Liquid multicomponent systems of the type H,O-oil-amphiphile-electrolyte are of growing interest, both in industry and in research. In applications technology, there are two problems to solve: I. To prepare stable homogeneous solutions of H,O and nonpolar liquids with as little amphiphile as possible which can be diluted with H 2 0 in all proportions without phase separation; e.g., concentrated solutions of drugs, herbicides, or insecticides. 2. To prepare stable mixtures of an aqueous, an amphiphile, and an oil phase with as little amphiphile as possible which are employed in tertiary oil recovery and in the pharmaceutical industry. Furthermore, such systems may be used for performing chemical reactions in heterogeneous liquid mixtures with continuously variable properties. In research, such systems are of interest for both experimental and theoretical studies of critical phenomena, especially near so-called tricritical points. Last but not least, their properties may stimulate further research in the field of associated solutions. In this paper we summarize the results of our studies on the phase behavior of ternary systems with nonionic amphiphiles, in particular with respect to the evolution of liquid three-phase bodies. The results suggest that the tricritical points in such systems should be regarded as kinds of pivot points from which the phase behavior evolves. If this were an approach to reality, the phase behavior would be governed in more or less good approximation by universal scaling laws, irrespective of the particular microstructure of the solutions. Finally, we discuss the effect of electrolytes on the phase behavior both in a quaternary phase tetrahedron and a pseudoternary phase prism representation. Although in practice most systems consist of mixtures of oils, amphiphiles, and electrolytes, an understanding of the phase behavior of truly ternary and quaternary systems with chemically well-defined components permits at least qualitative predictions with respect to the phase behavior of the multicomponent mixtures encountered in practice.

Microemulsions, that is, stable colloidal dispersions of water and nonpolar solvents stabilized by amphiphiles, are of growing interest in research and industry. The phase behavior of the multicomponent mixture is essentially determined by the features of corresponding binary mixtures. The efficiency of an amphiphile in solubilizing the solvents reaches its maximum in the temperature interval in which the mixture separates into three coexisting liquid phases. The domain size of the dispersion is determined by the interfacial tension between the aqueous and the oil-rich phase in the presence of a saturated monolayer. Because the interfacial tension reaches its minimum in the three-phase interval and, furthermore, decreases with increasing amphiphilicity, the transition from weakly structured solutions to microemulsions is gradual. It is, therefore, suggested that microemulsions be defined as stable colloidal dispersions of domains sufficiently large for the dispersed solvent to exhibit the properties as, e.g., the dielectric number of a bulk phase.

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In part 1 of this series (ref 1) we presented a qualitative thermodynamic description of the phase behavior of quaternary mixtures of water, nonpolar solvents, nonionic amphiphiles, and salts. The phase behavior of such mixtures follows general patterns that are essentially determined by the phase diagrams of the corresponding binary and ternary mixtures. In this paper (part 2) the nonionic amphiphiles are replaced by ionic amphiphiles. The general patterns of these mixtures, in particular the phase sequence with rising temperature, are inverse to those with nonionic amphiphiles? It is shown that this inverse behavior originates from the difference between the phase diagrams of binary H20-nonionic amphiphile and those of H20-ionic amphiphile mixtures.

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