| The characterization of imidazolium-based room temperature ionic liquids was performed using three complementary surface techniques; sum frequency generation (SFG) spectroscopy, 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 these ions identified as having the anions 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. Varying the length of the alkyl functionality in alkylimidazolium alkylsulfate ionic liquids showed significant effects on the measured quantities due to the possible aggregation of the chains leading to enhanced dispersion interactions as well as considerable dipole chain contribution. Surface tension decreased with the increase in alkyl chain length, while the surface potential showed observable dipole contribution from alkyl chains consisting of at least four carbons. Factors affecting the arrangement of ions on the surface are ionic interactions of the headgroups, dipole contribution from the alkyl chain, and of the headgroups, affinity of the ions toward each other, and sizes of the ions. A possible model on the configuration of ions at the gas-liquid interface of ionic liquids is described based on the concurring results from these three surface characterization techniques. |
