How to explain microemulsions formed by solvent mixtures without conventional surfactants
Significance Beginning over 40 y ago, a curious type of microemulsions as transparent dispersion of two immiscible liquids separated by an interfacial film at thermodynamic equilibrium has been described as "pre Ouzo," "detergentless," or surfactant free microemulsions. The experiments in ternary systems containing one hydrotropic cosolvent were ambiguous, and therefore, there was no need to come up with a general theory. Recent evidence obtained by specific deuteration in neutron scattering established the need for the extension of self assembly theories based on Derjaguin Landau Verwey Overbeck theory (DLVO) bending, or phase transfer energy. Here, we introduce a general free energy expression for weak self assembly, where solvation effects and entropy compete without the influence of film bending, that explains all experimental results for this class of microemulsions. Ternary solutions containing one hydrotrope (such as ethanol) and two immiscible fluids, both being soluble in the hydrotrope at any proportion, show unexpected solubilization power and allow strange but yet unexplained membrane enzyme activity. We study the system ethanol water octanol as a simple model of such kinds of ternary solutions. The stability of "detergentless" micelles or microemulsions in such mixtures was proposed in the pioneering works of Barden and coworkers [Smith GD, Donelan CE, Barden RE (1977) J Colloid Interface Sci 60(3):488 496 and Keiser BA, Varie D, Barden RE, Holt SL (1979) J Phys Chem 83(10):1276 1281] in the 1970s and then, neglected, because no general explanation for the observations was available. Recent direct microstructural evidence by light, X ray, and neutron scattering using contrast variation reopened the debate. We propose here a general principle for solubilization without conventional surfactants: the balance between hydration force and entropy. This balance explains the stability of microemulsions in homogeneous ternary mixtures based on cosolvents.
Proceedings of the National Academy of Sciences