Phase Behavior of Surfactant Systems

Applied Surfactants Principles and Applications. Tharwat F. Tadros Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30629-3 

With nonionic surfactants of the ethoxylate type, micellar growth with increasing concentration is more marked the shorter the EO chain. With 4 to 6 EO units there is a dramatic growth, whereas with 8 or more EO units there is negligible growth. These nonionic surfactants show much more pronounced growth at higher temperatures, i.e. opposite to the case of ionic surfactants. 

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Wade WH, Morgan J, Schechter RS, Jacobson JK, Salager JL (1978) Interfacial tension and phase behavior of surfactant systems. Soc Petrol Eng J 18 242... 

Finally, it should be mentioned that a combination of COSMO-RS with tools such as MESODYN [127] or DPD [128] (dissipative particle dynamics) may lead to further progress in the area of the mesoscale modeling of inhomogeneous systems. Such tools are used in academia and industry in order to explore the complexity of the phase behavior of surfactant systems and amphiphilic block-co-polymers. In their coarse-grained 3D description of the long-chain molecules the tools require a thermodynamic kernel... 

The phase behavior of surfactant systems is particularly complex because of the existence of numerous lyotropic (solvent-induced) liquid crystal phases (3). These phases, like liquids and crystals, are discrete states of matter. They are fluids, but their x-ray patterns display sharp lines signifying the existence of considerable structure. They are often extremely viscous because of their high viscosities and for other reasons they are difficult to study using conventional methods. This is evident from the fact that serious errors in the presumably well-established classical aqueous phase diagrams of soaps, sodium alkyl sulfates, monoglycerides, and... 

J. L., and Schechter, R. S., "Interfacial Tension and Phase Behavior of Surfactants Systems," Paper SPE 6844, presented at the 52nd Annual Fall Meeting of the SPE, Denver, Colorado, (October 1977). 

This book, which deals with many diverse topics of self-organized surfactant structures, will be valuable to many research workers who deal with the phase behavior of surfactant systems. It can also be of much value for industrial researchers who are interested in application of these structures in their formulation, in particular in the area of cosmetics and pharmaceuticals. 

Microstructure and phase behavior of surfactant systems strongly depend on H. Counting curvature toward apolar domains as positive, direct micelles are found when Hg 0, and reverse micelles when H kkO. For the ternary... 

K. Thalberg, B. Lindman, and G. Karlstrom Phase Behavior of a System of Cationic Surfactant and Anionic Polyelectrolyte The Effect of Salt. J. Phys. Chem. 95, 6004(1991). 

The quantification of the formulation of SOW systems was studied in detail during the 1970 s when a considerable research drive was dedicated to enhanced oil recovery by surfactant flooding methods [2], The basic concepts came from Winsor s work on the phase behavior of SOW systems and its R ratio of interactions between the surfactant molecules adsorbed at interface and oil and water [3], which has been presented thoroughly in a review book [4]. 

Winsor reported that the phase behavior of SOW systems at equilibrium could exhibit essentially three types, so called Wl, Wll and Will, illustrated by the phase diagrams indicated in Fig. 1. In the Wl (respectively, Wll) case, the surfactant bears a stronger affinity for the water (respectively, oil) phase and most of it partitions into water (respectively, oil). As a consequence, the system exhibits a two-phase behavior in which a microemulsion is in equihb-rium with excess oil (respectively, water). 

Figure 7 indicates the phase behavior of SOW systems containing ternary nonionic surfactant mixtures that in turn contain a very hydrophilic surfactant (Tween 60 Sorbitan -i- 20 EO stearate), a very hpophihc surfactant (Span 20 Sorbitan monolaurate), and an intermediate (Tween 85 Sorbitan 20 EO trioleate or Nonylphenol with an average of 5 EO groups). The two intermediate surfactants correspond exactly to an optimum formulation in the physicochemical conditions, i.e., they exhibit three-phase behavior with the system 1 wt. % NaCl brine-heptane-2-butanol. As the intermediate hy-drophihcity surfactant is replaced by an equivalent mixture of the extreme ... 

Fig. 7 Phase behavior of SOW systems containing ternary surfactant mixtures. After [40]...

Salager JL, Marquez N, Anton RE, Graciaa A, Lachaise J (1995) Retrograde Transition in the Phase Behavior of Surfactant-oil-water systems produced by an alcohol scan. Langmuir 11 37-41... 

Ysambertt F, Anton RE, Salager JL (1997) Retrograde Transition in the phase behavior of surfactant-oil-water systems produced by an oil EACN scan. Colloid Surf A 125 131-136... 

It is well known that the aqueous phase behavior of surfactants is influenced by, for example, the presence of short-chain alcohols [66,78]. These co-surfactants increase the effective value of the packing parameter [67,79] due to a decrease in the area per head group and therefore favor the formation of structures with a lower curvature. It was found that organic dyes such as thymol blue, dimidiiunbromide and methyl orange that are not soluble in pure supercritical CO2, could be conveniently solubihzed in AOT water-in-C02 reverse microemulsions with 2,2,3,3,4,4,5,5-octafluoro-l-pentanol as a co-surfactant [80]. In a recent report [81] the solubilization capacity of water in a Tx-lOO/cyclohexane/water system was foimd to be influenced by the compressed gases, which worked as a co-surfactant. 

We saw in Section 8.6 that phase diagrams are an effective way of representing the complex behavior of surfactant systems. Let us take a look at microemulsions in terms of phase diagrams. It turns out that nonionic surfactants form microemulsions at certain temperatures without requiring cosurfactants. Since only three components are present, these have somewhat simpler phase diagrams this kind of system offers a convenient place to begin. 

A correlation of the detergency performance and the equilibrium phase behavior of such ternary systems is expected, based on the results presented by Miller et al. (3,6). The phase behavior of surfactant - oil - water (brine) systems, particularly with regard to the formation of so-called "middle" or "microemulsion" phases, has been shown by Kahlweit et al. (7,8) to be understandable in teims of the... 

Figure 11, which is based on the phase behavior of surfactant/ oil/water systems, illustrates just a few of the many different patterns of phase behavior that may be encountered. On the left is a simple, "well-behaved" system, such as is implicitly assumed in most mobility control studies on "foams." Barring unforeseen wettability problems, the system can be expected to form a "C02 in-water foam."... 

Reverse micelle and microemulsion solutions are mixtures of a surfactant, a nonpolar fluid and a polar solvent (typically water) which contain organized surfactant assemblies. The properties of a micelle phase in supercritical propane and ethane have been characterized by conductivity, density, and solubility measurements. The phase behavior of surfactant-supercritical fluid solutions is shown to be dependent on pressure, in contrast to liquid systems where pressure has little or no effect. Potential applications of this new class of solvents are discussed. 

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. 

In the studies described here, we examine in more detail the properties of these surfactant aggregates solubilized in supercritical ethane and propane. We present the results of solubility measurements of AOT in pure ethane and propane and of conductance and density measurements of supercritical fluid reverse micelle solutions. The effect of temperature and pressure on phase behavior of ternary mixtures consisting of AOT/water/supercritical ethane or propane are also examined. We report that the phase behavior of these systems is dependent on fluid pressure in contrast to liquid systems where similar changes in pressure have little or no effect. We have focused our attention on the reverse micelle region where mixtures containing 80 to 100% by weight alkane were examined. The new evidence supports and extends our initial findings related to reverse micelle structures in supercritical fluids. We report properties of these systems which may be important in the field of enhanced oil recovery. 

The reverse micelle phase behavior in supercritical fluids is markedly different than in liquids. By increasing fluid pressure, the maximum amount of solubilized water increases, indicating that these higher molecular weight structures are better solvated by the denser fluid phase. The phase behavior of these systems is in part due to packing constraints of the surfactant molecules and the solubility of large micellar aggregates in the supercritical fluid phase. 

The phase behavior of surfactant formulations for enhanced oil recovery is also affected by the oil solubilization capacity of the mixed micelles of surfactant and alcohol. For low-surfactant systems, the surfactant concentration in oil phase changes considerably near the phase inversion point. The experimental value of partition coefficient is near unity at the phase inversion point (28). The phase inversion also occurs at the partition coefficient near unity in the high-surfactant concentration systems (31). Similar results were also reported by previous investigators (43) for pure alkyl benzene sulfonate systems. 

Kahlweit, M., Lessner, E. andStrey, R. (1984) Phase behavior of quaternary systems ofthe type H20-oil-nonionic surfactant-inorganic electrolyte. 2.J. Phys. Chem., 88(10), 1937-1944. 

The phase behavior of surfactants in water and hydrocarbon is the key to understanding the water- and oil-dissolving power of certain surfactant systems and the interfacial tension between the phases that form in these systems (1, 2). Ultralow tensions less than lOyN/m (0.01 dyn/cm) are required by one of the important mechanisms in various processes for enhancement of petroleum recovery. Much information is now in the literature (3 r4 r5 r6) t but most of the data are for commercial surfactants which are complex mixtures of surface-inactive as well as surface-active components (7 ). ... 

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