How to explain microemulsions formed by solvent mixtures without conventional surfactants
Résumé
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-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: balance between hydration force and entropy. This balance explains the stability of microemulsions in homogeneous ternary mixtures based on cosolvents.