How cation and diluent determine nanostructure in surface-active ionic liquids

Small-angle X-ray scattering is used to investigate how cation and diluent control nanostructure and swelling in bis-2,2'-ethylhexylsulfosuccinate (AOT) surface-active ionic liquids (SAILs). Alkylammonium and alkylmethylimidazolium AOT salts form sponge-like liquids with curved amphiphilic bilayers whose periodicity and order depend on cation chain length and branching, and on cation head group. Increasing chain length is consistent with a transition from cations segregated in polar domains towards intercalation within the bilayer. Water dilution enhances intercalation, yielding highly swellable sponge and bilayer-based lyotropic mesophases. In contrast, dilution with the ionic liquid ethylammonium nitrate favours cation partitioning into the polar domains, which alters swelling and favours long-range composition fluctuations. These results establish the molecular parameters for designing nanostructured SAILs.