Small-angle X-ray scattering study on nano-scale structures controlled by water content in a binary water/ionic liquid system

Abstract

We report the water-in-ionic-liquid microemulsions (ME) formed in a binary water/ionic liquid system, without organic solvents, using a surfactant ionic liquid (SAIL) based on 1-butyl-3-methylimidazolium (C₄mIm⁺) as the cation and dioctyl sulfosuccinate (AOT⁻) as the anion. Small-angle X-ray scattering (SAXS) revealed that MEs were stably formed in the binary water/SAIL solutions in the low water content region (water volume fraction, ϕ_w < 0.1), and the ME size systematically increased with increasing ϕ_w. We further investigated the nanostructures of the high ϕ_w region using a combination of SAXS and rheological measurements and found that the MEs changed to a stacked lamellar structure comprising SAIL bilayers and water phases at ϕ_w > 0.12. At the largest water content, ϕ_w = 0.99, vesicle structures were obtained.