Fluorine-free and hydrophobic room-temperature ionic liquids, tetraalkylammonium bis(2-ethylhexyl)sulfosuccinates, and their ionic liquid–water two-phase properties

Abstract

Fluorine-free and hydrophobic room-temperature ionic liquids (RTILs) composed of the bis(2-ethylhexyl)sulfosuccinate (BEHSS) ion, which is known as the surface-active anion constituting Aerosol OT^®, and symmetric tetraalkylammonium ions ((C_nH_2n+1)₄N⁺; n = 4–8), have been prepared. Physicochemical properties of the water-saturated RTILs such as density, conductivity, viscosity and mutual solubility with water (W) have been measured. The RTIL|W interface is polarizable for the RTILs with n = 5–8 in spite of the high water content, 3.6–8.9 wt% in the water-saturated RTILs. The width of the polarized potential window of the RTIL|W interfaces is quantitatively correlated with the solubility of the RTIL in W. The RTIL–W two-phase systems are not spontaneously emulsified and no reverse micelles are formed in the water-saturated RTILs, although BEHSS⁻ is known to form stable water-in-oil emulsions and reverse micelles in oil–water two-phase systems.