How to explain microemulsions formed by solvent mixtures without conventional surfactants.

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 Sci60(3):488-496 and Keiser BA, Varie D, Barden RE, Holt SL (1979)J Phys Chem83(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.

Eco-solvents--cluster-formation, surfactantless microemulsions and facilitated hydrotropy.

In this paper we consider clusters in the ternary systems water-benzyl alcohol and ethanol, ethyl lactate or γ-valerolactone as found with the help of dynamic and static light scattering experiments. These ternary mixtures are powerful solvent media and consist only of low-toxic solvents of natural origin. In a recent work we have shown that surfactantless microemulsions are formed in the water-ethanol-n-octanol system. By contrast, in the systems studied here the sizes of the aggregates are too small to be considered as micelles. It can be postulated that the presence of clusters or larger structures as in surfactantless microemulsions is strongly influenced by the most hydrophobic compound. The phenomenon of facilitated hydrotropy is also investigated in the above-mentioned systems. In this context, ethanol is considered as the primary hydrotrope and the more hydrophobic benzyl alcohol as the facilitating secondary hydrotrope. The hydrophobic dye Disperse Red 13 is used as a marker of facilitated hydrotropy. The results suggest that the degree of self-association of eco-solvent has a significant influence on the hydrotropic efficiency of benzyl alcohol.

Structure and solubility in surfactant-free microemulsions.

Cheers! The combination of static and dynamic light scattering is demonstrated to deliver an unambiguous proof that well-defined domains of two fluids of clearly different composition can exist in surfactant-free ternary mixtures (see picture for diagram), such as Ouzo.