Microemulsions phase behavior

Salager JL (1977) Physico-chemical properties of surfactant-oil-water mixture phase behavior, microemulsion formation and interfacial tension. PhD Dissertation, University of Texas at Austin... 

Table I summarizes the qualitative changes in the phase behavior of microemulsions containing ionic surfactants. Some details of the effects of different variables are available in Ref. 13 and various chapters in this book. The phase transitions are generally understood in terms of relative strengths of hydrophilic and hydrophobic properties of the surfactant film in the microemulsion. The phase behavior depends strongly on the type and structure of the surfactant. For example, microemulsions containing nonionic surfactants are less sensitive to salinity but are more sensitive to temperature than those with ionic surfactants. The partitioning of cosolvents such as alcohols between the surfactant film, the organic phase, and the aqueous phase also affects the phase behavior. Microemulsions can be tailored for specific applications by adjusting an appropriate variable. For example, as indicated in Table 1, the effect of salinity on the phase behavior can be counterbalanced by an increase in the pH of an appropriate microemulsion [18,19].

L. Salager, in "Physico-Chemical Properties of Surfactant-Water-Oil Mixtures Phase Behavior, Microemulsion Formation and Interfacial Tension," Ph.D. Dissertation, The University of Texas at Austin (1977). 

These fascinating bicontinuous or sponge phases have attracted considerable theoretical interest. Percolation theory [112] is an important component of such models as it can be used to describe conductivity and other physical properties of microemulsions. Topological analysis [113] and geometric models [114] are useful, as are thermodynamic analyses [115-118] balancing curvature elasticity and entropy. Similar elastic modulus considerations enter into models of the properties and stability of droplet phases [119-121] and phase behavior of microemulsions in general [97, 122]. 

Lattice models for bulk mixtures have mostly been designed to describe features which are characteristic of systems with low amphiphile content. In particular, models for ternary oil/water/amphiphile systems are challenged to reproduce the reduction of the interfacial tension between water and oil in the presence of amphiphiles, and the existence of a structured disordered phase (a microemulsion) which coexists with an oil-rich and a water-rich phase. We recall that a structured phase is one in which correlation functions show oscillating behavior. Ordered lamellar phases have also been studied, but they are much more influenced by lattice artefacts here than in the case of the chain models. 

Aughel and coworkers [63] studied the phase behavior of hydrocarbon-water mixtures in the presence of alkyl(aryl)polyoxyethylene carboxylates for enhanced oil recovery and found good salt tolerance with an alkyl ether carboxy-late (C13-C15) with 7 mol EO and a good microemulsion forming effect with the 3 EO type. 

Physical-chemical studies require traces of additives (reactants, catalysts, electrolytes) with respect to the concentration of the basic components of the microemulsion, and this causes only a minor change in the phase behavior of the system. However, when the amounts of additives are on the scale used in organic synthesis, the phase behavior, which is very sensitive to the concentration of the reactants, is sometimes difficult to control and the reaction is carried out in a one-, two- or three-phase state. 

FIG. 2 Low-frequency conductivity at 45°C as a function of composition, a (weight fraction decane relative to decane and brine) for brine, decane, and AOT microemulsions exhibiting the phase behavior illustrated in Fig. 1. The breakpoint at a = 0.85 corresponds to the onset of percolation. This conductivity increases by two orders as a decreases from 0.85 to 0.7. (Reproduced by permission of the American Institute of Physics from Ref. 37.)... 

Recently, the phase equilibria of a microemulsion were reported. The phase behavior of a microemulsion formed with food-grade surfactant sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) was studied. Critical microemulsion concentration (cpc) was deduced from the dependence of the pressure of cloud points on the concentration of... 

Practical Surfactant Mixing Rules Based on the Attainment of Microemulsion-Oil-Water Three-Phase Behavior Systems... 

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). 

In the Will case, provided that there is enough surfactant but not too much, e.g., 1 wt. %, the system splits into three phases, i.e., a microemulsion in equilibrium with excess water and excess oil. At a higher surfactant concentration than the top vertex of the 3

single phase microemulsion often called WW behavior is attained. However, this occurrence generally requires a large amount of surfactant, e.g., 20 wt. %, which is in most practical cases too much for cost reasons. At a very low surfactant concentration, around the CMC, only two phases are in equilibrium, and the tension is not necessarily very low. Hence, the convenient surfactant concentration to carry out a phase behavior study is in the range 0.5-3 wt. % for which three-phase behavior and a very low inter facial tension is exhibited in most Will cases. 

Skauge A, Fotland P (1990) Effect of Pressure and Temperature on the Phase Behavior of Microemulsions. SPE Reserv Engin 5 601-608... 

Upadhyaya A, Acosta EJ, Scamehorn JF, Sabatini DA (2006) Microemulsion phase behavior of anionic-cationic surfactant mixtures Effet of tail branching. J Surfact Deterg 9 169-179... 

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. 

For a given surfactant, the ability to form a single-phase w/o microemulsion is a function of the type of oil, nature of the electrolyte, solution composition, and temperature (54-58). When microemulsions are used as reaction media, the added reactants and the reaction products can also influence the phase stability. Figure 2.2.4 illustrates the effects of temperature and ammonia concentration on the phase behavior of the NP-5/cyclohexane/water system (27). In the absence of ammonia, the central region bounded by the two curves represents the single-phase microemulsion region. Above the upper curve (the solubilization limit), a water-in-oil microemulsion coexists with an aqueous phase, while below the lower curve (the solubility limit), an oil-in-water water microemulsion coexists with an oil phase. It can be seen that introducing ammonia into the system results in a shift of the solubilization... 

Co is called the spontaneous curvature. The spontaneous curvature is a more general parameter than the surfactant parameter Ns, defined by Eq. (12.4). It makes it easier to discuss the phase behavior of microemulsions because we get away from the simple geometric picture. 

Von Corswant, C. et al. (1997) Microemulsions based on soybean phosphatidylcholine and triglycerides. Phase behavior and microstructurbangmuir, 13 5061-5070. 

M. Kahlweit and R. Strey. Phase-behavior of ternary-systems of the type H20-oil-nonionic amphiphile (microemulsions). Angewandte Chemie. International edition in English, 24(8) 654—668,1985. 

Figure 8.4. Phase behavior of water/C02/surfactant systems studied to date. r=35°C, P = 414 bar and O T= 35°C, P=138 bar. PFPE COO NH4+ (Johnston et al., 1996) PFPE COO NH4+ (Zielinsky et al., 1997) di-HCF4 (Holmes et al., 1998). The one-phase microemulsion region is to the right of each curve.

Figure 1 shows changes in the system phase behavior as its HLB value is systematically adjusted. The left side of the diagram represents a two-phase system with micellar-solubilized oil in equilibrium with an excess oil phase (Winsor Type I) (Winsor 1954). The right side of the diagram represents a different two-phase system with reversed micellar-solubilized water. In-between these two systems a third phase coemerges which contains enriched surfactant with solubilized water and oil. This new thermodynamically stable phase is known as a Winsor Type HI middle phase microemulsion. 

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... 

Understanding surfactant phase behavior is important because it controls physical properties such as rheology and freeze-thaw stability of formulations. It is also closely related to the ability to form and stabilize emulsions and microemulsions. Micelles, vesicles, mi-croemulsions and liquid crystal phases have all been used as delivery vehicles for perfumes or other active ingredients. 

Similar attempts were made by Likhtman et al. [13] and Reiss [14]. Reference 13 employed the ideal mixture expression for the entropy and Ref. 14 an expression derived previously by Reiss in his nucleation theory These authors added the interfacial free energy contribution to the entropic contribution. However, the free energy expressions of Refs. 13 and 14 do not provide a radius for which the free energy is minimum. An improved thermodynamic treatment was developed by Ruckenstein [15,16] and Overbeek [17] that included the chemical potentials in the expression of the free energy, since those potentials depend on the distribution of the surfactant and cosurfactant among the continuous, dispersed, and interfacial regions of the microemulsion. Ruckenstein and Krishnan [18] could explain, on the basis of the treatment in Refs. 15 and 16, the phase behavior of a three-component oil-water-nonionic surfactant system reported by Shinoda and Saito [19],... 

Calculations were carried out for a system consisting of the anionic surfactant sodium dodecyl sulfate, 1-pen-tanol (cosurfactant), cyclohexane, and water containing 0.3 M NaCl. As mentioned atthe very beginning, thechoice of this system was dictated by the possibility of identifying various types of phase behaviors for the same chemical components by merely changing the amount of added alcohol. In all calculations, we assumed the coexistence of an excess dispersed phase. This means that the droplet microemulsion phase is part of a two-phase system and that the amount of dispersed phase present in the droplet is the maximum achievable. 

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