Amphiphilicity Controls Nanostructure in Surfactant-Free Ionic Liquid Microemulsions

Traditional microemulsions are ubiquitous nanostructured liquids stabilized by high surfactant concentrations that can be a significant drawback. Here, we show that surfactant-free microemulsions can be formulated when amphiphilicity is incorporated directly into a protic ionic liquid solvent. Using combined high-resolution X-ray and neutron scattering with isotopic labeling, we show that an amphiphilic ionic liquid, propylammonium nitrate, functions both as a polar solvent and hydrotrope, generating polar-apolar nanostructured domains without a conventional surfactant monolayer. In contrast, a non-amphiphilic ionic liquid, ethanolammonium nitrate, simply acts as a polar solvent, driving solvophobic segregation in purely non-aqueous mixtures. The balance between Coulomb interactions, hydrogen bonding, and amphiphilicity determines domain structure and phase coexistence. These results show that cation amphiphilicity is the molecular parameter that controls the nanostructure and phase behavior in surfactant-free microemulsions. This behavior is expected to apply to systems in which amphiphilicity is embedded within the solvent.