Orientation correlation of sulfosuccinate-based room-temperature ionic liquids studied by polarization-resolved...

Hyper-Rayleigh scattering (HRS) of sulfosuccinate-based room-temperature ionic liquids (RTILs), tetraalkylammonium salts of bis(2-ethylhexyl)sulfosuccinate (BEHSS(-)) and di(isobutyl)sulfosuccinate (DIBSS(-)), shows strong orientation correlation between sulfosuccinate ions for both BEHSS(-)-based and DIBSS(-)-based RTILs. The hyperpolarizability and polarization resolved HRS, giving information at the dipolar electric order and at the quadrupolar electric order, respectively, have been measured as a function of the distance between sulfosuccinates by changing the size of the tetraalkylammonium ions constituting the ionic liquids. The variations obtained for the dipolar electric and quadrupolar electric components of the HRS intensity show the orientation correlation between sulfosuccinate ions. In particular, the weight parameter eta introduced for molecular assemblies deviates from a pure dipolar electric order that is generally expected for a homogeneous molecular solution. Nonmonotonic variations of eta with increasing the alkyl chain length of tetraalkylammonium ions indicate a transition in the organization of the anions from an homogeneous to an heterogeneous arrangement with increasing the intermolecular distance between sulfosuccinate ions due to the competition between the short- and long-range molecular interactions.