularly negative impact on the removal of
Oily soils from synthetic fabrics. Commonly
encountered examples of this troublesome
soil-fabric combination are dirty motor oil,
cooking oil, or sebum on 100% polyester or
polyester-cotton blends. Many studies have been
performed to visualize the mechanisms by
which such oils are removed from synthetic
fabrics [8-12]. Although in practical situations
the oily soils are trapped in the interstices
between fabric fibers and as thin films along the
fiber surfaces, most work has focused on the
removal of oil drops from flat surfaces or
individual fibers with the assumption that
similar removal mechanisms would be relevant
to removal from fabric. In fact, good correlation
between Oil removal from flat films and from
chemically similar fabric has been reported [8].
Of relevance to this type of study is Young 5
equation, which relates the interfacial tension
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
between the surfactant solution and oil (γow), oil
and solid substrate (γos) and surfactant solution
and solid substrate (gws) to the equilibrium
contact angle q measured through the soil
cos θ =
γ ws − γ os
γ ow
(1)
For quite hydrophilic surfaces like cotton, gws
is smaller than g0s, and a contact angle greater
than go is commonly achieved. Anionic
surfactants which adsorb on the fabric with their
negatively charged head groups oriented toward
the detergent solution are particularly effective
in reducing gws. in this case, the roll-up
mechanism is operative: the water preferentially
wets the fabric, causing the oily stains to be
entirely lifted off the fibers into the washing
solution. This behavior, shown schematicaily in
Fig. 1(b) for soil removal from a flat surface. is
enhanced on cotton fabric due to swelling of the
cotton fibers with water which increases the
hydrophilicity of the fabric surfaces [9,13].
For low surface energy, i.e. hydrophobic,
materials such as polyester, a contact angle of
less than 90º is usually observed, and small
portions of the oily soil may be removed by
hydraulic currents at the soil-water interface, as
shown in Fig. 1(a). In Fact, if the fabric surface
is initially completely covered by oily soil, no
location is available for the surfactant solution
Fig. 1. Mechanisms of liquid soil removal: (a)
emulsification; (b) roll-up.
171
to reach the fiber surface and undercut the soil.
Observation of this "necking" or emulsification
mechanism has been made by many
investigators for mineral oils and mineral
oil-polar soil mixtures on hydrophobic flat films
and fibers [8-12]. Removal in this manner is
enhanced by low interfacial tension at the
oil-water interface which allows the oil film to
be deformed easily to form small emulsion
droplets.
Several factors have been studied with regard
to their effect on the emulsification mechanism
for the removal of mixtures of mineral oil and
polar organic alcohols or acids from polyester
[8-10]. Such model systems, depending on the
ratio of the non-polar and polar constituents, can
be considered to be representative of sebum
soils from the skin. The rate of emulsification of
mineral oil-oleic acid mixtures from polyester
(Mylar) films was found to change as the oleic
acid content was varied [8], Other factors such
as electrolyte concentration and temperature
were also found to have large effects on the rate
of soil removal by this mechanism [8,9]. In
some
situations,
emulsification
of
non-polar-polar soil mixtures without external
agitation, i.e. spontaneous emulsification, has
been observed [ 13,14]. Emulsification, roll-up,
and other adhesion and detachment phenomena
involving oily soils and solid surfaces are
reviewed in the accompanying paper [ 15].
Another mechanism of oily soil removal
involves the formation of intermediate phases at
the
detergent
solution-oil
interface
[11,13,14,16]. Apart from the recent studies
described below, this mechanism has most often
been reported for the removal of soils containing
large quantities of polar constituents. The
growth of liquid crystal occurs in these systems
due to interaction at the interface between the
polar soil constituents and the adsorbed
surfactants. After growing to a sufficient extent,
the intermediate phase is broken off by agitation
and emulsified into the aqueous solution
allowing fresh contact of the remaining soil with
the detergent solution.
Direct solubilization of oily soils into
surfactant micelles can also occur to a
significant extent if a large excess of surfactant
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relative to oil is present and if the surfactant is
above its CMC. The solubilization of very small
oil drops from polymer fibers has been
visualized for a variety of non-polar oils
representative of liquid laundry soils [17].
However, soil solubilization rates are often
enhanced when surfactant-rich phases, either
isotropic or liquid crystalline in nature, are
present in the washing solution. Such phases
exist, for instance, when non-ionic surfactants
are above their cloud points. These phases can
either solubilize oily soils directly or interact
with soil to form intermediate surfactant-rich
phases such as microemulsions containing large
amounts of oil. Under favorable conditions, the
intermediate phases can be emulsified into the
washing bath. A detailed discussion of this
mechanism of soil removal is the subject of this
review.
Clear evidence exists that solubilization and
emulsification are major factors in removal of
oily soils from hydrophobic, synthetic fabrics
[18,19]. Unlike roll-up, in which the interaction
of the fabric with the oily soil and water is most
critical, the solubilization-emulsification mechanism occurs primarily at the soil-detergent
solution interface and is therefore directly
infiuenced by the phase behavior of the
corresponding oil-water-surfactant system. For
example, the formation of intermediate liquid
crystalline phases in fatty acid-surfactant-water
systems has been explained by the equilibrium
phase behavior of those systems [ 16]. Also,
spontaneous emulsification phenomena in
oil-water-surfactant systems have been shown to
be predictable from equilibrium phase behavior
[20]. Therefore, an understanding of the phase
behavior in these systems is needed to predict
the effectiveness of and/or to optimize detergent
solutions for specific soil compositions and
washing conditions. Available information on
phase behavior is reviewed in the next section.
In subsequent sections, systematic studies of
dynamic contacting between aqueous surfactant
solutions and oils as well as detergency studies
using the same systems are reviewed in order to
further explain the role of the solubilizationemulsification mechanism in practical detergency processes.
3. Equilibrium phase behavior
As indicated above, some knowledge of the
equilibrium
phase
behavior
of
soil-water-surfactant systems is needed to
understand
solubilizationemulsification
mechanisms of detergency. In this section, we
review such phase behavior with emphasis
given to model systems with well-defined
components. Results are given for one- and
twocomponent oils consisting of hydrocarbons,
triglycerides, and/or long-chain alcohols or
acids, representing soils such as lubricating oils,
cooking oils and sebum. In many cases
single-component specific alcohol ethoxylates
are the surfactants. However, most features of
the behavior described should be applicable to
more complex systems as well, e.g. those
containing
multicomponent
commercial
surfactants.
3. 1. Phase behavior of soil-free washing baths
We begin with the fluids used for washing,
i.e. rather dilute mixtures of surfactant and water
with inorganic salts and/or various additives
also present in some cases. As is well known, a
typical hydrophilic surfactant above its Krafft
temperature forms micelles in water at
concentrations above its CMC. If the
temperature or composition of a micellar
solution is varied in such a way that the
surfactant becomes less and less hydrophilic, the
separation of another phase eventually results.
If. for instance, the temperature of a micellar
solution of an ethoxylated alcohol is increased, a
second liquid phase begins to appear when the
so-called cloud point temperature is reached. As
Fig. 2 for the n-dodecyl pentaoxyethylene
monoether (C12E5)-water system [21] shows,
the cloud point is a function of surfactant
concentration since clouding occurs when the
coexistence curve forming the upper boundary
of the aqueous surfactant solution L1 is crossed
during heating. At temperatures well above the
cloud point, the lamellar liquid crystalline phase
La and yet another liquid phase L3, widely
considered to consist of bilayers arranged in a
sponge-like structure, are seen in this system for
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Fig. 2. Phase diagram of C12E5-water system [21]. L1,
L2, and L3 denote isotropic liquids; Lα, H1, and V1
denote lamellar, hexagonal and viscous isotropic
liquid crystalline phases, respectively. Reprinted with
permission of the Royal Society of Chemistry.
relatively low surfactant concentrations. As the
particles of a liquid crystal do not coalesce as
readily as liquid drops, the dispersions of La are
frequently less turbid than those of L3, a
property which can be used to locate phase
transition temperatures at which the La and L3
phases form [22]. At the highest temperatures
shown in Fig. 2 the surfactant-rich liquid phase
L2 coexists with water.
For more hydrophilic surfactants such as
n-dodecyl hexaoxyethylene monoether (C12E6),
clouding occurs at higher temperatures.
Moreover, the Lα and L3 phases do not appear
at low surfactant concentrations; the La phase
transforms continuously into L2, and the cloud
point curve is the only feature of this part of the
phase diagram (see Fig.3). Phase diagrams for
various binary nonionic surfactant-water
systems are given by Mitchell et al. [23].
Temperature effects are weaker for ionic
surfactants and generally act in the opposite
direction. Since the Debye length, a measure of
the electric double layer thickness, is
proportional to (kT)1/2, where kT is the
characteristic free energy of random thermal
173
Fig. 3. Phase diagram of C12E6-water system [23].
The symbols for the phases are as in Fig. 2 except
that S is a solid phase and W is a water-rich liquid
phase. Reprinted with permission of the Royal
Society of Chemistry.
motion, higher temperatures make ionic
surfactant films more hydrophilic, with a greater
tendency to curve toward an oil-in-water
configuration. However, the addition of
inorganic salts has the opposite effect,
compressing electric double layers and causing
ionic surfactant films to become less
hydrophilic. In some cases, a- second liquid
phase is ultimately formed as salinity increases,
i.e. the behavior is similar to clouding of
non-ionic surfactant solutions discussed above.
This phenomenon was observed by McBain
many years ago for aqueous soap solutions
[23b]. Another example is shown along the
upper boundary of Fig. 4 [24], with NaCl added
to the sodium salt of a commercial ethoxylated
sulfate based on a C12-C13 alcohol and
containing an average of three ethylene oxide
groups (Neodol 23-3S). As Fig. 4 indicates,
multiphase regions involving the lamellar liquid
crystal La are observed at even higher salinities.
In other systems, for instance the Aerosol
OT-NaCl-water system, the first phase formed
upon increasing the salinity is the lamellar liquid
crystalline phase [25,26]. Indeed, such behavior
is typical for anionic surfactant-short-chain
alcohol systems investigated for possible use in
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C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Fig. 4. Phase behavior of mixtures of C12E3, Neodol
23-3S, and NaCl brine, with the temperature and total
surfactant concentration fixed at 30ºC and 5.4 wt.%,
respectively [24]. B, NaCl brine; W(non), weight
fraction of non-ionic surfactant in the surfactant
mixture. Reprinted with permission from Dr. Dietrich
Stemkopff Verlag.
enhanced oil recovery, with the L3 phase found
at still higher salinities [27]. The addition of
divalent cations (i.e. an increase in hardness)
produces the same effects in these systems,
although generally at lower electrolyte
concentrations [27].
Whether a liquid phase or the liquid crystal
forms upon the addition of salt apparently
depends on the relative importance of reducing
the effective electrical repulsion between nearby
ions within a micelle and reducing it between
micelles. If the former is the dominant effect,
the micelle shape changes from spherical to
cylindrical to planar as the effective surfactant
head group area decreases, a sequence in
accordance with well-understood surfactant
packing considerations in micelles [28]. The
large, bilayer sheets corresponding to the planar
configuration with nearly equal head and tail
areas arrange themselves into the lamellar
phase. In contrast, if reducing the repulsion
between relatively small micelles is the more
important effect, the micelles eventually
flocculate to form a "coacervate" or micelle-rich
liquid phase.
It appears from the available evidence that
coacervation is the likely outcome of increasing
salinity for anionic surfactants that are rather
hydrophilic, while liquid crystal formation is
probable for surfactants whose hydrophilic
characteristics only slightly outweigh their
lipophilic characteristics in the absence of salt.
Figure 4 provides an example. The addition of
less than 20% of the non-ionic surfactant
n-dodecyl trioxyethylene monoether (C12E3)
reduces the hydrophilic nature of the surfactant
mixture sufficiently such that the liquid crystal
phase forms instead of a coacervate when the
NaCl concentration is increased. Such behavior
can be explained as follows. Hydrophilic
surfactants such as Neodol 23-3S require large
concentrations of salt for the surfactant
aggregates to become planar. Repulsion
between small, nonplanar micelles is apparently
reduced sufficiently so as to produce
coacervation before the salinity increases
enough for planar micelles to form. For less
hydrophilic surfactants, less salt is needed to
produce planar aggregates, and formation of the
lamellar phase occurs before coacervation.
The cloud point phenomenon discussed above
for non-ionic surfactants may also be viewed as
coacervation of a micellar solution induced by
making the surfactant less hydrophilic. In this
case. raising the the temperature reduces the
interaction between the ethylene oxide chains
and water and thereby reduces the repulsion
between micelles or even reverses it to an
attraction. Of course, the effective area of the
ethylene oxide head group is also reduced, and a
change from spherical to cylindrical micelles,
which would facilitate coacervation by an
entropic mechanism, can occur before the cloud
point is reached. Unlike the situation for anionic
surfactants, however, there do not seem to be
any reports of the lamellar liquid crystalline
phase forming directly from micellar solutions
Or ethoxylated alcohols before clouding occurs
as the temperature is raised in dilute binary
systems.
The cloud point of a non-ionic surfactant
natulrally depends on its structure, increasing
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
175
for longer ethylene oxide and shorter
hydrocarbon chains. Shifting the point of
attachment of the ethylene oxide chain from the
end to the central portion of the hydrocarbon
chain depresses the cloud point. The addition of
many common salts, e.g. sodium and potassium
chlorides and sulfates, lowers the cloud point
although the effects are much smaller than for
ionic surfactants. However, some salts cause the
cloud point to increase. The former effect is
generally considered to stem from reduced
hydration of the ethylene oxide chains resulting
from competition with the added ions for the
available water molecules. The latter effect
occurs for ions such as I-, SCN- and most
multivalent cations Which break the structure of
water. For some salts the anion and cation have
opposite effects, with the stronger determining
the direction of the cloud point shift. These
effects have been recently discussed by Mackay
[29].
Other additives also influence the cloud point
and other phase boundaries in non-ionic
surfactant-water systems. Long-chain alcohols
make the system less hydrophilic, as Fig. 5
shows for the case of n-dodecanol added to
mixtures of C12E5 and water [30].
In this case, the cloud point is lowered by some
23ºC when the alcohol content is only 10 wt.%
relative to the surfactant. The temperatures for
the other phase transitions are lowered as well.
For the binary surfactant-water system, the
phase rule constrains the three-phase regions,
e.g. W + L1 + Lα, to a single temperature, but
the addition of the alcohol provides an
additional degree of freedom and allows these
regions to span a finite temperature range, as
Fig. 5 indicates.
The additive can, of course, be another
surfactant. It is well known that the addition of
an ionic surfactant greatly increases the cloud
point of an ethoxylated alcohol by adding an
electrical repulsion between micelles and
thereby inhibiting coacervation [31]. The
opposite occurs when a second but more
lipophilic non-ionic surfactant is added, e.g.
C12E3 to C12E6, as shown in Fig. 6 [32]. This
system is particularly interesting because, as
noted previously, the Lα and L3 phases do not
occur in dilute mixtures of water and pure C12E6.
However, both these phases appear when only a
few per cent of the more lipophilic surfactant
has been added, due to some rather complex
Fig. 5. Phase behavior resulting from the addition of
small amounts of n-dodecanol to I Wt'% C12E5 in
water [30]; 30 denotes a three-phase region.
Reprinted with permission from Academic Press.
Fig. 6. Phase behavior of mixtures of C12E6 and
C12E3 in water. The total surfactant concentration is
fixed at 1.0 wt.% [32].
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C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
phase behavior in the region indicated by the
box. Although details are not given here, an
interesting feature of this behavior is the
existence of a four-phase region, which is
constrained to a single temperature in this
ternary system by the phase rule and which
involves W, L1, L3 and La, phases [32]. Figure 6
also shows a transition from W + L3 to W + L2 at
58ºC for the C12E3-water system, which was
clearly seen in this study using optical
microscopy but does not appear in the existing
phase diagram [23].
The addition of a non-ionic surfactant to an
anionic surfactant makes the surfactant mixture
more lipophilic and causes a shift from L1 to
Lα, to L3, as shown in Fig. 4. This sequence is,
not surprisingly, consistent with that shown in
Fig. 6 for increase in the C12E3 content of
C12E6-C12E3 mixtures at constant temperature.
The same sequence is found when a lipophilic
alcohol is added to an anionic [27] or a
zwitterionic [33] surfactant system.
Although cationic surfactants are rarely used
for cleaning purposes, they are useful for
neutralizing charge build-up on fabric surfaces
and for fabric softening. As an important trend
in detergent formulation is combining all
ingredients into a single mixture in order to
eliminate the need for the separate addition of
bleaches, fabric softeners, etc. during the
washing process, it seems useful to include
some information on phase behavior with
cationic surfactants present. Moreover, some
recent work suggests that mixtures of cationic
and nonionic surfactants may be useful in
removing oily soils, though not by solubilization
mechanisms [34]. Because anionic and cationic
surfactants attract one another, surfactant
aggregation occurs readily with substantial
neutralization of charge. When either the
anionic or the cationic surfactant is present in
substantial excess, the result is mixed micelles
but with a much lower CMC than for the
individual surfactants. When the two surfactants
are present in almost equal amounts, the
formation of a solid or liquid crystalline phase
can be expected if the hydrocarbon chain
lengths are sufficiently long. In some cases,
vesicles have been found to form spontaneously
upon
mixing
aqueous
solutions
with
intermediate ratios of anionic and cationic
surfactants [35].
3.2. Phase behavior of
water-surfactant-hydrocarbon systems
An important feature of the phase behavior of
systems containing water, surfactants, and
hydrocarbon soils is the existence of
microemulsions, thermodynamically stable
liquid phases containing substantial amounts of
both water and oil. The formation of
microemulsions
requires
that
the
surfactant-films which separate oil and water
microdomains be rather flexible, and that the
hydrophilic and lipophilic properties of the
surfactant be roughly balanced. However, within
conditions satisfying these overall constraints,
the microstructure is quite sensitive to changes
in the relative strength of hydrophilic and
lipophilic interactions. In systems which contain
comparable volumes of oil and water and which
are dilute in surfactant but not so dilute as to
preclude aggregation, packing considerations
dictate that oil-in-water microemulsions coexist
with excess oil for hydrophilic surfactants and
water-in-oil microemulsions with excess water
for lipophilic surfactants. Drop size' are of the
order of 5-50 nm, increasing in size as the
temperature, pressure, or system composition is
changed to shift the surfactant closer to the
condition of precise balance between
hydrophilic and lipophilic properties. Very near
this balance. the microemulsion becomes
continuous in both phases and coexists with
both excess water and excess oil. Interfacial
tensions of this 'middlephase" microemulsion
with both excess phases are frequently below
0.0 1 mN m-1 - in some systems by an order of
magnitude or more.
The condition for which the hydrophilic and
lipophilic properties are exactly balanced and
the surfactant films have no spontaneous
tendency 10 curve in either direction has been
called the phase inversion temperature (PIT) or
hydrophile-lipophile balance (HLB) temperature
by Shinoda and Friberg [36] for the case of
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
non-ionic surfactants for which temperature is
usually the variable of greatest interest. For
ionic surfactants it is more common to speak of
"optimal" conditions, e.g. optimal salinity [37].
Whatever one calls it, several criteria have been
used to define the condition for balance in terms
of readily measured experimental quantities.
The most common criterion is equal volumetric
solubilization in the microemulsion of the oil
and water phases. The differences between the
"optimal" conditions given by this and other
criteria are small for practical purposes and will
be ignored here.
The effects of temperature and inorganic salts
on making the surfactant more or less
hydrophilic are basically the same as those
described in the preceding section, and so are
the effects of adding alcohols or additional
surfactants, except that one additional factor
must be considered - the relative solubilities of
the surfactants and additives in the oil phase. It
is the composition of the surfactant films
separating oil and water domains that
determines
the
microstructure
of
the
microemulsion. In a mixture of two non-ionic
surfactants the more lipophilic surfactant has a
higher solubility in the oil phase and the
surfactant films are thus more hydrophilic than
the overall surfactant mixture. The magnitude of
this effect for a given pair of surfactants
depends on both the overall surfactant
concentration and the water-to-oil ratio.
Kunieda, Shinoda and co-workers have
developed equations for predicting the
dependence of the PIT on system composition
for mixtures of two non-ionic surfactants [38]
and for mixtures of an anionic and a non-ionic
surfactant [39]. For instance, in the latter case
the following relationship must be satisfied at
the PIT
Wn = Ssn + SonRow [(1 - Ssn)/(1 - Son)] (X -1)
177
is the mass fraction of oil in the oil-water
mixture, and X is the total surfactant mass
fraction in the system. The solubility of the
anionic surfactant in the excess oil has been
neglected.
It is clear from this equation that a plot of Wn
as a function of (X-1 - 1) at constant Row and
temperature should yield a straight line from
which values of Ssn and Son can be extracted.
Figure 7 shows such plots for mixtures of C12E3
and Neodol 23-3S at various temperatures along
with the corresponding values of Ssn and Son.
The oil phase is n-hexadecane and the aqueous
phase is water containing 1 wt.% NaCl. As
might be expected, nonionic surfactant
solubility in the oil phase Son increases with
increasing temperature. In contrast, the fraction
Ssn of nonionic surfactant in the films decreases.
Since increasing temperature makes the nonionic surfactant less hydrophilic, it is reasonable
that less of it would be required to achieve the
(2)
where Wn is the mass fraction of non-ionic
surfactant in the overall mixture, Ssn is the mass
fraction of non-ionic surfactant in the surfactant
films, Son is the mass fraction of non-ionic
surfactant in the excess hydrocarbon phase, Row
Fig. 7. PIT results for the C12E3-Neodol 23-3S-1
wt.% NaCl brine-n-hexadecane system [24]; X is the
total surfactant mass fraction in the system.
Reprinted with permission from Dr. Dietrich
Steinkopff Verlag.
178
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
balance between hydrophilic and lipophilic
properties at high temperatures.
The PIT is also influenced by the composition
of the oil phase, being higher for hydrocarbons
with longer chains. The reason is that their
penetration into the hydrocarbon chain region of
the surfactant films tends to make the films
curve toward a water-in-oil configuration. Such
penetration
is
less
for
longer-chain
hydrocarbons [40], probably due primarily to an
entropic effect [41] except for short-chain
hydrocarbons where energy effects have
recently been shown to be important as well
[42]. Reed and Puerto [43] have developed a
scheme relating optimal conditions to the molar
volume of the oil and the solubilization at
optimal conditions.
The other factor mentioned above as being
necessary for microemulsion formation is the
existence of flexible films. Films are most rigid
for
surfactants
having
long,
straight
hydrocarbon chains. Flexibility can be increased
by promoting less ordered packing in the
hydrocarbon chain region of the films, e.g. by
using branched-chain surfactants or mixtures of
surfactants with different chain lengths, or by
adding short-chain alcohols. Increasing the
temperature also promotes flexibility. Too much
flexibility can be undesirable, however, because
it reduces the solubilization capacity of a
microemulsion. However, when the surfactant
films become too rigid, the lamellar liquid
crystalline phase forms. Barakat et al.
determined the conditions in several systems for
which the lamellar phase formed instead of a
middle-phase microemulsion because of film
rigidity [44]. Hackett and Miller investigated the
detailed phase behavior near the transition [45].
The liquid crystal can also form when the
amount of oil or water present becomes too low
[46] or when the surfactant concentration of a
middle-phase microemulsion becomes too high
[47].
Kunieda and Shinoda [47] have presented
ternary diagrams at several temperatures ranging
from below to above the PIT for the
C12E5-water-n-tetradecane system. We discuss
below the use of such diagrams in interpreting
the dynamic behavior which occurs when
surfactantwater mixtures contact oil.
3.3. Water-non-ionic surfactant- triglyceride
systems
Extensive studies have been made of
microemulsions in hydrocarbon systems. Much
less information is available for oils which are
liquid triglycerides. An important difference is
that triglycerides such as triolein which are of
interest for detergency are of much higher
molecular weight than simple straight-chain
liquid hydrocarbons such as n-hexadecane. The
higher molecular weight makes it much more
difficult to incorporate such triglycerides into
surfactant films, the result being a major
reduction in solubilization in many systems.
Figure 8 shows the general form of a
water-nonionic surfactant-liquid triglyceride
ternary diagram [48-50]. Deviations from this
behavior occur at sufficiently high temperatures,
a matter discussed further below. As indicated
on the diagram, the phase designated D' is the
same as that usually called L3 in binary
surfactant-water systems, mentioned in section
3.1.
While,
like
the
middle-phase
microemulsions discussed above, the D' phase
coexists with both excess water and excess oil
for suitable overall system compositions, it
differs from the microemulsions in that it can
Fig. 8. Schematic phase diagram for non-ionic
surfactant-water-triolein system at low temperatures
[48]. O denotes a triolein-rich phase.
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
solubilize only small amounts of the oil phase.
Note that solubilization of triglyceride in the
lamellar liquid crystalline phase is also low.
Behavior of the W + D' + O three-phase
triangle has been studied as a function of
temperature for pure non-ionic surfactants and
triolein. Figure 9 shows the results for C12E3
[48], a lipophilic surfactant that is already above
its cloud point at 0ºC, well below the
experimental temperature range. The surfactant
content of the D' phase increases rapidly with
temperature, the same as for the L3 phase in
binary
surfactant-water
systems,
but
solubilization of triolein remains low. The
solubility of surfactant in the triolein phase is
substantial - more than 10% by volume even at
the lowest temperature studied (30.5ºC) - and
increases with temperature. For comparison, we
note that the solubility of C12E3 in excess
n-hexadecane
in
equilibrium
with
microemulsions at 30ºC is about 3% by volume
(see Fig. 7).
At about 40ºC the rate of increase with
temperature of surfactant solubility in the
triolein phase increases significantly. Simultaneously, water solubilization in this phase,
previously rather low, rises dramatically. Such
formation of water-in-oil microemulsions is one
way the system can depart at high temperatures
from the behavior shown in Fig. 8.
Another way is illustrated in Fig. 10 by the
Fig. 9. The W + D' + 0 region at several temperatures
in the C12E3-water-triolein system [48].
179
Fig. 10. The W + D' + O and W + D + O regions at
several temperatures in the C12E5-water-triolein
system [48].
corresponding C12E5 diagram [48]. Just above
64ºC a phase transformation occurs, and at
higher temperatures the diagram shows a new W
+ D + O three-phase region instead of the W +
D' + O region. The D phase is able to solubilize
considerable triolein and is thus more favorable
for detergency than D' if it forms as an
intermediate phase during washing. According
to Fig. 10, the composition of the D phase shifts
to become richer in oil with increasing
temperature in a manner similar to that seen for
microemulsion systems [47] although the
surfactant concentration of about 40% is well
above that observed in typical microemulsions.
Details of the phase behavior in the temperature
region where the D phase first appears,
including the existence of two four-phase
regions at two closely spaced temperatures, are
given for another system by Kunieda and
Haishima [50].
As indicated above, the inability of the
hydrocarbon chain region of the surfactant films
to incorporate the large triglyceride molecules is
the chief reason for the poor solubilization.
Similar poor solubilization and phase behavior
have been seen in systems containing the
anionic surfactant Aerosol OT, hydrocarbons
with chain lengths of twelve and above, and
NaCl brine [26]. Recently, Binks [51] has
investigated further the phase behavior of some
of these systems.
If the surfactant films were made more flexi-
180
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
ble, i.e. if chain packing in this region were
made more disordered, one might expect
solubilization to increase. Clearly in some
systems such as water-C12E5-triolein (Fig. 10),
increasing temperature provides sufficient
disorder for the D phase to form. In other
systems such as water-C12E3-triolein (Fig. 9), the
D phase does not form at any temperature. One
might expect that adding amphiphilic
compounds with chain lengths different from the
surfactant or with branched chains would
promote less ordered packing and formation of
the D phase. As Fig. 11 shows, the addition of
tert-amyl alcohol (TAA) does, in fact, favor
formation of the D instead of the D' phase in the
water-C12E4-triolein system [52]. In the absence
of TAA the D phase is seen only over a range of
about 0.2ºC near 55ºC, and not at all for the
somewhat lower temperatures of Fig. 12 [48].
For the purposes of improving detergency, the
use of TAA to promote solubilization of
long-chain liquid triglycerides at relatively low
temperatures is not attractive because its high
solubility in water requires that it be used at
Fig. 12. Partial phase diagram of the C12E4-watertriolein-n-hexadecane system [48]. W. and 0. denote
water-continuous and oil-continuous microemulsions.
rather high concentrations. The same
disadvantage applies to the use of hydrotopes, a
possibility considered by Friberg and Rydhag
[53]. Alander and Warnheim [54] managed to
solubilize a mixture of medium-chain
triglycerides (C8-C10) in aqueous solutions of a 1
:2 mixture of sodium oleate and n-pentanol at
25ºC, but they had less success with long-chain
triglycerides.
Recent results suggest that the use of
doublechain surfactants with varying chain
lengths, e.g. secondary alcohol ethoxylate
surfactants, instead of straight-chain surfactants
may prove effective for forming the D phase
and thereby solubilizing reasonable amounts of
triolein at temperatures suitable for warm water
washing [55]. Here, too, the basic idea is that
solubilization should be improved for surfactant
films with disordered packing in the
hydrocarbon chain region.
3.4. Mixtures of hydrocarbons and triglycerides
Fig. 11. Partial phase diagram of the C12E4-tertiary
amyl alcohol-water-triolein system: surfactant
content, 16 wt.%; equal volumes of water and triolein
[52]; LC denotes the lamellar or Lα phase.
Microemulsion formation is easier in mixed
soils of hydrocarbon and triglyceride than for
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
pure triglyceride soils. In the mixed soil
systems, hydrocarbon molecules presumably
penetrate the surfactant films, allowing oil
droplets or oil microdomains of other shapes to
form. Triglyceride and hydrocarbon are jointly
solubilized within the microdomains. Figure 12
shows phase behavior in the water-n-dodecyl
tetraoxyethylene monoether (C12H4)-triolein-nhexadecane system for a particular overall
surfactant concentration and equal volumes of
oil and water phases [48]. The transition from
the existence of a middle-phase microemulsion
(D phase) in hydrocarbon-rich systems the D'
phase in triolein-rich systems is clear. Note that
the sequence of phases seen with increasing
temperature over one temperature range for pure
triolein is, omitting the excess oil phase, Lα, L2
+ D'. D', W + D'. The same sequence occurs in
,he oil-free system at modest surfactant
concentrations. That is, the sequence of phases
with triolein present is the same as that when it
is absent, except that an excess oil phase is
present and the transition temperatures are
somewhat lower. This behavior is to be
expected when solubilization is low.
181
Table 1
Compositions in volume fractions of four coexisting
phases of the C12E4-water-triolein-n-hexadecane
system at 39.2ºC [48]
to about 0.005 mN m-1, which is a reasonable
value for a system near its PIT. In contrast, the
tension reached after some 3 h is about 0.2 mN
m-1 for pure triolein. The higher tension is
expected in view of the low solubilization of
triolein. Mixed oils have intermediate values of
interfacial tension.
At intermediate oil compositions, D phase
formation can be promoted by increasing either
the temperature or the hydrocarbon content of
the oil. By making the surfactant less
hydrophilic,
increasing
the
temperature
promotes a surfactant film configuration where
the hydrocarbon chains diverge, and thereby
facilitates the solubilization of triolein. The
transition between the W + D' + O and W + D +
0 regions occurs by means of a narrow
four-phase region W + D'+ D + O. Table I gives
compositions of the four-phase region at one
temperature. Clearly, solubilization of both
hydrocarbon and triglyceride is greater in the D
phase. Moreover, hydrocarbon is solubilized in
preference to triolein in both phases. Similar
phase behavior has been reported for another
triglyceride [49].
Figure 13 shows interfacial tensions measured
with the spinning drop apparatus at 30ºC for 1
wt.% C12E4 with various mixtures of
n-hexadecane and triolein [48]. For the pure
hydrocarbon the tensions drops after 10 minutes
Fig. 13. Interfacial tensions (IFT) at 30ºC for 1 Wt'%
C12E4 with various mixtures of triolein and
n-hexadecane [48].
182
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
3.5. Water-surfactant-polar soil systems
Hydrocarbons and triglycerides are frequently
referred to as non-polar soils, while long-chain
fatty acids and alcohols are termed polar soils.
In this section we consider only the case of pure
polar soils.
Ekwall has studied the phase behavior of
many anionic surfactant-water-polar soil
systems [56]. Figure 14 is a partial ternary
diagram at 50ºC for the water-sodium
octylsulfonate-n-hexanol system, which has
been investigated in recent years by Kunieda
and Nakamura [57]. A matter of interest for later
sections of this paper is that the region of
coexistence of L1 and L2 phases near the
water-hexanol axis is bounded by a three-phase
triangle involving these phases and the lamellar
liquid crystal La. In other cases, including the
water-sodium octanoate-n-decanol system that
was studied extensively by Ekwall's group,
careful examination of the dilute region reveals
that the D' (or L3) phase replaces Lα, in the
three-phase triangle which terminates the L1-L2
region [58]. In this system a second three-phase
triangle D'-Lα-L2 also exists, the arrangement
being similar to that shown in Fig. 8 for
triglyceride systems. In almost all diagrams of
this type involving relatively long-chain
compounds, the lamellar phase and its
associated multiphase regions are prominent
although other liquid crystalline phases may be
present as well at high surfactant concentrations [56].
Kunieda and Nakamura [57] showed that the
addition of NaCl to the system of Fig. 14 caused
the D' phase to appear and the dilute portion of
the phase diagram to resemble Fig. 8. The
higher salinity is likely to make the
surfactant-alcohol bilayers more flexible and
enables them to assume the locally saddleshaped configuration necessary for formation of
the sponge-like microstructure of the D' phase
for slightly lipophilic conditions. Apparently
only the lamellar structure is possible for more
rigid bilayers.
When the surfactant is non-ionic, the same
behavior, i.e. the appearance of the D' phase and
a shift from a diagram resembling Fig. 14 to one
resembling Fig. 8, can be effected by increasing
the temperature, as Kunieda and Miyajima [591
showed for the water-n-dodecyl octaoxyethylene monoether (C12E8)-n-decanol system.
Here. too. the ability to form the D' phase at
temperatures above about 14ºC is probably the
result of increased flexibility of the
surfactant-alcohol bilayers.
3.6. Mixtures of non-polar and polar soils
Fig. 14. Phase behavior of the sodium
octylsulfonate-water-nhexanol system in the dilute
region at 50ºC [57]. Reprinted with permission of the
American Chemical Society.
As mentioned in section 3.2, the addition of
rather lipophilic amphiphilic compounds reduce,
the PIT of non-ionic surfactant-water-hydrocarbon systems. Long-chain alcohols and
(undissociated) fatty acids ate of this type, as
Fig. 15 shows for the addition of oleyl alcohol
to systems containing water, n-hexadecane, and
several non-ionic -surfactants [12]. Note that 5%
oleyl alcohol in the system by oil phase reduces
the PIT in the C12E6 about 35ºC. Similar results
were found by the same authors for oleic acid.
When enough polar soil is present that the
system is above its PIT but the surfactant is
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Fig. 15. PIT values for non-ionic surfactant-water-nhexadecane-oleyl alcohol systems [12]. Reprinted
with permission of the American Oil Chemists'
Society.
below its cloud point, one might expect that
phase behavior in the dilute region would be
similar to that described in the preceding
section, i.e. the L1-L2 region would terminate in
a three-phase region involving either the D' or
the La phase. Experiments at 30ºC with C12E7
and oils having ratios of n-hexadecane to oleyl
alcohol of 3/1 and 1/1, respectively, showed that
such behavior did, in fact, occur, with the third
183
phase being the lamellar liquid crystal Lα [60].
However, in contrast to the situations shown in
Fig. 8 and 14 where the oil phase solubilizes
modest amounts of water, L2 phases in these
systems extended to compositions containing up
to 75-80% water which were in equilibrium with
aqueous micellar solutions. Evidently, the
presence of hydrocarbon and of the double bond
in the alcohol chain makes the surfactant films
sufficiently flexible so that the L2 phase can
invert continuously and become water
continuous, ultimately reaching compositions
comparable to those of the D' phase in systems
such as that shown in Fig. 8. The relevance of
this behavior to detergency is discussed later.
When the long-chain alcohol is mixed with a
liquid triglyceride instead of with a
hydrocarbon, multiphase regions containing the
D' phase are prominent, as is shown in Fig. 16
[61] for the C12E6-water-triolein-oleyl alcohol
system. The sequence of phases observed with
increasing temperature in Fig. 16 for oils having
oleyl alcohol contents exceeding about 20% is
the same as was found for the water-non-ionic
surfactant-triolein systems discussed in section
3.3, as may be seen, for instance, along the
right-hand boundary of Fig. 12 for water-C12E4-
Fig. 16. Partial phase diagram of C12E6-water-triolein-oleyl alcohol system with 10 wt.% surfactant, 45 wt.%
water, and 45 wt.% mixed oil [61]. The symbol IV denotes the four-phase region W + D'+ D + O.
184
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
triolein. The D phase is seen, however, for
surfactant concentrations well above that of Fig.
16. Note that the amounts of oleyl alcohol
needed to depress the temperatures of the
various phase boundaries are much greater than
are shown in Fig. 15 for hydrocarbon systems.
A likely explanation is that much of the oleyl
alcohol is dissolved in the bulk triglyceride
phase, leaving relatively little alcohol in the
surfactant films which, to a large extent,
determine the basic phase behavior by
controlling the aggregate shape.
4. Diffusion path analysis
Being of rather short duration and typically
involving small quantities of soils, detergency
processes are strongly influenced by dynamic,
diffusional phenomena which occur on a
microscopic scale. Oily soil removal, in
particular, depends on phase transitions which
occur at the oil-washing solution interface. The
preceding section described equilibrium phase
behavior in both water-surfactant systems,
representing the washing solution, and
oil-water-surfactant systems, As demonstrated
below, such equilibrium phase behavior can be
combined with the theory of diffusion processes
to interpret certain dynamic behavior such as
intermediate phase formation and spontaneous
emulsification that occurs during detergent
processes.
A mathematical technique called diffusion
path analysis has been used with success in
predicting certain dynamic phenomena in
multicomponent solid and liquid systems when
two phases not in equilibrium are brought into
contact with one another [62-64]. Essentially, a
time-invariant path of compositions can be
plotted across an equilibrium phase diagram
by-solving component transport equations with
certain assumptions and boundary conditions.
First, convection in the system from any source
is assumed to be negligible. Second, the two
phases are assumed to be semiinfinite in extent.
This assumption simplifies the mathematical
analysis and is probably reasonable at least for
short times after contacting. Third, diffusion of
each species is assumed to be dependent only on
its own concentration gradient with a uniform
diffusion coefficient in each phase. Also, local
equilibrium is assumed at all interfaces which
form; this means that the compositions at the
interfaces are defined by equilibrium tie lines.
Diffusion path analysis is most conveniently
applied to three-component, i.e. ternary,
systems. In this situation, the phase diagram can
be represented in the form of a two-dimensional
triangle as, for instance, in Fig. 8, with
two-phase regions shown as regions of varying
shape containing equilibrium tie lines and
three-phase regions represented as triangles in
which the compositions of the equilibrium
phases are shown as the vertices. The analysis in
this case consists of solving in each phase the
following transport equations for two of the
species
(∂wi/∂t) = Di(∂2wi/∂x2)
i = 1,2
(3)
where x is the distance from the initial surface
of contact, t is time and wi and Di are the mass
fraction and diffusivity of species i,
respectively. The value of W3 for the third
species is found by invoking Σwi = 1. The
semi-infinite phase assumption allows transformation of the above equations to ordinary
differential equations in the similarity variable
ηi = [x/(4Dit)1/2]. Integration yields the following
error function solutions for the diffusion path
segment in each phase
wi = Ai + Bi erf ηI
i = 1,2
(4)
where Ai and Bi are constants which are
evaluated from the boundary conditions. Since
ηi varies from - ∞ to + ∞ at each value of time,
the set of compositions given by Eq. (4) is
independent of time although the position x of a
specific composition does vary with time. It is
often convenient to plot the compositions or
"diffusion path" directly on the equilibrium
phase diagram.
In evaluating Ai and Bi for phases in contact.
iteration on a tie line is performed until the
individual species mass balances at the interface
are satisfied. In addition to obtaining the path of
compositions that forms between the initial
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
phases, one may also calculate the relative
velocities of all interfaces, and therefore the
growth rates of intermediate phases. More
detailed descriptions of the mathematical
analysis and the specific error function solutions
can be found elsewhere for a ternary system
forming a single interface [63] or two or more
interfaces [65].
The utility of diffusion path theory in ternary
liquid systems was first shown for predicting the
occurrence of spontaneous emulsification in
alcohol-water-oil systems [63]. Specifically,
when an alcohol-oil mixture denoted d in Fig.
17 is brought into contact with water,
spontaneous emulsification of oil drops in the
water phase is observed. In this situation, the
construction of a diffusion path between the
initial compositions shows the formation of an
interface with equilibrium compositions b and c
connected by the tie line represented by the
broken line. Spontaneous emulsification in the
aqueous phase can be explained by the passage
of that path segment from b to W through the
corner of the two-phase envelope, thereby
predicting the formation of small drops of
oil-alcohol mixture below the interface.
Experiments showed that, in the absence of
interfacial turbulence, interfacial displacement
Fig. 17. Schematic diffusion path in alcohol(A)water(W)-oil(O) system showing supersaturation
leading to spontaneous emulsification.
185
is proportional to the square root of time, as
predicted by the theory [65,66]. This diffusion
mechanism of spontaneous emulsification is
distinct from other modes of spontaneous
emulsification in which interfacial instability
results in the mechanical dispersion of one
phase in another [67].
Diffusion path analysis was later applied to
oil-water-surfactant systems [20,64,68]. In these
cases, the use of pseudoternary phase diagrams
was required. For example, commercial
surfactants are almost always complex mixtures
containing numerous species of surfactants.
Rather than solving the diffusion equations for
each species, one can sometimes combine all
surfactant components together and treat them
as
a
pseudocomponent.
Mixtures
of
hydrocarbons can also be considered as
pseudocomponents. Although diffusion path
studies are typically performed when
single-phase systems are originally present, the
ability to calculate diffusion paths in which one
of the initial compositions is a stable dispersion
of one phase in another, e.g. a liquid crystalline
dispersion, has also been demonstrated [64].
5. Dynamic contacting studies
Direct observation of the dynamic phenomena
that occur when non-equilibrated liquid phases
are brought into contact can be made in various
ways. On a macroscopic scale, a liquid can be
gently placed on top of another liquid in a tube,
and rather large-scale phenomena can be
observed. This simple technique was used in the
early studies of spontaneous emulsification in
oil-water-alcohol systems [63] and has been
used with surfactant systems to monitor the
formation of microemulsion and liquid
crystalline phases between oil and surfactant
solutions [68-71]. In these cases, the oil is
gently layered on top of the aqueous phase, and
dynamic phenomena are observed without
magnification. Crossed polarizers aid in the
identification of birefringent liquid crystalline
phases. A shortcoming of this technique is the
inability to observe events which occur
immediately after the contacting of the two
186
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
phases. Also, the experimental scale increases
the possibility of convection occurring due to
the formation of adverse density gradients.
Nevertheless, intriguing phenomena including
oscillations in the interfacial position and abrupt
changes in the interfacial velocity have been
observed by Friberg and co-workers [70,71] in
systems in which the lamellar liquid crystal is
present, although generally at rather long times
(a few days) after initial contact. For various
reasons, diffusion path theory is not applicable
for describing such phenomena, as these
workers have pointed out.
To facilitate the observation of dynamic
phenomena at short times after contact, a
microscopic technique was developed. A key
aspect of the technique is the use of rectangular
glass capillaries, having a path width 200 gm,
tohold the sample [68]. Figure 18 shows a
diagram of the sample cell that is 50 min in
length. After the aqueous phase is imbibed
about half way into the capillary, that end is
sealed by a resin curable by ultraviolet light. The
oil phase is then injected into the other end by
use of a syringe. Initially, the resulting diffusion
phenomena were observed using a conventional
microscope with the cell in a horizontal
configuration. However, due to the density
difference between the two phases, overriding of
the oil over the surfactant solution occurred,
causing distortion of the interface and
complicating the interpretation of the results
[68].
A vertical-stage microscope was then
designed that allowed the cell to be placed in a
vertical configuration in a controlled
temperature environment. Details of the
microscope and contacting technique are given
elsewhere [20]. In this configuration in a stable
region of contact between the two initial phases
can be maintained, allowing easy viewing. The
vertical-configuration
microscope
was
subsequently improved and equipped with a
video imaging system [48,72]. The use of video
taping and image analysis allows detailed
review of phenomena which may be missed
initially in real time, and improved
determination of, for example, the velocities of
interfaces and rates of formation of intermediate
phases.
The vertical-contacting technique was first
used to study the dynamic contacting in
water-anionic
surfactant-oil
systems
representative of those used in enhanced oil
recovery processes [20]. Intermediate phase
formation and spontaneous emulsification
experimentally observed in a brine-petroleum
sulfonate-hydrocarbon system were found to be
predictable from calculated diffusion paths
based on relevant phase diagrams [64]. Widely
varying but predictable phenomena were found
as the salinity of the aqueous phase was varied.
6. Diffusional phenomena in detergent systems
6.1. Water - alcohol ethoxylate - hydrocarbon
systems
Fig. 18. Rectangular glass capillary cell used in
vertical-stage contacting experiments.
Studies of diffusional phenomena in systems
having direct relevance to detergency processes
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
have recently been performed. Experiments
were designed to investigate the effects of
changes in temperature on the dynamic
phenomena which occur when aqueous
solutions of pure non-ionic surfactants contact
hydrocarbons such as tetradecane and
hexadecane [18,72]. These oils can be
considered to be models of non-polar soils such
as lubricating oils. The dynamic contacting
phenomena, at least immediately after contact,
are representative of those which occur when a
detergent solution contacts an oily soil on a
synthetic fabric surface. The following is a
summary of the observed behavior interpreted
through the use of schematic diffusion paths.
Detailed phase behavior in such systems has
been reported previously and was used in
construction of the diffusion paths [47].
With C12E5 as the non-ionic surfactant at a 1
wt.% level in water, quite different phenomena
were observed below, above, and well above the
cloud point when tetradecane or hexadecane
was carefully layered on top of the aqueous
solution. Below the cloud point temperature of
31ºC, very slow solubilization of oil into the
one-phase micellar solution was observed. An
interesting phenomenon was observed at 20ºC
involving a "volcanolike" instability which
caused flow of the aqueous solution to the oil
interface. This flow column, which is believed
to have resulted from an adverse density
gradient within the aqueous phase, is shown in
Fig. 19 [72]. The upper tip of the column was
observed to oscillate, probably due to gradients
in interfacial tension along the oil-water
interface (Marangoni flow). Of more importance
to a detergency process, the schematic diffusion
path shown in Fig. 20(a) explains why no
intermediate phase formed between the water
and oil. Also, due to the low solubility of oil in
the dilute aqueous surfactant solution in this
region of the ternary phase diagram, it predicts
the quite slow solubilization of oil into the
surfactant solution. At temperatures just below
the cloud point temperature, an intermediate
phase depleted in surfactant did form between
the micellar solution and the oil. The schematic
diffusion path in this case is shown in Fig.
20(b). Once again, instabilities in the aqueous
187
Fig. 19. "Volcano" instability in C12E5-water-ntetradecane system at 20ºC. The image is out of focus
to allow observation of refractive index variations
[72]. Reprinted with permission of Academic Press.
Fig. 20. (a) Diffusion path well below the cloud point
showing no intermediate phase formation; (b)
diffusion path slightly below the cloud point showing
the formation of intermediate phase W [72].
Reprinted with permission of Academic Press.
ous phase occurred, in this case due to a density
difference between the original micellar solution
and the intermediate phase, causing flow of
surfactant solution to the oil interface.
Nevertheless, at all temperatures studied below
the cloud point, only very slow solubilization of
oil into the surfactant solution was observed.
At 35ºC, which is just above the cloud point, a
much different behavior was observed. The
surfactant-rich L1 phase separated to the top of
the aqueous phase prior to contacting by
hexadecane.
188
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Upon addition of the oil, the drops of the L1
phase rapidly solubilized the hydrocarbon to
form an oil-in-water microemulsion containing
an appreciable quantity of hydrocarbon. After
depletion of the larger surfactant-containing
drops, a front developed as smaller drops were
incorporated into the microemulsion phase. This
behavior is shown schematically in Fig. 21.
Unlike the experiments carried out below the
cloud point temperature, an appreciable
solubilization of oil was observed in the time
frame of the study, as indicated by upward
movement of the oil-microemulsion interface.
Similar phenomena were observed with both
tetradecane and hexadecane as the oil phases.
When the temperature of the system was
raised to just below the phase inversion
temperatures of the hydrocarbons with C12E5
(45ºC for tetradecane and 50ºC for hexadecane),
two intermediate phases formed when the initial
dispersion of L1 drops in the water contacted
the oil. One was the lamellar liquid crystalline
phase Lα (probably containing some dispersed
water). Above it was a middle-phase
microemulsion. In contrast to the studies below
the cloud point temperature, appreciable
solubilization of hydrocarbon into the two
intermediate phases, shown 4 min after
contacting in Fig. 22, was observed. A diagram
of the phenomena observed is shown in Fig. 23.
A similar progression of phases was found at
35ºC using n-decane as the hydrocarbon. At this
temperature, which is near the phase inversion
Fig. 22. Video frame showing intermediate phases 4
min after contact in C12E5-water-n-tetradecane
system at 45ºC near the PIT [72]. Reprinted with
permission of Academic Press.
Fig. 23. Schematic diagram showing the conversion
of L1 phase into a middle-phase microemulsion and a
liquid crystal dispersion, for the experiment depicted
in Fig. 22 [72]. Reprinted with permission of
Academic Press.
temperature of the water-C12E5-decane system,
the existence of a two-phase dispersion of Lα
and
water
below
the
middle-phase
microemulsion was clearly evident.
Fig. 21. Schematic diagram showing conversion of
the L1 phase into an oil-in-water microemulsion at
temperatures above the cloud point [72]. The symbol
me denotes a microemulsion. Reprinted with
permission of Academic Press.
To compare the rate of tetradecane
solubilization at temperatures below and near
the phase inversion temperature, verticalcontacting experiments were performed between
the L1 phase and oil at 40 and 48ºC. In both
cases, the surfactant-rich phase was the
equilibrium phase that separated from a 10%
aqueous solution at that temperature. At 40ºC a
liquid crystalline phase and an oil-in-water
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
micro emulsion formed between the L1 and oil
phases. At 48 º C, a similar phase progression
was observed, with a middle-phase microemulsion forming in place of the oil-in-water
microemulsion. As shown in Fig. 24, plots of
the position of the oil-micro emulsion interface
versus the square root of time were straight lines
in both cases indicating diffusion-controlled
mass transfer. In Fig. 24, an arbitrary constant
has been added to each position for ease of
comparison, i.e. x = 0 is not the initial contact
position. At any given time, the relative slopes
of the two lines are indicative of the relative
rates of oil solubilization. In this case, the rate of
solubilization near the PIT is 2.2 times greater
than at 40ºC.
Well above the phase inversion temperatures
for both hexadecane and tetradecane, 1 wt.%
C12E5 exists as a dispersion of liquid crystal Lα
in water. Rapid movement of the liquid crystal
to the oil occurred upon contacting, causing
extensive spontaneous emulsification of water in
the oil phase. Eventually a layer depleted in
liquid crystal formed near the oil interface.
Figure 25 shows the spontaneous emulsification
that occurred. Interpretation of the phenomena
Fig. 24. Variation of oil- microemulsion interface
with time at 40ºC and 48ºC following contact of the
L1 phase with oil for the C12E5-water-n-tetradecane
system.
189
Fig. 25. Video frame showing spontaneous
emulsification observed 18 min after initial contact in
the C12E5-water-n-hexadecane system at 60ºC [72].
Reprinted with permission of Academic Press.
by diffusion path analysis indicated that the oil
was being converted into a waterin-oil
microemulsion at this high temperature; this
means that very little solubilization of oil into
the aqueous phase was taking place. The
spontaneous emulsification occurred due to the
passage of the oil-phase diffusion path segment
across the twophase water-micro-emulsion
region, as shown in Fig. 26.
Experiments similar to those described above
were also performed using C12E4 as the
surfactant [18]. This surfactant is more hydrophobic than C12E5 and therefore has a lower
Fig. 26. Schematic diffusion path for the experiment
depicted in Fig. 25. Point d represents the
composition of the initial water-surfactant mixture;
HC, S, and tie denote hydrocarbon, surfactant, and
microemulsion. The last is oil-continuous in this
case. Some multiphase regions are not shown [72].
Reprinted with permission of Academic Press.
190
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
cloud point (7ºC) and PIT with hexadecane
(30ºC). Contacting experiments were performed
below, at, and above the PIT. In contrast to
C12E5, the structure of the 1 wt% C12E4 solution
was a lamellar liquid crystalline dispersion in
water at all the temperatures studied. With
regard to the intermediate phases which formed
at the different temperatures, the following
results were obtained. Below the PIT, an
oil-in-water microemulsion formed between the
lamellar liquid crystalline dispersion and oil. At
the PIT, the behavior was dependent upon the
concentration of liquid crystalline material at the
initial oil-surfactant solution interface. At the
low concentration of liquid crystal present at a
1% surfactant level, no intermediate phase
formation was observed when the aqueous
dispersion was contacted with hexadecane. To
provide a higher level of liquid crystal at the
initial point of contact, La drops were allowed to
cream to the air-water interface prior to the
contacting studies. These drops converted to
concentrated La domains upon heating to 30ºC.
With this initial aqueous phase structure, the
formation of a middle-phase microemulsion
layer was observed upon contacting with oil.
Finally, if a pure lamellar liquid crystalline
phase containing 25% surfactant was contacted
with oil, swelling of the liquid crystalline phase
was observed, as shown in Fig. 27.
Fig. 27. Video frame showing swelling of the
lamellar liquid crystalline phase 43 min after initial
contact for the C12E4-water-n-hexadecane system at
the PIT of 30ºC [18], Reprinted with permission of
Academic Press.
Dynamic contacting studies were also
performed
with
hydrophobic
additives
combined with C12E5 [30]. C12E3 and
n-dodecanol were added to C12E5 in proportions
to yield cloud points near that of C12E4
(approximately 7ºC). This study was conducted
An interpretation of these results was made by
comparing the qualitative diffusion paths,
shown in Fig. 28, resulting from the different
aqueous starting compositions [18]. The
differences in behavior for the dilute (C),
concentrated (B) and pure liquid crystal (A)
cases were attributed to the relatively high
solubility of C12E4 in the oil phase at this
temperature, which influenced whether the
diffusion path passed above or below the
various three-phase regions that are present on
the phase diagram.
At temperatures of 40-50ºC, which are above
the PIT, behavior similar to that for C12E5 was
observed. Dissolution of the lamellar liquid
crystalline phase into the oil resulted in the
formation of a water-in-oil microemulsion and
spontaneous emulsification of water in the oil
phase.
Fig. 28. Schematic diffusion paths representing
different beha- vior observed at different surfactant
concentrations for the C12E4-water-n-hexadecane
system at the PIT of 30ºC [18] The symbol (mp µe)
denotes a middle-phase microemulsion. Reprinted
with permission of Academic Press.
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
ducted to determine whether formation of
intermediate microemulsion phases containing a
high proportion of oil could be obtained at
temperatures lower than those for C12E5 alone.
However, despite exhibiting phase behavior in
the absence of oil similar to that of C12E4, the
C12E5-C12E3 mixture behaved in a way
intermediate to the behavior seen with C12E4 and
C12E5 upon being contacted with hexadecane.
Also, the microemulsion phases formed at
various temperatures when the C12E5-dodecanol
system contacted oil were essentially unchanged
from those seen in the C12E5 system without any
additive present. For example, rather than
forming a middle-phase microemulsion with
hexadecane at 30ºC, the two systems formed
oil-in-water micro-emulsions. The contacting
temperature had to be increased to 40ºC in the
case of the C12E5-C12E3 System and 50ºC in the
case of the C12E5-dodecanol system before
middlephase microemulsion formation was
observed. These differences between the two
systems were attributed to differences in
partitioning of the additive and the more
water-soluble C12E5 between the oil and the
microemulsion phases. The observed differences
between the two additive systems would be less
if smaller quantities of oil relative to the
surfactant solution had been present.
6.2. Water-alcohol ethoxylate-triglyceride
(+ hydrocarbon) systems
Triolein is a pure triglyceride suitable for use
as a model for kitchen soils such as vegetable
oils. Dynamic contacting studies similar to those
described above for hydrocarbons were
performed with triolein using aqueous solutions
containing the three alcohol ethoxylates C12E3,
C12E4 and C12E5 [48]. As described in the phase
behavior section, ternary triolein - water-nonionic surfactant systems exhibit different phase
behavior than those containing straight-chain
hydrocarbons. Specifically, the large size of the
triolein molecules inhibits solubilization and
formation of microemulsion phases.
191
At low temperatures, schematic phase
behavior like that shown in Fig. 8 is observed in
which two three-phase regions are present in the
ternary diagram. At these temperatures, the
surfactantwater mixture is a dispersion of the
liquid crystal La in water. When this dispersion
is contacted with triolein, a water layer forms
between the liquid crystal and the oil, and
extensive spontaneous emulsification occurs in
the oil phase [48]. This behavior can be
explained in terms of a diffusion path which
passes below the bottom three-phase region in
Fig. 8. Spontaneous emulsification occurs in the
oil phase due to passage of that diffusion path
segment across the two-phase water-oil region.
In general, insufficient surfactant is available at
the interface to form an intermediate L3 or D'
phase due to the high solubility of non-ionic
surfactants in triolein at these conditions. At still
higher temperatures where C12E3 and C12E4 are
in the form of aqueous dispersions of L3 (greater
than 30ºC for C12E3 and 55ºC for C12E4), similar
behavior occurs except that even more vigorous
spontaneous emulsification is observed.
C12E5 exhibited behavior at approximately
65ºC quite comparable to that observed with
hydrocarbon systems near the PIT. In fact, as
shown in Fig. 10, the D phase in that system
contains almost equal volumes of triolein and
water at 65ºC. When a concentrated lamellar
liquid crystalline dispersion contacted triolein at
that temperature, the D phase formed, first as
lenses along the water-oil interface, then as a
continuous layer. A schematic diffusion path
corresponding to this behavior is shown in Fig.
29.
The dynamic contacting of C12E4. solutions
with oil mixtures containing varying proportions
of triolein and hexadecane was also studied
[48]. For 3/1 hexadecane-triolein mixtures,
behavior comparable to that found with the pure
hydrocarbon system was obtained. Similarly,
3/1 triolein-hexadecane systems behaved in the
contacting experiments like the pure triglyceride
system. However, contacting of the concentrated
liquid crystalline dispersion at 38ºC with a 1/1
mixture of triolein and hexadecane resulted in
the formation of transient middle-phase
microemulsion droplets. The failure to form a
192
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Fig. 29. Diffusion path corresponding to observed
behavior for the C12E5-water-triolein system at
64.5ºC [48].
complete microemulsion layer during the course
of the experiment probably resulted from the
high solubility of the surfactant in the oil
mixture.
The latter experiment was repeated with
tertiary amyl alcohol (TAA) added to the
concentrated La dispersion at three different
TAA/C12E4 ratios: 0.05, 0.10, and 0.20 by
weight [52]. All three experiments showed the
rapid formation of an intermediate phase,
presumably the D or microemulsion phase. The
higher the ratio of TAA to surfactant, the faster
was the formation of the intermediate phase as a
continuous layer. As indicated in the previous
paragraph, the formation of a continuous
middle-phase microemulsion layer did not occur
in the absence of TAA.
7. Oil drop-contacting experiments
As discussed in the preceding section,
contacting experiments using vertically oriented
cells, and their interpretation using diffusion
path theory, have provided a fundamental
understanding of the conditions for the
occurrence of intermediate phase formation and
spontaneous emulsification for a variety of
systems containing water, pure surfactants, and
non-polar oils. However, when either surfactant
mixtures, e.g. the C12E5-additive systems
discussed above, or commercial surfactants that
are themselves complex mixtures, are used, the
conditions for which intermediate phases form
during a vertical cell-contacting experiment are
frequently rather different from those expected
during washing experiments in the same system
at the same temperature. The reason is
differential partitioning of different surfactant
species into the oil phase. As explained
previously in section 3, the extent of differential
partitioning depends on both the overall
surfactant concentration in the aqueous phase
and the water-to-oil ratio (see Eq. (2)). In
particular, the surfactant remaining after some
partitioning into the oil has occurred, the factor
which largely controls whether an intermediate
phase will form, is less hydrophilic for a
washing experiment in which the water-to-oil
ratio is very large than for a contacting
experiment in which the volumes of oil and
water are comparable. It should be noted that
this problem does not arise for systems
containing mixtures of anionic surfactants and
hydrocarbon oils, because none of the individual
surfactant species has appreciable solubility in
the hydrocarbon phase.
When the oil consists of one or more polar
components or of both polar and non-polar
components, there is another limitation of the
vertical cell-contacting technique that is
significant even when pure surfactants are used.
Basically, both experiment and diffusion-path
theory yield information on the behavior of the
system during the early stages of contact when
the oil phase remains at its initial composition
except in a region near the surface of contact.
However, when the volume of the oil phase is
small, the diffusion of polar material out of
and/or diffusion of surfactant into the oil can
cause the composition of the entire oil phase to
vary with time, i.e. no location exists where the
oil has its initial composition. This effect is
especially important when inverse micelles or
other aggregates form in the oil that have mixed
films of surfactant and polar compounds. As a
result, situations occur in which an intermediate
phase does not form on initial contact but de-
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
193
washing synthetic fabrics at low temperatures.
The oil dropcontacting technique was used to
determine
whether
an
intermediate
microemulsion phase would form near the PIT
for these mixed surfactant systems with
hydrocarbon soils [24] in a manner similar to
that described above for pure non-ionic
surfactants with the vertical cell technique.
Fig. 30. Schematic illustration of contacting
experiment in which a small oil drop is injected into
an aqueous surfactant solution.
velops later in the experiment when the oil
composition becomes suitable. Examples of
such behavior are discussed below.
An oil drop-contacting technique was
developed in which the water-to-oil ratio is
large, as in practical washing situations. As
shown in Fig. 30, a drop of oil, usually some
10-100 mm in diameter, is injected into a
horizontal rectangular glass cell by means of a
very thin hypodermic needle. The cell, which is
400 Rm thick, is inside a thermal stage modified
to enable the drop to be observed by
videomicroscopy from the moment of injection
[24,60]. Since the drop must be viewed through
the surfactant solution in which it is immersed,
this technique works best when the surfactant
solution is below its cloud point temperature.
However, some experiments have been
successfully carried out in which the initial
surfactant solution was a dispersion of the
lamellar liquid crystal in water, as discussed
below. It is noteworthy that no similar limitation
exists for the vertical cell technique, and indeed
almost all of the experiments described above
for non-polar oils were conducted above the
cloud point temperature.
7. 1. Experiments with surfactant mixtures and
nonPolar oils
Mixtures of anionic and non-ionic surfactants
are now almost universally used in liquid
detergents for laundry applications since they
are more effective than anionics alone for
The pure non-ionic surfactant C12E3 and the
commercial anionic surfactant Neodol 23-3S
were used in this study, i.e. the same
combination as discussed above in the phase
behavior section. The use of a commercial
mixture rather than a pure anionic surfactant had
minimal effect on the differential partitioning
since all the individual anionic species in the
mixture had very low solubilities in
n-hexadecane, the hydrocarbon used. Because
the volume of the oil drop injected was small, it
dissolved little non-ionic surfactant, and the
relevant PIT was that for which the surfactant
composition Ssn in the films within the
microemulsion phase was the same as the
overall surfactant composition in the system.
Data on Ssn for this system when the aqueous
phase contains 1 wt.% NaCl are given in Fig. 7.
As may be seen from Fig. 4, the initial washing
bath, i.e. the oilfree mixture of the surfactant
and a 1 wt.% NaCl solution, forms a dispersion
of the lamellar liquid crystal in brine for
surfactant compositions equal to the relevant
values of Ssn.
Contacting experiments were conducted for a
surfactant mixture containing 78 wt.% of the
nonionic surfactant [24]. According to Fig. 4,
the relevant PIT is 30ºC. At 25ºC, no
intermediate phase was observed and the drop
diameter did not decrease appreciably with time.
Thus, solubilization of hydrocarbon by the
liquid crystalline phase was very slow at this
temperature below the PIT. At 30ºC, an
intermediate
microemulsion
phase
was
observed. Its volume continued to increase until
the oil phase disappeared. At 40ºC, the drop
diameter increased with time, the expected
behavior above the PIT as the oil phase takes up
surfactant and water. The liquid crystalline
particles surrounding the drop made it difficult
to discern whether spontaneous emulsification
194
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
in the oil occurred under these conditions, as
would be expected based on observations above
the PIT described above for the vertical cell
experiments. The conclusion reached from these
experiments is that mixed surfactant systems
behave in basically the same manner as pure
surfactant systems for hydrocarbon oils,
provided that the PIT used to interpret the
behavior is as defined above.
Valuable results were also obtained by the oil
drop technique for the C12E4-water-triolein-TAA
system. As discussed in Section 3, the addition
of TAA reduced the temperature at which the D
phase formed in this system and also promoted
formation of the D instead of the D' phase. The
latter has considerably less ability to solubilize
triolein than the former. When a drop of pure
triolein was injected into an alcohol-free
mixture of C12E4 and water at 50ºC, spontaneous
emulsification was observed within the drop as
well as the growth of small myelinic figures at
the drop surface [52]. The drop volume
decreased slowly with time, an indication that
some triolein was being solubilized into the
liquid crystalline particles present initially. This
behavior is, as expected, the same as was seen
with the vertical cell technique for the same
system and temperature.
In contrast, with TAA added to the
surfactantwater mixture in an amount equal to
20 wt.% of the surfactant, an intermediate liquid
phase, presumably the D phase, formed when a
triolein drop was injected at the same
temperature. As shown in Fig. 31, the drop
shrank considerably during the first few minutes
of the experiment as triolein was solubilized into
the D phase. After about 5 min, the triolein drop
had disappeared completely. The contacting
experiments thus confirmed the conclusion
reached from the phase behavior results that
TAA promoted the formation of the D phase,
which is capable of solubilizing considerable
triolein.
7.2. Experiments with pure long-chain alcohols
It can be shown using diffusion path theory
that when the initial surfactant concentration in
Fig. 31. Video frames showing the dynamic behavior
of a drop of triolein contacted with 5 Wt-% C12E4
with added TAA at 50-C (TAA/C12E7=0.20).
Obtained from Ref. 52.
an aqueous micellar solution is sufficiently low,
no intermediate phases form upon initial contact
of this solution with a long-chain alcohol.
However, an intermediate phase does form
when the surfactant mass fraction exceeds a
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
critical value ws* given by the following
equation [73]
ws* = wsB F1(woC) + wsC F2 (woC)
(5)
Here wsB and wsC are the surfactant mass
fractions at the L1 and L2 ends of the limiting tie
line forming one boundary of the two-phase
region for these phases, and woC is the oil mass
fraction at the L2 end. The quantity wsB is often
called the "limiting association concentration"
or LAC [56]. The functions F1 and F2 are
defined as follows
where Do and Ds are the diffusivities of the oil
and the surfactant in the L2 phase, D's is the
diffusivity of the surfactant in the L1 phase and
hoC is the (constant) value of the similarity
parameter [x/(4D.t) 1/2 ] at the L2 end of the
limiting tie line (see discussion of diffusion path
analysis above).
Vertical cell-contacting experiments gave
results in reasonable agreement with Eq. (3) for
the sodium octanoate-n-decanol-water system
[73]. One might expect that detergency would
be improved when the intermediate phase - in
this case the lamellar liquid crystal - is formed
since more of the alcohol is solubilized. Indeed,
Kielman and van Steen [11] observed such
behavior in the potassium octanoate-n-decanolwater system.
The focus of this section is the time-dependent
behavior of the system if a drop of alcohol is
injected into a solution whose surfactant
concentration is below the critical value. The
question to be answered is whether an
intermediate phase will form at some time after
initial contact.
A series of such experiments was performed
for the C12E5-water-oleyl alcohol system at 30ºC
[74]. The L1-L2 coexistence curve and some
195
interfacial tensions between these two phases
are shown in Fig. 32. Note that the coexistence
curve terminates at a point F corresponding to a
water content of about 70 wt.%, which is one
vertex of the L1-Lα-L2 three-phase triangle.
Video frames taken at various times during an
experiment
in
which
the
surfactant
concentration was 1 wt.% are shown in Fig. 33.
Note that the drop, initially some 70 mm in
diameter, swells as it takes up water and
surfactant. After about 23 min, the lamellar (La)
phase begins to develop as myelinic figures
which grow into the aqueous solution.
Eventually, nearly all the alcohol is converted to
liquid crystal.
The order of magnitude of the time required
for diffusion within the drop is the ratio of the
square of its radius to the diffusion coefficient.
As this time is much less than that of the
experiment, a quasi-steady state scheme may be
used to model drop behavior. Basically, this
means that drop composition may be viewed as
TIE -LINE
1
2
3
4
INTERFACIAL TENSION (dyne/cm)
2.52
1.03
0.25
0.03
Fig. 32. Partial ternary phase diagram for the
C12E5-water- alcohol system at 30ºC showing the
L1-L2 coexistence curve and the limiting tie line EF.
The oil drop composition varies as indicated by the
arrows. The interfacial tensions are between
pre-equilibrated phases for the four tie lines shown
[74]. Reprinted with permission of Plenum Press.
196
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Fig. 33. Video frames showing dynamic behavior following contact of 1.0 wt.% solution of C12E5 with a drop of
pure oleyl alcohol at 30ºC [74]: (a) alcohol drop about 2 min after injection; (b) the same, about 20 min later; (c)
initial formation of lamellar phase about I min later; (d) growth of myclinic figures into the surrounding aqueous
phase about 3 min later; (e) almost complete conversion of alcohol into liquid crystal about 8 min later. Reprinted
with permission of Plenum Press.
moving along the coexistence curve in the
direction of the arrows in Fig. 32. Since the
long-chain alcohol has low solubility in the
aqueous phase, the drop volume increases
during this process as its contents of water and
surfactant increase. The result is swellingas
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
observed during the experiment. When the drop
composition reaches point F at the end of the
coexistence curve, the driving force for
diffusion of the surfactant and water into the
drop remains. gut because the drop cannot
remain in the L2 region, according to the phase
diagram, the lamellar phase begins to form.
A theory has been developed which predicts
that the time t from the start of the experiment
until the liquid crystal starts to form is given by
the following expression [60]
t = Ks (R02/Dsws∞)
(6)
Here R0 is the initial radius of the drop, Ds and
are the diffusivity and bulk concentration of the
surfactant in the aqueous solution and Ks is a
constant that depends only on the shape of the
coexistence curve and the location of point F.
The predicted proportionality between t and Ro
2 has been confirmed by experiment (see Fig.
34), as has the inverse relationship between t and
the bulk surfactant concentration ws∞. With a
197
value of 0.514 for K. calculated using the phase
behavior of Fig. 32, Eq. (6) and the measured
values of t were used to estimate Ds. A value of
about 4 x 10-11 m2 s-1 was obtained, which is
reasonable for a micellar solution.
A similar equation has been developed for the
case of a uniform layer of oil on a flat solid
surface immersed in a stirred aqueous surfactant
solution [74]
t = Kp (h0d/Dsws∞)
(7)
where ho is the initial thickness of the oil layer
and d is the thickness of the diffusion boundary
layer adjacent to the oil. Like Ks in Eq. (6), Kp
depends only on the shape and terminal point of
the coexistence curve between the L1 and L2
phases.
For the C12E8-water-n-decanol system at
temperatures above 14ºC, the three-phase
triangle bounding the L1-L2 region has D'
instead of Lα as the additional phase [59]. When
a contacting experiment was conducted at 27ºC
in this system, with a drop initially about 60 µm
in diameter, a liquid intermediate phase
developed after about 14 min and surrounded
the initial alcohol drop [55]. As the intermediate
phase grew during the next 8 min to a diameter
of about 83 mm, the alcohol drop shrank slightly
to a diameter of about 83 µm. Presumably, this
growth process could be analyzed using a
"shrinking core" model with the quasi-steady
state approximation. However, as the available
data on phase behavior [59] do not include
coexistence curves for the L1-D' and L2-D'
two-phase regions, it is not currently possible to
make quantitative comparisons between
predictions of the analysis and the experimental
results.
7.3. Experiments with mixtures of hydrocarbons
and long-chain alcohols
Fig. 34. Plot of the square root of the time t required
to initiate liquid crystal formation as a function of
initial drop size for the system of Fig. 32. The
surfactant concentration in the aqueous phase is 1.0
wt.% [74]. Reprinted with permission of Plenum
Press.
More interesting for detergency applications
than pure alcohols are mixtures of polar and
nonpolar oils, which are representative of
sebum-like soils. Here we discuss a series of
experiments in which drops of various mixtures
of n-hexadecane and oleyl alcohol, typically
198
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
containing at least 50 wt.% hydrocarbon, were
injected into dilute aqueous solutions of pure
non-ionic surfactants at temperatures below
their cloud points [60]. PIT measurements for
these systems were given previously (Fig. 15).
A general pattern of behavior was observed.
At temperatures below the PIT, as given by Fig.
15, no intermediate phase was seen at any time
during the experiment and the drop volume
decreased very slowly with time owing to
solubilization of the alcohol and hydrocarbon
into the micellar solution. Above the PIT, the
behavior was similar to that described in the
preceding section for the C12E5-water-oleyl
alcohol system. That is, the drop would swell,
and, at a particular time, the lamellar liquid
crystalline phase could be seen growing as
myelinic figures. However, the myelinic figures
were shorter, smaller in diameter and more
numerous, and they grew faster than in the pure
alcohol system. The differences can be seen by
comparing Fig. 35 for a drop initially containing
85 wt.% hydrocarbon and 15 wt.% alcohol
immersed in an aqueous solution containing
0.05 Wt-% C12E8, with Fig. 33 for the pure
alcohol
system.
The
low
surfactant
concentration for the mixed oil experiment is
typical of those used in household laundry
processes.
Coexistence curves for the L1-L2 region were
determined at 30ºC for systems containing
n-dodecyl heptaoxyethylene monoether (C12E7)
and oils with 50% and 75% n-hexadecane,
respectively [60]. In both cases the curves
extended to water contents of 75-80 wt.%.
Contacting experiments for various initial drop
sizes and surfactant concentrations in the latter
system confirmed that the dynamic behavior
was consistent with Eq. (6). Here, too, liquid
crystalline intermediate phases were seen for
drops in contact with solutions containing only
0.05 wt.% surfactant.
It was also noted that the rate of swelling of
the drops increased markedly as the limiting tie
line was approached. Since the coexistence
curves exhibited nearly constant alcohol-tosurfactant ratios near the limiting tie lines, this
behavior was expected as little surfactant must
diffuse into the drop for it to experience a
substantial increase in volume. The basic
analysis leading to Eq. (6) confirmed
quantitatively that rapid swelling should occur
for these conditions.
Values of the parameter Ks were found from
the phase behavior data to be 0.313 and 0.0516
for the two systems. That is, for a given initial
drop size and surfactant concentration,
formation of the liquid crystal occurred more
rapidly in the system containing less alcohol,
presumably because less surfactant had to
diffuse into the drop to balance hydrophilic and
lipophilic properties of the surfactant films -to
the extent that conditions favorable for
formation of the lamellar phase were created.
Indeed, a general result of the contacting
experiments with various surfactants and oils
was that more time was required for liquid
crystal formation when the system was made
less hydrophilic by increasing temperature or
the alcohol content of the drop or by reducing
the ethylene oxide chain length of the surfactant
[60].
One may ask why the intermediate phase
formed during the contacting experiments
should be the lamellar liquid crystal instead of a
microemulsion, in cases where the drops
contained substantial amounts of hydrocarbon.
After all, diffusion of surfactant into the drop
should cause the surfactantalcohol films within
it to become more hydrophilic, as indicated
above. Eventually, one might expect film
composition to approach that corresponding to
the PIT at the experimental temperature and
thereby cause formation of a middle-phase
microemulsion. The answer is that the drop
itself becomes a microemulsion as it takes up
water and surfactant. The occurrence of low
interfacial tensions supports this conclusion. For
instance. a tension of about 0.03 mN m-1 was
measured by the spinning drop technique about
50 min after the start of the experiment for an
oil phase initially containing 75% n-hexadecane
and 25% oleyl alcohol and an aqueous solution
containing 0.1 wt% C12E7 at 30-C [60].
The coexistence curve for this system
indicates that the water-to-hydrocarbon ratio
during an oil drop contacting experiment varies
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
199
Fig. 35. Video frames showing dynamic behavior following the contact of 0.05 Wt'% C12E8 solution with a drop
of 5.67/1 n-hexadecane-oleyl alcohol at 50ºC [60]: (a) oil drop about 13 min after injection; (b) the same about 8
min later; (c) growth of myelinic figures less than 6 s later; (d) further growth of myelinic figures into the aqueous
phase 12 s later. Reprinted with permission of the American Chemical Society.
from its initial value of zero to about 5 when
the drop composition eventually reaches the
end of the coexistence curve (corresponding to
point F of Fig. 32). No intermediate phase
forms until point F is reached because the
hydrocarbon content does not exceed the
solubilization limit of the microemulsion.
However,
at
point
F,
where
the
hydrocarbon-to-amphiphile ratio has dropped
to about 1.5, the microemulsion structure
apparently cannot be sustained and an
intermediate lamellar phase begins to form.
That it would form at such ratios of the various
components and when the composition of the
surfactant-alcohol films is approximately
balanced between the hydrophilic and
lipophilic properties is generally consistent
with phase behavior results for other systems
reported by Ghosh and Miller [46]. Indeed,
lamellar phases with even higher oil contents
have been reported near the PIT of the
C12E4-water-n-hexadecane system [75].
200
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
When the alcohol content of the oil is low and
the temperature is near or only slightly above
the PIT, only a small amount of surfactant need
diffuse into the drop in order for hydrophilic and
lipophilic properties of the surfactant-alcohol
films formed there to be almost balanced. In this
case, the hydrocarbon solubilization limit is
exceeded and drops of oil form within the
original drop, which has become a middle-phase
microemulsion. Such behavior was observed,
for example, when a drop containing 90 wt.%
n-hexadecane was injected into a solution
containing 0.05 wt.% C12E7 at 40ºC, which is
near the PIT for an oil of this composition. As
the oil content of the microemulsion decreased
during the experiment owing to this spontaneous
emulsification, and as the surfactant content
continued to increase, a point was eventually
reached when the hydrocarbon-to-amphiphile
ratio was too low to sustain the microemulsion
structure and the lamellar phase again developed
as myelinic figures [76].
Finally, it is noteworthy that when drops of
oil containing 75% n-hexadecane and 25% oleyl
alcohol were contacted with a surfactant
solution containing 0.05 Wt-% C12E7 at
temperatures above 40ºC, it appeared that the
first intermediate phase formed was a liquid,
presumably D' [76]. That is the system
apparently experienced between 30. and 400 the
transition discussed in the phase behavior
section above which D' replaces La as the third
phase in the three-phase region bounding the
region of coexistence between La and L2.
Detailed phase behavior at 40º was not
determined, however. The intermediate D' phase
in the contacting experiment was later converted
to liquid crystal (myelinic figures).
7.4. Experiments with mixtures of triolein and
longchain alcohols
Drops containing various mixtures of triolein
and oleyl alcohol were injected into 0.1 wt.%
solutions of C12E6 at 40ºC, which is about 10º
below the cloud point temperature of dilute
solutions of this surfactant [61]. As discussed in
section 3 and shown in Fig. 16, the sequence of
phases seen with increasing temperature in this
system is similar to that found with pure triolein,
the various transition temperatures being lower
for higher oleyl alcohol contents. For drops
containing less than 25 wt.% oleyl alcohol, a
scarcely perceptible change in drop size with
time was observed, an indication that the oil was
being slowly solubilized into the surfactant
solution. As indicated above, similar behavior
was seen for hydrocarbon- alcohol drops below
the PIT. Figure 16 confirms that the system is
quite hydrophilic under these conditions.
A liquid intermediate phase, presumably D',
formed after a few minutes for a drop containing
50 wt.% oleyl alcohol (Fig. 36(b)). Careful
study of the image at various depths of focus
revealed that this phase did not surround the
original drop but instead formed a drop in
contact with the original drop but larger in
diameter, as the figure shows. Later, myelinic
figures began to grow outward into the aqueous
solution from the D'phase (Fig. 36(c)).
Ultimately, what was left of the original oil drop
- its volume was only about a third of the initial
value - became detached from the intermediate
liquid phase and showed no further changes
with time.
Although this system has four components,
with the result that its phase behavior cannot be
adequately described by a triangular diagram
similar to Fig. 32, an explanation can be given
for the dynamic behavior observed. As in the
hydrocarbon-alcohol
systems,
surfactant
diffuses into the drop, and the surfactant-alcohol
films which form there become more
hydrophilic with time. Eventually, the boundary
of the L1-L2 coexistence region is reached, and
an intermediate D' phase forms as surfactant
continues to diffuse into the drop. As this new
phase
solubilizes
little
triolein,
the
alcohol-to-triolein ratio in the original drop
decreases greatly. However, the D' phase is
itself in contact with the surfactant solution and
accordingly experiences an increase in
surfactant content with time. When the
hydrophilic and lipophilic properties of its
surfactant - alcohol films are balanced, the D'
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
201
Fig. 36. Video frames from an experiment in which a drop containing equal amounts of triolein and oleyl alcohol
was injected into a 0.1 Wt'% C12E6 solution at 40ºC [61]. (a) Shortly after injection; (b) approximately 9 min later
(the intermediate phase has formed); (c) about 5 min later (the intermediate phase has grown, myelinic figures are
starting to form); (d) about I min later (a drop of unsolubilized oil separates from the intermediate phase); (e) the
drop is never completely solubilized. Reprinted with permission of Marcel Dekker publishers.
202
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
phase, whose films are known to be slightly
lipophilic, cannot persist, and the lamellar phase
forms as myelinic figures. Since the triolein-rich
drop does not dissolve, it may well be that the
system enters a four-phase region at the time the
myelinic figures develop.
For a drop containing 75% oleyl alcohol
behavior was more complex. Two intermediate
liquid phases formed at various times during the
experiment and greater solubilization was
observed [61]. Ultimately, the liquid crystalline
phase was seen as well.
When the drop is pure oleyl alcohol, no D'
phase is seen, and the first intermediate phase is
the lamellar liquid crystal. It develops as many
short myelinic figures, the appearance being
more similar to that of Fig. 35 for
hydrocarbon-oleyl alcohol drops than to that of
Fig. 33 for the C12E5-water-oleyl alcohol system.
8. Fabric detergency test methods
Laboratory evaluations of laundry detergency
can range from use of an actual washing
machine with a capacity of several gallons to
use of a Terg-O-Tometer mini-laundry machine.
The Terg-0-Tometer is a washing machine
which replicates on a small scale the cleaning
action of an agitatortype washing machine [77].
The capacity of a single Terg-O-Tometer pot is
approximately 11. The temperature can be
controlled quite accurately through the use of a
water bath surrounding a bank of four or six
pots. The small size of a Terg-O-Tometer allows
rapid comparative studies of detergent
formulations under the same washing
conditions. Also, formulations containing
expensive ingredients, e.g. pure alcohol
ethoxylates, or experimental surfactants of
limited quantity, can be evaluated at reasonable
costs. For these reasons, the use of
Terg-O-Tometer machines in detergent research
laboratories has become quite common.
The measurement of soil removal from fabrics
can be performed in several ways [78]. The
most straightforward technique involves visual
inspection for determination of cleanliness.
More sophisticated techniques involve the use
of spectrophotometry, chemical analyses, and
radiotracer methods. As with visual inspection,
the measurement of light reflectance from fabric
does not quantify actual soil removal but
depends on factors such as the distribution of
soil on the fabric, and the particle sizes.
Analyses of weight gain or loss through the
chemical extraction of soils from the fabric as
well as radiotracer methods both yield
measurements of actual soil removal and do not
depend on the change of appearance of the
fabric. Although not representative of real-world
visualization of cleanliness, both techniques
provide
complementary
information
to
reflectance measurements and are more useful
for mechanistic studies.
Specific advantages of the radiotracer
detergency method are given elsewhere [79,80].
This method makes use of mildly radiolabeled
soils for highly sensitive quantitative
determination of soil removal by simple
radiochernical techniques. Several radioisotopes
may be used as labels. Generally, polar oily
soils such as alcohols and acids are tagged with
small amounts of carbon-14 labeled material.
The use of two labels in a soil such as artificial
sebum, which contains both polar and non-polar
components, allows the removal of both types of
soil to be monitored simultaneously. The recipe
for such a labeled artificial sebum has been
given elsewhere [80]. Radiolabelled particulate
and protein soils have also been developed and
utilized [80].
The Shell Development Company has utilized
a radiotracer detergency method for many years
to evaluate laundry detergents and to study the
fundamental nature of oily soil removal [81,82].
In the case of oily soils, a 4-in square fabric
swatch is soiled with 28 mg of radiolabeled oil.
The soil is applied to the fabric in a toluene
solution, which is allowed to air dry. The swatch
is washed under controlled conditions in a
Terg-0-Tometer. Aliquots of wash water are
removed for radioactive counting, with the level
of soil remaining on the swatch determined by
difference. In addition to the sebum oil
described above, oily soils which have been
commonly utilized include tritium-labelled
hexadecane (cetane), 1/1 hexadecane-squalane
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
(a C30 branched hydrocarbon) and triolein.
Also, hydrocarbon-fatty acid (or alcohol) soils
of varying composition have been studied in
which both carbon- 14 and tritium labels are
utilized to allow the discrimination of removal
of both components. The following are results
of fundamental detergency studies using some
of these soils, which were performed to allow
correlation to the phase behavior and dynamic
contacting studies described above.
9. Fabric detergency - non-polar
hydrocarbon soils
Results of radiotracer detergency studies of
removal of hexadecane from 65/35 permanent
press polyester-cotton fabric are shown in Fig.
37 [18]. This soil can be considered a model for
non-polar hydrocarbon-based soils such as
lubricating oils. The washing solutions
contained 0.05 wt.% surfactant, resulting in a
fabric-to-soil weight ratio of
Fig. 37. Removal of n-hexadecane from 65/35
polyester-cotton fabric using 0.05 wt.% aqueous
solutions of C12E4 and C12E5 [18]. The arrows show
the cloud point temperatures of the surfactants.
Reprinted with permission of Academic Press.
203
40/1 and a surfactant-to-soil ratio of 9/1.
Triethanolamine (50 ppm) was included as a
buffer, and no water hardness was present. The
washing time in the Terg-0-Tometer was 10
min.
Of particular interest in these studies was the
effect of temperature and non-ionic alcohol
ethoxylate surfactant structure on the levels of
soil removal. As shown in Fig. 37, a strong
dependence on temperature was observed with
the highest levels of soil removal occurring
almost 200C above the cloud point of the
washing solution, a temperature regime in which
the washing solution structure is a liquid
crystalline dispersion. In fact, the optimum
detergency temperature (ODT) in each case
occurred very near the PIT of the
water-surfactant-hexadecane system. Although
not shown in Fig. 37, very poor detergency
occurred in the temperature range of interest
with C12E3 as the surfactant. Also, when a 1/1 by
weight mixture of hexadecane and squalane was
used as the soil, a similar peak in performance
was found for each surfactant near its PIT for
the mixed soil. These PITs were approximately
7ºC higher than the corresponding values for
hexadecane alone, i.e. 37 and 58ºC versus 30
and 52ºC. The equivalence of the PIT and ODT
in this type of detergency system has been
confirmed by the work of Schambil and
Schwuger [22] as well as Solans et al. [83].
Also, the same correspondence has been found
for the removal of hydrocarbon by the same
surfactant from 100% polyester fabric [84]. The
high levels of soil removed near the PIT can be
attributed to the ultralow interfacial tensions
achieved near that temperature, and to the high
rates of oily soil solubilization into
middle-phase microemulsions, as was visualized
in the dynamic contacting studies described
previously.
When the detergent properties of mixtures of
C12E5 with the hydrophobic additives C12E3 and
n-dodecanol (C12E0) were studied, optimum
detergency was found at temperatures lower
than the ODT for C12E5 alone but somewhat
higher than the ODT for C12E4. Detergency
data for the two additive systems, which
exhibited the same cloud point as C12E4, are
204
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Fig. 38. Removal of n-hexadecane from 65/35
polyester-cotton fabric using 0.05 wt.% aqueous
solutions of a 90/10 blend of C12E5 and n-dodecanol
[30]. Reprinted with permission of Academic Press.
shown in Figs. 38 and 39 [30]. The optimum
temperatures correspond quite closely to the
phase inversion temperatures extrapolated to the
low soil-to-surfactant ratios used in the
detergency studies. In this regard, C12E3 was
somewhat more effective than dodecanol in
lowering the optimum detergency temperature.
Of interest here is that the detergency results
differed from the behavior observed in the
verticalcontacting studies in which, as discussed
above, somewhat higher temperatures were
required for fastest oil solubilization due to
partitioning of the additive into the oil.
Practical detergency applications utilize
commercial alcohol ethoxylate surfactants
which contain a broad range of species having
varying hydrophobe lengths and levels of
ethylene oxide, Detergency studies were
performed in the manner described above for the
single ethoxylate and ethoxylate-additive
systems using commercial ethoxylates based on
a blend of predominantly normal C12-C13
alcohols and containing an average of 3, 4 and 5
mol of ethylene oxide, respectively [85]. These
materials are denoted N23-3, N23-4 and N23-5.
Table 2 compares the ODTs for these systems to
those of the corresponding specific alcohol
ethoxylate systems having the same average
structure. The ODTs of the commercial
materials and their molar-average equivalents
are the same in all three cases. The ODTs of the
commercial systems, in fact, match the PITs for
those systems with hexadecane extrapolated to
very low levels of oil. The PIT data are shown
in Fig. 40. Shown for comparison are the PIT
vs. soil-surfactant ratio plots for C12E4 and
C12E5. These are flat, indicative of the fact that
the PITs for ternary specific alcohol ethoxylatewater-hydrocarbon systems are independent of
the oil-surfactant ratio. Bercovici and Krussman
Table 2
Correlation of PIT
temperature ODT
Fig. 39. Removal of n-hexadecane from 65/35
polyester-cotton fabric using 0.05 wt.% aqueous
solutions of a 60/40 blend of C12E5 and C12E3 [30].
Reprinted with permission of Academic Press.
Surfactant
Specific ethoxylate
C12E5
C12E4
C12E3
Broad-range Ethoxylate
N23-5
N23-4
N23-3
to
optimum
detergency
PIT (ºC)
ODT (ºC)
52
31
< 20
50
30
< 20
51a
26a
< 20a
50
30
< 20
The data shown are for cetane removal from 65/35
polyester-cotton with 0.05% surfactant [85].
a
Evaluated at cetane-surfactant ratio → 0.
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Fig. 40. PIT as a function of n-hexadecane(cetane)surfactant weight ratio for commercial (broad-range)
and specific alcohol ethoxylates [85].
[86] have shown that the addition of long-chain
alcohols to commercial alcohol ethoxylates, as
described above for the specific ethoxylate
C12E5, reduces the PIT and optimum detergency
temperature for nonpolar soil removal by those
surfactants. As with the specific ethoxylate and
surfactant- additive systems, optimum soil
removal with commercial nonionic surfactants
occurs near the extrapolated PIT because the
balanced surfactant system provides high rates
of oil solubilization and low interfacial tensions.
The effects of the addition of anionic
surfactants to non-ionics on the relationship of
soil removal to washing temperature have also
been investigated [87]. In this case, the same
commercial sodium alcohol ethoxysulfate
mentioned in the phase behavior section
(Neodol 23-3S) and the sodium salt of C12 linear
alkylbenzenesulfonate (denoted C12LAS) were
used as the anionic surfactants. As noted
previously, C12E3 itself is not effective at
removing hexadecane from 65/35 polyestercotton at temperatures between 20 and 70ºC.
This result can be attributed to C12E3 being too
hydrophobic. However, the addition of
appropriate amounts of hydrophilic anionic
surfactant was found to improve its performance
in a 1% NaCl solution, as shown in Figs. 41 and
42. The total surfactant concentration in the
205
Fig. 41. Removal of n-hexadecane (cetane) by
C12E3-N23-3S blends [87]. Reprinted with
permission of the American Oil Chemists' Society.
Fig. 42. Removal of n-hexadecane by C12E3-C12LAS
blends [87]. Reprinted with permission of the
American Oil Chemists' Society.
washing solution in these experiments was
0.05% by weight, the same as in the non-ionic
surfactant studies discussed above.
Optimum detergency temperatures were
increased to approximately 30ºC by the
substitution for C12E3 of 22% and 24% N23-3S
and LAS, respectively, and to approximately
50ºC by the substitution of 33% and 38%
N23-3S and LAS. Consistent with these results
was the finding of optimum detergency at 35ºC
with a 76.5/23.5 mixture of C12E3 and N23-3S
[24]. Thus, in contrast to the hydrophobic
206
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
additives such as dodecanol described above,
the addition of anionic surfactants increases the
temperature for optimum detergency. However,
as found for the hydrophobic additives, the ODT
still corresponds closely to the PIT at a low
oil-surfactant weight ratio. In the case of the
anionic surfactants, the ratio of anionicto-nonionic surfactant yielding phase inversion at a
given temperature was determined rather than
the PIT for a given surfactant composition, as
the oil-surfactant ratio was varied [87]. The
phase inversion composition plots determined
by electrical conductivity measurements and
used for comparison with the detergency data
are shown in Figs. 43 and 44 [87]. These results
are consistent with those of other phase behavior
studies with the same system (see Fig. 7). At a
given temperature, the extrapolated phase
inversion composition at a very low
soil-surfactant ratio matches the detergent
composition yielding optimum detergency at
that temperature. The high levels of soil removal
can be explained by the solubilization of oil into
a middle-phase microemulsion at the optimum
conditions, as described in the oil
dropcontacting section above.
In summary, for the removal of n-hexadecane
soils by a variety of pure non-ionic surfactant
systems, non-ionic surfactant-hydrophobic addi-
Fig.44.
Phase
inversion
compositions
for
C12E3-C12LAS system [87]. Reprinted with
permission of the American Oil Chemists' Society.
tive systems, and non-ionic-anionic surfactant
systems, optimum detergency can be related to
the PIT. More specifically, the optimum
detergency temperature is the extrapolated PIT
at low oil-surfactant ratios where the
composition of the surfactant films matches the
composition of detergent in the washing
solution. In all cases the surfactant solutions at
the optimum conditions are dispersions of
surfactant-rich phases, most commonly lamellar
liquid crystals in water.
10. Fabric detergency - triglycerides and
hydrocarbon-polar soil mixtures
Fig. 43. Phase inversion compositions for
C12E3-N23-3S system [87]. Reprinted with
permission of the American Oil Chemists' Society.
Radiotracer detergency studies of triolein soil
removal from 65/35 permanent press
polyester-cotton fabric have been performed
under the same conditions as. described in the
previous section [48]. The triolein, which
models more complex triglyceride mixtures
such as those found in cooking oils, was tagged
with tritium-labeled triolein. Figure 45 shows
the results for triolein removal using 0.05%
solutionso of C12E3, C12E4, and C12E5. For C12E3,
triolein removal is very low at all temperatures
studied. This result is consistent with that found
for the removal of hexadecane by the same
surfactant. C12E4 provides higher and almost
constant soil removal between 25 and 50 ºC.
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
Fig. 45. Removal of triolein from 65/35 polyestercotton fabric by washing with pure non-ionic
surfactants [48].
Above 50ºC, detergency drops off sharply. C12E5
yielded optimum detergency at still higher
temperatures, with a rather sharp peak in
performance occurring at 65ºC.
The optimum detergency temperature ranges
for both C12E4 and C12E5 are higher than the
ODTs found with hexadecane soil. At 65ºC with
C12E5, optimum detergency corresponds to the
temperature at which the D phase forms in that
system, i.e. the PIT. With C12E4, no sharp peak
in performance is noted because the D phase
forms in that system only for a very narrow
temperature range [48]. The decrease in
detergency at high temperatures (above 50ºC for
C12E4 and 65ºC for C12E5) corresponds to the
regime where the surfactant solubility in the
triolein phase increases significantly and a
water-in-oil microemulsion forms, i.e. where
little if any solubilization of oil into the washing
solution occurs.
Detergency studies have also been performed
in which TAA was added to the washing
solution at levels up to 20% relative to the
surfactant [52]. This study was performed to
determine if the addition of TAA could increase
triolein removal by promoting the formation of
the D phase during the washing process. The
207
levels of soil removal were found by extracting
the washed fabric swatches (both 100%
polyester and 65/35 polyester-cotton) with
hexane. For C12E4, triolein removal was poor at
50ºC and was unaffected by the addition of
TAA. Soil removal was much greater at 35ºC,
with the addition of 20% TAA providing only a
slight improvement when compared with
detergency with C12E4 alone. The lack of
significant performance enhancement was
attributed to the high water solubility of TAA,
which limits its effectiveness in highly dilute
surfactant solutions. Formation of the D phase
in the oil drop-contacting studies (see Fig. 31)
apparently occurred due to the higher
concentration of surfactant and TAA used in
that study.
A similar dependence of soil removal on
temperature was observed when the same
surfactants were used to remove a soil mixture
containing 1/1 hexadecane-triolein. In this
study, the triolein was labeled with tritium and
the hexadecane was labeled with carbon-14 to
allow independent measurement of the removal
of the two components. The results of that study
are shown in Fig. 46. When compared with the
results in Fig. 45, somewhat higher removal
levels of triolein were found from the soil
mixture. Also of interest is that removal of hexa-
Fig. 46. Removal of triolein and n-hexadecane from
65/35 polyester-cotton fabric by washing with pure
non-ionic surfactants [48]. Soil contains 50 wt.%
triolein.
208
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
decane was consistently higher than that of
triolein at all temperatures. The regions of
optimum detergency occur near the PITs of the
surfactants with the oil blend. The preferential
removal of hexadecane can be attributed to the
higher solubility of hexadecane in the
middle-phase microemulsion D (see Table 1).
The removal of blends of hexadecane with
oleyl alcohol and oleic acid by 0.05% solutions
of alcohol ethoxylates has also been studied
[12]. These binary soil mixtures are models of
sebum-like soils containing both non-polar and
polar fractions. In general, somewhat more
hydrophilic surfactants are required to obtain
high removal of these soils compared to the
optimum surfactants for nonpolar hexadecane
removal. Results for the removal of hexadecane
from 9/1 and 3/1 blends of hexadecane and oleyl
alcohol are shown in Figs. 47 and 48. N25-9 is a
broad-range commercial ethoxylate based on a
predominantly linear C12-C15 alcohol and
containing an average of nine ethylene oxide
(EO) units. In the case of the 9/1 soil at 20ºC,
detergency was found to increase with
decreasing EO content of the pure non-ionic
surfactants, while the reverse trend was true at
the higher temperatures. For both soils,
optimum detergency occurred over a
temperature range below the cloud points of the
surfactants, behavior which contrasts markedly
Fig. 48. Removal of n-hexadecane for mixed 3/1
n-hexadecane-oleyl alcohol soil [12]. Reprinted with
permission of American Oil Chemists' Society.
Fig. 47. Removal of n-hexadecane for mixed 9/1
n-hexadecane-oleyl alcohol soil [12]. Reprinted with
permission of the American Oil Chemists' Society.
Fig. 49. Effect of oleyl alcohol content on
n-hexadecane removal [12]. Reprinted with
permission of the American Oil Chemists' Society.
markedly with the results discussed above for
hydrocarbon and triolein soils. The cleaning
efficiency of the commercial broad-range
ethoxylate is almost identical at all temperatures
to that of the specific ethoxylate (C12E8) having
nearly the same cloud point temperature,
indicating, once again, that surfactant
partitioning effects in the detergency studies are
negligible.
The effect of the soil composition on soil
removal can be seen more clearly in Fig. 49.
While the detergency peaks near 80ºC with the
pure hexadecane soil, good detergency is
obtained at much lower temperature with the
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
mixed soils. Maximum detergency is reached
near 40ºC with 10% oleyl alcohol in the soil
while high levels of cetane removal are attained
down to 20ºC with 25% oleyl alcohol present.
The temperatures at the lower limit of the
plateaus in detergency are the respective phase
inversion temperatures. For C12E7, PIT values
can be obtained from the curves in Fig. 15.
Above the PIT, soil removal remains at a high
level until the cloud point of the surfactant is
reached. As implied in the oil drop-contacting
section, the formation of an intermediate liquid
crystalline phase plays the key role in the
soilremoval process in the plateau regime.
The relative amount of polar soil also affects
the rate of soil removal. Despite being removed
at a comparable level to that of the 3/1
cetane-oleyl alcohol soil after 10 min at 50ºC,
the 9/1 soil is actually removed much more
quickly, as indicated in Fig. 50. This very fast
removal can be attributed to that soil having a
PIT of approximately 50ºC with C12E7. A 1/1
cetane-oleyl alcohol soil is removed very slowly
at 50ºC, although a high level of removal is
ultimately attained. Removal of the hexadecane
soil reaches a plateau at a lower level since the
washing temperature is well below the optimum
detergency temperature for its removal by C12E7,
i.e. approximately 80ºC.
209
Similar results to those described for
hexadecane-oleyl alcohol soils were obtained
for hexadecane-oleic acid soils having the same
ratio of nonpolar and polar constituents. These
studies were performed in the absence of
triethanolarnine to insure that the oleic acid was
in the non-ionized state. Also, the oleic acid was
tagged with 14C-labeled material to allow
measurement of its removal as well as that of
hexadecane. The results for 3/1 cetane-oleic acid
soil are shown in Fig. 51. High levels of soil
removal were found over a wide temperature
range from 20ºC to the cloud point of the
surfactant. As with the oleyl alcoholcontaining
soils, optimum removal was found between the
PIT and cloud point temperature. Interestingly,
the data show that the removal of oleic acid
paralleled that of cetane, although at levels
10-20% higher. This behavior results from the
higher ratio of oleic acid to hexadecane found in
the intermediate liquid crystalline phase
compared to that in the original soil.
Presumably, preferential removal of oleyl
alcohol would have been found in the
hexadecane-oleyl alcohol experiments if the
alcohol had been radioactively tagged.
11. Discussion
As shown above, maximum removal of liquid
hydrocarbon soil from synthetic fabrics in non-
Fig. 50. Kinetics of n-hexadecane removal for mixed
n-hexadecane-oleyl alcohol blends [12]. Reprinted
with permission of the American Oil Chemists'
Society.
Fig. 51. Oil removal for mixed 3/1 n-hexadecaneoleic acid soil [12]. Reprinted with permission of the
American Oil Chemists' Society.
210
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
ionic and anionic-non-ionic surfactant systems
occurs near an appropriately defined PIT because solubilization of hydrocarbon into intermediate microemulsion or liquid crystalline phases
is high and interfacial tensions are low. The low
tensions facilitate emulsification of the intermediate phases into the agitated washing bath.
Hydrocarbon soil removal is low at
temperatures significantly below the PIT
because, as is well known, the amount of oil that
can be solubilized by microemulsion phases
under these conditions is well below the
solubilization
capacity
of
middlephase
microemulsions near the PIT, and interfacial
tensions are significantly higher. Moreover, the
initial rate of solubilization is less than that
found near the PIT, as shown in Fig. 24 above.
At temperatures significantly above the PIT,
the explanation for the poor detergency is
different. In this case both surfactant and water
diffuse into the oil phase, so that it actually
increases in volume instead of being solubilized.
Moreover, extensive spontaneous emulsification
of water in the oil occurs, as seen, for example,
in Fig. 25. Solans and Azemar [19] reported that
polyester-cotton fabric washed at temperatures
above the PIT often experienced a net gain in
weight, a result consistent with the above
statements. They further observed that
"aggregates", which apparently contained both
oil and surfactant, remained on the fabric
surfaces after washing. They suggested that
additional water drops might be incorporated
into the initial spontaneously formed emulsion
as a result of agitation during the washing
process, the result being an emulsion with a
viscosity so high that it is not easily removed.
Indeed, they proposed that the disperse phase
content could become so large that a so-called
high internal phase ratio (HIPR) emulsion could
be formed on the fabric with the drops of water
becoming polyhedral and the emulsion
developing a yield stress.
Yang and Rathman [88] confirmed that
increases in weight occurred for polyestercotton fabrics washed above the PIT, and they
found a surfactant-to-oil ratio of about 0.5 in
material extracted from the washed fabric in one
system.
They also found that clean fabric became
soiled if it was washed with soiled fabric above
the PIT. Thus, redeposition of soil, probably in
the form of an oil-continuous emulsion or
microemulsion, must be considered a significant
factor in the washing process under these
conditions.
In the studies discussed so far, the effects of
electrolytes on detergency performance were not
investigated. Recent results have shown that the
optimum detergency temperature and the PIT
are equivalent but lower when salts such as
sodium citrate, a common builder for liquid
detergent formulations, are present in appreciable amounts in the washing solution [89]. Similar effects with other salts would be expected.
For the hydrocarbon soils and surfactants of
interest for detergency, the PIT is invariably
above the surfactant cloud point, so that the
washing bath is a dispersion of the
surfactant-rich liquid L1 or the lamellar liquid
crystalline phase La in water. In most contacting
experiments near the PIT an intermediate
microemulsion phase was observed near the
surface of contact. However, as mentioned
above in connection with the vertical cellcontacting experiments for C12E4 and n-hexadecane at 30ºC, no microemulsion phase was
seen when the dispersion of the lamellar phase
was sufficiently dilute. In this case, small
particles of liquid crystal striking the oil-water
interface apparently dissolved in the oil,
although it is likely that tiny drops of microemulsion quickly formed and dissolved during
the process. However, if only a small quantity of
oil had been present in this experiment, as in
practical washing situations, it would have
rapidly become saturated with surfactant by
taking up such particles, and subsequent
particles striking the interface would have
produced an intermediate microemulsion phase.
The experiment where the pure lamellar phase
was contacted with oil for the same system at
the same temperature is also of interest. Here
considerable oil was solubilized directly by the
liquid crystal although, as the dark region in Fig.
27 and the diffusion path of Fig. 28 indicate,
some microemulsion did form as a dispersion
with the liquid crystal at a location away from
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
the oil-liquid crystal interface. This experiment
demonstrates that the formation of an
intermediate phase is not necessary for high
solubilization if sufficient quantities of a
surfactant-rich phase are present initially.
Indeed, such solubilization into the lamellar
phase was apparently responsible for the
moderate removal of pure triolein from
polyester-cotton fabrics by C12E4 at temperatures
between about 25 and 50ºC (Fig. 45). The poor
detergency performance at high temperatures in
this case is, as with hydrocarbon soils, the result
of conversion of the oil phase into a water-in-oil
microemulsion accompanied by spontaneous
emulsification of the water there.
If two different types of hydrocarbon soils are
present, a useful strategy with a single pure
surfactant would be to wash near the higher of
the two PIT values and rinse near the lower.
Experiments have shown that substantial soil
removal does take place when washing occurs
above the PIT and rinsing near the PIT [83,88],
as would be the case in this example for the soil
with the lower PIT. One reason this method
works is that substantial surfactant remains on
fabric washed above the PIT, as mentioned
above, much of it probably dissolved in the
unremoved soil. If there are multiple
hydrocarbon soils and if a commercial
surfactant or surfactant mixture is used, washing
should again start at the highest PIT. In this
case, however, it would be best to decrease
temperature with time during the latter stages of
washing or during rinsing, so that the PIT values
for the various soils would all be reached at
some time during the process. Note that during
rinsing the PIT values will not be the same as
those for the initial washing bath owing to
differences in surfactant composition and
surfactant-to-oil ratio during washing and
rinsing. The concept of designing a process so
that the PIT is achieved at some time as the
system is gradually made more hydrophilic is
related to certain strategies that have been
proposed for enhanced oil recovery processes,
e.g. variation of the injected salinity [90]. The
concept of washing at high and rinsing at low
temperatures to improve detergency was
proposed by Rubingh and Stevens [91] although
211
their explanation did not involve the PIT and the
formation of intermediate phases.
Achieving excellent detergency for pure
liquid triglyceride soils and synthetic fabrics is
difficult because the large triglyceride molecules
are not readily solubilized. Indeed, substantial
solubilization apparently requires conditions
where an intermediate D phase forms. With
straight-chain ethoxylated alcohols, this seems
not to be possible except at undesirably high
temperatures, e.g. about 65ºC with C12E5 and
triolein (Fig. 10). Adding a short-chain alcohol
does seem to make the surfactant films more
disordered and thereby promote D phase
formation at slightly lower temperatures (Fig.
11), but the relatively high solubility of the
alcohols in water makes them ineffective for this
purpose when the dilute surfactant solutions of
interest for detergency are used. As indicated
previously, preliminary results with secondary
alcohol
ethoxylate
surfactants,
whose
double-tailed structure with different chain
lengths also creates disordered films, look more
promising as a means of forming the D phase at
temperatures existing during warm and cold
water washing [55]. Further study of these
systems is in progress. Of course, the D phase
could also be formed by using washing baths
containing solubilized hydrocarbon (see Fig.
12), but this scheme seems less attractive than
using the secondary alcohol ethoxylate
surfactants.
Results presented above demonstrate that
liquid mixtures of hydrocarbons and long-chain
alcohols or undissociated fatty acids can be
removed effectivcly at temperatures below the
cloud point of the surfactant but equal to or
above the PIT of a system whose excess oil
phase coexisting with the microemulsion has the
same composition as the soil. Under these
conditions, the soil takes up surfactant and water
from the washing bath, as shown in the oil
drop-contacting experiment of Fig. 35, the
results being the lowering of the interfacial
tension between the soil and surfactant solution
and the eventual formation of an intermediate
lamellar liquid crystalline phase as filaments or
myelinic figures. These, presumably, are broken
off and dispersed into the washing bath as a
212
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
result of agitation. Note that Fig. 48 and 49
show no significant change in detergency
between 40 and 60ºC for C12E7 and the soil
containing 75% cetane even though the
contacting experiments mentioned above for this
system indicated that an intermediate D' phase
formed first and only later the liquid crystal
formed. Below the PIT no intermediate phase
was seen in all the systems, the drops were
slowly solubilized into micelles in the washing
bath, and soil removal was lower.
For all the systems discussed above for which
both videomicroscopy observations and soil
removal measurements have been performed,
intermediate phase formation and low interfacial
tensions leading to good detergency occurred
when the hydrophilic and lipophilic properties
of the surfactant or surfactant -alcohol films of
the intermediate phase were approximately
balanced. This balance may be present initially,
as in the experiments with hydrocarbon soils
near the PIT, or it may develop during washing
as a result of mass transfer, as in the
experiments with mixtures of hydrocarbons and
long-chain alcohols. Note that if mixed
surfactants and lower surfactant-to-oil ratios had
been used in the hydrocarbon washing
experiments, mass transfer effects would most
likely have been important there as well, i.e. the
system could have moved closer to or further
from the PIT over time. A similar conclusion on
the need for balance was reached by Malmsten
and Lindman [92], although without the proviso
that a system not balanced initially may achieve
balance during washing. It should also be
recognized that, even when balance is achieved,
the intermediate phase formed must have the
capability of solubilizing considerable soil, a
property clearly lacking in some of the
non-ionic surfactant-triolein systems discussed
above.
If a system contains soils with several
different mixtures of hydrocarbons and
long-chain polar compounds on the fabrics, an
effective strategy would seem to be to choose a
washing temperature and surfactant so that the
temperature is initially above the PIT of all soil
compositions. The surfactant must be
hydrophilic enough to be somewhat below its
cloud point but lipophilic enough to meet the
above PIT criterion. Mass transfer during the
washing process will bring the various soils to a
balanced condition (though at different times) at
which they can be removed by some
combination of intermediate phase formation
and emulsification promoted by low interfacial
tensions. This strategy is, of course, similar to
that suggested above for multiple hydrocarbon
soils except that no variation of temperature is
required.
For hydrocarbon soils the phase behavior of
the surfactant-water mixture in the initial
washing bath did not seem to have a significant
influence on detergency. Maximum soil removal
occurred at the system PIT whether the washing
bath consisted of a dispersion of the L1 phase or
of the Lα phase. In contrast, soil removal
dropped sharply when the cloud point
temperature of the surfactant solution was
reached for the mixed hydrocarbon-alcohol soils
above their PITs. It is likely that water formed
as in the intermediate phase for temperatures
above the cloud point temperature and hindered
the mass transfer necessary to achieve the
balanced condition discussed above. That is, the
diffusion path is most likely similar to that of
Fig. 26 except that the surfactant-rich phase is
L1 instead of La.
In the oil drop-contacting experiments it was
not unusual for 20 min or even more to elapse
before liquid crystal formation began. Such long
times are unsatisfactory for washing processes.
It should be kept in mind, however, that the
contacting experiments were deliberately
conducted with no imposed mixing to facilitate
the interpretation of the observations in terms of
diffusion theory. In actual washing situations,
mixing would greatly speed up mass transfer,
and liquid crystal could be expected to form
after much shorter times. Further discussion,
including some quantitative estimates of the
enhancement in mass transfer rates, may be
found in the original papers [60,74]. When
mixing occurs, the times required for liquid
crystal formation were found to be reasonable
for practical processes, a conclusion confirmed
by the excellent soil removal obtained in the
washing experiments described above.
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
In the preceding section, it was mentioned
that the time required for removal of mixed
hydrocarbon-long-chain alcohol soils increases
when the initial state of the system is further
above the PIT. This result is consistent with the
intuitive expectation that surfactant would have
to diffuse into the oil for a longer time before
liquid crystal formation occurred for systems
further above the PIT as well as with the
quantitative values of the parameter Ks in Eq.
(6) found for soils with different compositions.
It is also noteworthy that in some of the
contacting experiments, convection arose
spontaneously near the interface, an effect that
would also speed up mass transfer. The
Marangoni flow mentioned in connection with
Fig. 19 for the C12E5-water-n-tetradecane
system at 20ºC is one example, although it
occurred for a situation when soil removal is
minimal owing to a low solubilization capacity
of the surfactant solution. Perhaps more relevant
to detergency, vigorous convection was
sometimes seen during the early stages of
formation of a non-wetting intermediate phase,
i.e. one that forms preferentially as lenses rather
than as a continuous layer. Intermediate phase
formation in the C12E5-water-n-tetradecane
system at temperatures just below and above the
cloud point is an example. If non-uniformities in
intermediate
phase
thickness
develop
immediately following the initial contact when
the phase is a still a thin liquid film, disjoining
pressure effects will promote flow from thin to
thick portions of the film, thus increasing the
discrepancies in thickness. Diffusion processes
will continue to form more of the intermediate
phase in the thin regions, and disjoining
pressure gradients will continue to drive the
newly formed material to thicker regions.
No soil removal experiments are currently
available for the systems discussed above
containing mixed triolein-oleyl alcohol soils and
a pure nonionic surfactant. The contacting
experiments indicate, however, that the first
intermediate phase formed is D', which
solubilizes little triolein although readily
incorporating alcohol. It may be that some of the
same ideas suggested above to improve
detergency with pure triolein, e.g. use of
213
secondary alcohol ethoxylate surfactants, will be
needed to achieve high degrees of removal of
the triolein portion of these mixed soils. Of
course, liquid triglyceride soils normally contain
some fatty acids formed by triglyceride
hydrolysis. In addition, lipase enzymes are
incorporated into some detergent formulations
to promote the breakdown of triglycerides into
monoglycerides, diglycerides and fatty acids.
Increasing the pH can convert these acids to
soaps, which could well improve detergency.
This behavior is currently being studied by the
contacting techniques discussed above.
12. Conclusions
Solubilization-emulsification is a key
mechanism in the removal of oily liquid soils
from polyester and polyester-cotton fabrics.
Systematic studies, discussed above, utilizing
several model soils indicate that solubilization
into an intermediate phase formed during
washing is generally much more extensive and
rapid than solubilization into a micellar solution.
Accordingly, knowledge of the phase behavior
of surfactant-soil-water systems is needed to
make a rational choice of optimum surfactant
compositions and washing conditions. With
hydrocarbon soils, for instance, washing at
temperatures near the PIT is best. The
intermediate phase is a microemulsion although
the lamellar liquid crystalline phase may form as
well if it is not already present in the initial
washing bath. For commercial non-ionic
surfactants and their mixtures with anionic
surfactants, the appropriate PIT in the usual case
of washing with a high surfactantto-oil ratio is
that for which the composition of the surfactant
films in the middle-phase microemulsion is that
of the surfactant mixture in the initial washing
bath.
For mixed soils containing hydrocarbons and
long-chain alcohols or fatty acids, good soil
removal is achieved at temperatures above the
PIT but below the cloud point of the surfactant.
In this case the PIT is evaluated at high
surfactant-to-oil ratios as above with the
additional condition that the excess oil phase in
equilibrium with the microemulsion has the
214
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215
initial soil composition. The lamellar liquid
crystal is the intermediate phase. It begins to
grow as myelinic figures at a time after initial
contact between surfactant solution and soil
which can be predicted in simple cases using
diffusion theory and certain limited information
on system phase behavior.
In both these situations, the intermediate
phase, or phases, start(s) to grow when the films
of surfactant and long-chain polar compound, if
present, are roughly balanced with respect to
hydrophilic and lipophilic properties. This
balance, which may be present initially or may
occur at some later time during the washing
process as a result of mass transfer, facilitates
emulsification because interfacial tensions,
including those involving intermediate phases,
are low.
Long-chain liquid triglycerides are less
readily solubilized than the hydrocarbons of
interest in detergency owing to their higher
molecular volumes. The best prospect for
removing such triglycerides seems to be to
establish
conditions
under
which
an
intermediate D phase will develop during
washing. This phase, which is closely related to
the middle-phase microemulsion, can be
expected to form if sufficient hydrocarbon is
present with the triglyceride in the initial soil.
Otherwise, its formation can be promoted by
using mixtures of surfactants with varying
hydrocarbon chain lengths, so that surfactant
films with rather disordered packing in the chain
region are present.
Acknowledgments
The research at Rice University discussed in
this paper was supported by the National
Science Foundation, Shell Development
Company, and the Clorox Corporation.
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