Recently, we could prove experimentally that surfactant-free microemulsions (SFMs) do exist. They occur when two immiscible liquids are mixed with a third component that is miscible with both of these liquids. We call this phenomenon “the pre-Ouzo effect”, because it occurs in Ouzo-like mixtures, but in the phase region, where the mixture is still monophasic. In the case of water-oil-co-solvent mixtures we can explain and even predict the appearance of micellar-like dynamical aggregates by the balance of hydration forces and entropy, in agreement with scattering results.
When surfactants are added to these initially water-oil-co-solvent mixtures, first all structures and aggregates are destroyed. So surfactants have a destructuring effect on SFMs. Only at significantly high surfactant concentrations classical microemulsions are formed again.
In both cases, classical surfactant-based microemulsions and SFMs, the addition of salts has a significant effect. Depending on the cosmotropic or chaotropic nature of the salts, SFMs can be significantly strengthened in their structure or the structure can be destroyed. If the ions of the salt are of very different hydrophobicity/hydrophilicity the salt alone may even play the role of surfactants, a phenomenon that we call the Onuki effect.
As will be shown with the help of phase diagrams and X-ray scattering results, there can be a continuous cross-over between SFMs, classical surfactant-based microemulsions and “Onuki” microemulsions.
We can conclude that classical microemulsions are only a part of a much more general aggregation phenomenon and, in parallel, that DLVO is only a special case of a much more general theory.
werner.kunz@ur.de
