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.