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 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: the balance between hydration force and entropy. This balance explains the stability of microemulsions in homogeneous ternary mixtures based on cosolvents. Significance Beginning over 40 y ago, a curious type of microemulsions—as transparent dispersion of two immiscible liquids separated by an interfacial film—at thermodynamic equilibrium has been described as “pre-Ouzo,” “detergentless,” or surfactant-free microemulsions. The experiments in ternary systems containing one hydrotropic cosolvent were ambiguous, and therefore, there was no need to come up with a general theory. Recent evidence obtained by specific deuteration in neutron scattering established the need for the extension of self-assembly theories based on Derjaguin-Landau-Verwey-Overbeck theory (DLVO), bending, or phase transfer energy. Here, we introduce a general free energy expression for weak self-assembly, where solvation effects and entropy compete without the influence of film bending, that explains all experimental results for this class of microemulsions.

Sandwiched: The cobaltabisdicarbollide (mono-)anion ([3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻, COSAN⁻) forms monolayer vesicles at low concentrations in water (see picture). An increase in concentration leads to a Coulomb explosion of the closely packed vesicles into small micelles, which results in the coexistence of both aggregation states at higher concentrations.

Poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles can be prepared in the form of concentrated aqueous dispersions via polymerization-induced self-assembly (PISA). In the present study, these syntheses are conducted in the presence of varying amounts of silica nanoparticles of approximately 18 nm diameter. This approach leads to encapsulation of up to hundreds of silica nanoparticles per vesicle. Silica has high electron contrast compared to the copolymer and its thermal stability enables quantification of the loading efficiency via thermogravimetric analysis. Encapsulation efficiencies can be obtained using disk centrifuge photosedimentometry, since the vesicle density increases at higher silica loadings while the mean vesicle diameter remains essentially unchanged. Small angle X-ray scattering (SAXS) is used to confirm silica encapsulation, since a structure factor is observed at q ~ 0.25 nm-1. A new two-population model provides satisfactory data fits to the S...