On the Arrangement of Ions in Imidazolium-Based Room Temperature Ionic Liquids at the Gas−Liquid Interface, Using Sum Frequency Generation, Surface Potential, and Surface Tension Measurements

Abstract

The characterization of three ionic liquids [BMIM][BF4], [BMIM][DCA], and [BMIM][MS] having a common cation and with anions of varying sizes and shapes was performed with three complementary surface techniques: sum frequency generation-polarization mapping, surface tension measurement, and surface potential measurement. Custom vacuum cells were designed for each technique to be able to perform measurements in a highly controlled environment minimizing the presence of water and other contaminants, which may compromise measured values. SFG results show evidence of having anions and cations present on the surface with the butyl chain of the cation positioned toward the gas phase and the imidazolium ring mostly parallel to the surface plane. Results from the surface potential measurements reveal the relative positions of the ions where the anions are located at a slightly lower plane compared to the cations. Observed values from the surface tension measurements denote surface intermolecular interactions indicative of both van der Waals and Coulombic interactions suggesting the presence of alkyl chains as well as ions on the surface. A model on the gas−liquid interface of ionic liquids is described based on the concurring results from these three surface characterization techniques, as well as current literature.