Binary Surfactant Systems

Surfactant molecules commonly self-assemble in water (or in oil). Even single-surfactant systems can display a quite remarkably rich variety of structures when parameters such as water content or temperature are varied. In dilute solution they form an isotropic solution phase consisting of micellar aggregates. At more concentrated surfactant-solvent systems, several isotropic and anisotropic liquid crystalline phases will be formed [2]. The phase behavior becomes even more intricate if an oil (such as an alkane or fluorinated hydrocarbon) is added to a water-surfactant binary system and the more so if other components (such as another surfactant or an alcohol) are also included [3], In such systems, emulsions, microemulsions, and lyotropic mesophases with different geometries may be formed. Indeed, the ability to form such association colloids is the feature that singles out surfactants within the broader group of amphiphiles [4]. No wonder surfactants phase behavior and microstructures have been the subject of intense and profound investigation over the course of recent decades. 

In the phenomenological model of Kahlweit et al. [46], the behavior of a ternary oil-water-surfactant system can be described in terms of the miscibility gaps of the oil-surfactant and water-surfactant binary subsystems. Their locations are indicated by the upper critical solution temperature (UCST), of the oil-surfactant binary systems and the critical solution temperature of the water-surfactant binary systems. Nonionic surfactants in water normally have a lower critical solution temperature (LCST), Tp, for the temperature ranges encountered in surfactant phase studies. Ionic surfactants, on the other hand, have a UCST, T. Kahlweit and coworkers have shown that techniques for altering surfactant phase behavior can be described in terms of their ability to change the miscibility gaps. One may note an analogy between this analysis and the Winsor analysis in that both involve a comparison of oil - surfactant and water-surfactant interactions. 

In this chapter, a brief theoretical background on the rheological behavior of viscoelashc worm-like micelles is given. It is followed by a discussion on the temperature-induced viscosity growth in a water-surfactant binary system of a nonionic fluorinated surfactant at various concentrations. Finally, some recent results on the formation of viscoelastic worm-like micelles in mixed nonionic fluorinated surfactants in an aqueous system are presented. 

Tests with other pure ethoxylated surfactants have revealed that a discontinuity is observed with respect to oil removal versus temperature in cases of the existence of dispersions of liquid crystals in the water-surfactant binary system. Figure 3.29 shows that the detergency... 

Achilefu, S. et al., Monodisperse perfluoroalkyl oxyethylene nonionic surfactants with methoxy capping Synthesis and phase behavior of water/surfactant binary systems, Langmuir, 10, 2131, 1994. 

In conclusion, free water is formed not by detaching outer water layers through phase separation but rather by adding more and more water until its association with the surfactant is barely perceivable. Phase separation should then be regarded as a segregation of some (or all) of the free (pure) components (water, oil, or alcohol) of the microemulsion. Thus, the distribution of free and bound water in a microemulsion system (as well as in the related water-surfactant binary system) does not depend on the degree of phase separation [2,8,9,11]. 

Without going into this theory in detail, let us reproduce here the equation proposed by Rubingh for the activity factor of surfactant species making up mixed micelles in a binary system ... 

Taking Simultaneous Micellizadon and Adsorption Phenomena into Consideration In the presence of an adsorbent in contact with the surfactant solution, monomers of each species will be adsorbed at the solid/ liquid interface until the dual monomer/micelle, monomer/adsorbed-phase equilibrium is reached. A simplified model for calculating these equilibria has been built for the pseudo-binary systems investigated, based on the RST theory and the following assumptions ... 

The adsorption plateaus on this solid, determined with each of the surfactants (Table II) and the individual CMC values, were used to calculate the adsorption constants input in the model. Figure 3 compares the total adsorption (sulfonate + NP 30 EO) of the pseudo-binary system investigated as a function of the initial sulfonate fraction of the mixtures under two types of conditions (1) on the powder solid, batch testing with a solid/liquid ratio, S/L = 0.25 g/cc (2) in the porous medium made from the same solid, for which this solid ratio is much higher (S/L = 4.0 g/cc). 

Monomer—Micelle Equilibria. The distribution of surfactant components between micelles and monomeric state in aqueous solutions depends on surfactant structures as well as on overall solution composition. For example, for a binary system of surfactants A and B in solution, the micelle may contain SO mole % A/SO X B while the monomer may be 90 /. A/10 X B. Since either the monomer or the micelle composition may be crucial to behavior of the system, the ability to predict the relative distribution of surfactant components between monomer and micelle, given the overall solution composition, is an important one. 

For a binary system of surfactants A and B, the mixed micelle formation can be modeled by assuming that the thermodynamics of mixing in the micelle obeys ideal solution theory. When monomer and micelles are in equilibrium in the system, this results in ... 

Equations 1 and 2 provide the ability to make a priori predictions of mixed micellar behavior for binary systems of similar surfactants (easily extended to more than 2 components(10)) No mixture data is necessary to use these equations. If the overall concentration of the individual surfactants in solution are known. Equations 1 and 2 can be combined with a material balance to... 

In the case of surface adsorption, at constant surface tension ofsolutions, an equation of the same form as equation 12 is obtained for the binary surfactant mixture system (15,16) ... 

Scamehorn et. al. (19) reported the adsorption isotherms for a binary mixture of anionic surfactants. A formal adsorption model developed for single surfactant systems ( ) was extended to this binary system and shown to accurately describe the mixed adsorption isotherms (19). That theoretically based model was very complex and is probably not feasible to extend beyond two surfactant components. 

As already discussed in Chapter 1, the relative tendency of a surfactant component to adsorb on a given surface or to form micelles can vary greatly with surfactant structure. The adsorption of each component could be measured below the CMC at various concentrations of each surfactant in a mixture. A matrix could be constructed to tabulate the (hopefully unique) monomer concentration of each component in the mixture corresponding to any combination of adsorption levels for the various components present. For example, for a binary system of surfactants A and B, when adsorption of A is 0.5 mmole/g and that of B is 0.3 mmole/g, there should be only one unique combination of monomer concentrations of surfactant A and of surfactant B which would result in this adsorption (e.g., 1 mM of A and 1.5 mM of B). Uell above the CMC, where most of the surfactant in solution is present as micelles, micellar composition is approximately equal to solution composition and is, therefore, known. If individual surfactant component adsorption is also measured here, it would allow computation of each surfactant monomer concentration (from the aforementioned matrix) in equilibrium with the mixed micelles. Other processes dependent on monomer concentration or surfactant component activities only could also be used in a similar fashion to determine monomer—micelle equilibrium. 

The surfactant is cesium perfluorooctanoate, with cesium being the counterion. The CsPFO-H20 system has been well studied experimentally and is regraded as a typical binary system to exhibit micellization. As the details of the simulation have been discussed elsewhere [8], we directly proceed to the discussion of the significant results. 

For a binary system that contains only one surfactant at dilute concentration, the chemical potential can be replaced with the logarithm of the concentration of surfactant (c), the temperature (7), and the universal gas constant (/ ) to obtain ... 

Figure 1 shows the results obtained by Francois and Skoulios (27) on the conductivity of various liquid crystalline phases in the binary systems water-sodium lauryl sulfate and water-potassium laurate at 50 °C. As might be expected, the water-continuous normal hexagonal phase has the highest conductivity among the liquid crystals while the lamellar phase with its bimolecular leaflets of surfactant has the lowest conductivity. Francois (28) has presented data on the conductivity of the hexagonal phases of other soaps. She has also discussed the mechanism of ion transport in the hexagonal phase and its similarity to ion transport in aqueous solutions of rodlike polyelectrolytes. 

Previous work has shown that binary surfactant systems containing Dowfax 8390 and the branched hydrophobic surfactant AOT can form Winsor III systems with both PCE and decane whereas DOWFAX 8390 by itself cannot (Wu et. al. 1999). This binary surfactant system was used in conjunction with hydrophobic octanoic acid to help with phase behavior and lessen the required concentration of CaCl2. Since this formulation is rather complicated, questions about field robustness arise. Thus, for the phase behavior studies presented here, we used the simple binary system of the nonionic TWEEN 80 and the branched hydrophobic AOT, and we optimized the NaCl concentration to give the Winsor Type III system. The lesser electrolyte concentration requirement for the binary TWEEN 80/ AOT system helps to decrease the potential for undesirable phase behavior such as surfactant precipitation, thereby increasing surfactant system robustness. 

The micellar phenomenon cannot be discussed without considering a surfactant property which is intrinsically related to the very existence of micelles the so-called detergency, i.e., the ability of surfactant molecules to take up (= solubilize) polar material, for example, water in the polar core of the inverted micelles. Thus, micelli-zation and solubilization are competitive processes. It is obvious that the tendency to solubilize minute amounts of polar impurities, particularly, water is quite pronounced. In principle it must appear, therefore, doubtful whether it is reasonable at all to discuss true binary systems, i.e., surfactant plus solvent, except by way of... 

There is a common rule, called Bancroft s rule, that is well known to people doing practical work with emulsions if they want to prepare an O/W emulsion they have to choose a hydrophilic emulsifier which is preferably soluble in water. If a W/O emulsion is to be produced, a more hydrophobic emulsifier predominantly soluble in oil has to be selected. This means that the emulsifier has to be soluble to a higher extent in the continuous phase. This rule often holds but there are restrictions and limitations since the solubilities in the ternary system may differ from the binary system surfactant/oil or surfactant/water. Further determining variables on the emulsion type are the ratios of the two phases, the electrolyte concentration or the temperature. 

These deviations were accounted by Strey et al.,8 who carried out experiments with the binary water—C12E5 system, by noting that the amplitude of the thermal undulations increased with the repeat distance d. If one considers the total area of the interface a constants which depends only on the number of surfactant molecules, the projection So of the total area on a plane perpendicular to d will decrease with increasing d. Hence, the apparent area per surfactant molecule, which is defined as the ratio between So and the total number of surfactant molecules, decreases with increasing d, while the ideal dilution law implies that the apparent area per surfactant molecule is a constant. The excess area, defined as AS = S — So, was related to the bending modulus of the interface,8 and the experimental results for the deviations from the ideal dilution law were used to determine. Kc-3,11 However, it should be noted that there are binary systems for which the deviations from the ideal dilution law occur in the opposite directions. For instance, in the binary systems of fatty acid alkali soaps/water, the apparent area per headgroup increases with water dilution, because of the incorporation of water in the interface.1... 

It is not easy to match the required HLB value of the oil or the oil mixture with that of a single surfactant to form the most stable emulsion. The appropriate combination of surfactants (usually a binary system) should be chosen. The HLB value of the binary mixture of surfactants A ( HLBA ) and B ( HLBb ) is calculated by ... 

In the previous sections the results of surfactant accumulation by foam or purification of a solution from surfactants were considered mostly for binary systems. The aim was to increase the accumulation ratio or the degree of extraction of one of the components with the... 

Comparable to the binary systems (water-surfactant or oil-surfactant), self-assembled structures of different morphologies can be obtained ranging from (inverted) spherical and cylindrical micelles to lamellar phases and bicontin-uous structures. To map out these regions, a phase diagram is most useful. 

For a pure supercritical fluid, the relationships between pressure, temperature and density are easily estimated (except very near the critical point) with reasonable precision from equations of state and conform quite closely to that given in Figure 1. The phase behavior of binary fluid systems is highly varied and much more complex than in single-component systems and has been well-described for selected binary systems (see, for example, reference 13 and references therein). A detailed discussion of the different types of binary fluid mixtures and the phase behavior of these systems can be found elsewhere (X2). Cubic ecjuations of state have been used successfully to describe the properties and phase behavior of multicomponent systems, particularly fot hydrocarbon mixtures (14.) The use of conventional ecjuations of state to describe properties of surfactant-supercritical fluid mixtures is not appropriate since they do not account for the formation of aggregates (the micellar pseudophase) or their solubilization in a supercritical fluid phase. A complete thermodynamic description of micelle and microemulsion formation in liquids remains a challenging problem, and no attempts have been made to extend these models to supercritical fluid phases. 

The phase boundary lines for supercritical ethane at 250 and 350 bar are shown in Figure 2. The surfactant was found to be only slightly soluble in ethane below 200 bar at 37 C, so that the ternary phase behavior was studied at higher pressures where the AOT/ethane binary system is a single phase. As pressure is increased, more water is solubilized in the micelle core and larger micelles can exist in the supercritical fluid continuous phase. The maximum amount of water solubilized in the supercritical ethane-reverse micelle phase is relatively low, reaching a W value of 4 at 350 bar. 

Wilson and co-workers developed a statistical mechanical model for single component surfactant adsorption (29-31) and expanded it to a binary system (2,3). Different adsorption curves were generated by varying the Van der Waals interaction parameters. The mixed adsorption equations that were developed were very complex and were not applied to experimental data. 

Scamehorn et. al. expanded a single component adsorption equation ( ) to describe the adsorption of binary mixtures of anionic surfactants of a homologous series (1 1). Ideal solution theory was found to describe the system fairly well. The mixed adsorption equations worked very well in predicting the mixture adsorption, but the equations were complex and would be difficult to extend beyond a binary system. 

As in binary surfactant-water systems considered previously, two constraints on the geometry of the surfactant interface are active a local constraint, which is due to the surfactant molecular architecture, and a global constraint, set by the composition. These constraints alone are sufficient to determine the microstructure of the microemulsion. They imply that the expected microstructure must vary continuously as a function of the composition of tile microemulsion. Calculations show - and small-angle X-ray and neutron scattering studies confirm - that the DDAB/water/alkane microemulsions consist of a complex network of water tubes within the hydrocarbon matrix. As water is added to the mixture, the Gaussian curvature - and topology -decreases [41]. Thus the connectivity of the water networks drops (Fig. 4.20). 

The steadily growing interest in polymer-surfactant systems arises from their wide applications in painting, coating, inks, drug delivery, foodstuffs, cosmetic products, and oil recovery. In this respect, the physical properties of the binary system play an important role in determining the industrial products. 

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