Sum Frequency Generation Spectroscopy of Imidazolium-Based Ionic Liquids with Cyano-Functionalized Anions at the Solid Salt–Liquid Interface

A surface-sensitive nonlinear vibrational spectroscopic technique, sum frequency generation (SFG), has been used to study cyano-containing ionic liquids in contact with two different solid salt surfaces. Specifically, the interfacial chemistry of BaF2(111) single-crystal and solid NaCl{100} surfaces in contact with ionic liquids such as [BMIM]­[SCN], [BMIM]­[DCA], [BMIM]­[TCM], and [EMIM]­[TCB] has been investigated. Spectral features in both C–H and C–N stretching regions were assigned, with a detailed discussion of the nature of surface interactions and ordering of the ionic liquid ions at the interface of the different crystals. Results showed that [BMIM]+ cations adhered closely via Coulombic interactions to the negatively charged NaCl{100} surface, while [SCN], [TCM], and [DCA] anions revealed a strong electrostatic affinity to the positively charged BaF2(111) surface. Ions of the ionic liquid adsorbed to the solid salt surface to form a Helmholtz-like electric double layer. The linear [SCN] anion has a particularly strong affinity to the BaF2(111) surface, resulting in a first layer of anions directly in contact with BaF2(111) containing an effective negative surface excess charge. This promoted ordering of the cations in the second layer to counter the charge excess. At the BaF2(111)–[EMIM]­[TCB] interface, however, a strongly bound layer of anions populating the first layer resulted in a much larger counterion charge delivered near the crystal salt surface than required to effectively neutralize the initial surface charge from the crystal. As a result, strong resonances from the cation were observed at the BaF2(111) surface, suggesting a more complicated structure of the double layer at the interface than a simple Helmholtz-type model.