Nanoscopic microheterogeneities or pseudo-phase separations in non-conventional liquids

• Published in: Current opinion in colloid & interface science 2022-02, Vol.57, p.101535, Article 101535
• Main Authors: Blahnik, Jonas, Müller, Eva, Braun, Lydia, Denk, Patrick, Kunz, Werner
• Format: Article
• Language: English
• Subjects: Catanionics; COMPLET; Surfactant-free microemulsions; Three-block surfactants
• ISSN: 1359-0294; 1879-0399
• DOI: 10.1016/j.cocis.2021.101535

This article discusses new concepts in macroscopically monophasic colloidal systems, where entropy is a major driving force for very subtle interactions and structuring. First, we show how microemulsion-similar structures can be achieved with hydrotropes. These aggregates are less defined in structure and of shorter lifetime than classical micelles but still potentially useful microheterogeneities with internal interfaces. The other extreme case of strong interactions is given when cationic and anionic surfactants are mixed in equimolar ratios. It is well known that surfactants can be made more soluble when ethylene oxide groups are incorporated. This strategy is applied for such ‘catanionics’ to avoid surfactant precipitation. Finally, we consider the fact that ethylene oxide groups increase the size of the hydrophilic headgroups of carboxylates so that the geometrical constraints compel a direct spherical shape even in the absence of water. As a result, ‘water-free direct microemulsions’ with only charged surfactants and oil are conceivable. [Display omitted] •Examples, theory, applications of microemulsion-like structures without surfactants.•Stabilization of equimolar catanionic mixtures by insertion of ethylene oxide units.•Direct micelles with ethylene oxide-carboxylate surfactants at high concentrations in water.