The aggregation behavior in aqueous solutions of three ionic liquids based on the 1-alkyl-3-methylimidazolium cation has been investigated by means of surface tension, conductivity, and small-angle neutron scattering (SANS) measurements. From analysis of the SANS data, models for the shapes and sizes of aggregates have been proposed: the short-chain 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim] [BF4] system can be best modeled by treating it as a dispersion of polydisperse spherical aggregates that form above a critical aggregation concentration, whereas the 1-octyl-3-methylimidazolium iodide, [C8mim] [I], solutions can be modeled as a system of regularly sized near-spherical charged micelles that form above a critical micelle concentration. Solutions of 1-octyl-3-methylimidazolium chloride, [C8mim]-[Cl], display weak long-range ordering of possibly disklike particles culminating in the formation of structures with distinct long-range order at higher concentrations.

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Neutron reflectometry measurements suggest that the interfacial structure normal to the free surface of two short-chain amphiphilic 1-alkyl-3-methylimidazolium-based ionic liquids is inhomogeneous, with the inhomogeneity extending ∼40 Å. For the two liquids studied (1-butyl-3-methylimidazolium tetrafluoroborate and 1-octyl-3-methylimidazolium hexafluorophosphate), the data are consistent with a stratified surface model with distinct regions of alkyl chains and ionic headgroups.

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