Toward surfactant-free and water-free microemulsions.

It was recently demonstrated that a nano-clustering was present in the monophasic "pre-Ouzo" region of ternary liquid mixtures without surfactants. The goal of this work is to check if this nano-clustering is also present in the surfactant-free and water-free "green" microemulsions glycerol/ethanol/1-octanol and deep eutectic solvent/tetrahydrofurfuryl alcohol/diethyl adipate. The deep eutectic solvents used instead of water were ethylene glycol-choline chloride (molar ratio 4-1) and urea-choline chloride (molar ratio 2-1). To our knowledge this is the first time that deep eutectic solvents were used to formulate microemulsions. The surfactant-free and water-free microemulsions were studied using phase diagrams, dynamic light scattering, and small-angle X-ray scattering. The presence of aggregate fluctuations was demonstrated and they were found to be independent of molecular critical fluctuations, except when approaching the critical point where the critical phenomenon is superimposed to the signal. These structures have similarities to classical microemulsions but, in contrast to them, without having a sharp interface between the non-miscible phases, much as it was the case for systems previously investigated like water/ethanol/oil, where the oil was 1-octanol, fragrance molecules, or mosquito repellents.