10.1021/jp4019074.s001 Chariz Y. Peñalber Chariz Y. Peñalber Gary A. Baker Gary A. Baker Steven Baldelli Steven Baldelli Sum Frequency Generation Spectroscopy of Imidazolium-Based Ionic Liquids with Cyano-Functionalized Anions at the Solid Salt–Liquid Interface American Chemical Society 2013 DCA Coulombic interactions anion sum frequency generation BaF SFG sum Frequency Generation Spectroscopy SCN TCM surface interactions Spectral features surface charge crystal salt surface EMIM counterion charge BMIM salt surface salt surfaces 2013-05-16 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Sum_Frequency_Generation_Spectroscopy_of_Imidazolium_Based_Ionic_Liquids_with_Cyano_Functionalized_Anions_at_the_Solid_Salt_Liquid_Interface/2414599 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 BaF<sub>2</sub>(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]<sup>+</sup> cations adhered closely via Coulombic interactions to the negatively charged NaCl{100} surface, while [SCN]<sup>−</sup>, [TCM]<sup>−</sup>, and [DCA]<sup>−</sup> anions revealed a strong electrostatic affinity to the positively charged BaF<sub>2</sub>(111) surface. Ions of the ionic liquid adsorbed to the solid salt surface to form a Helmholtz-like electric double layer. The linear [SCN]<sup>−</sup> anion has a particularly strong affinity to the BaF<sub>2</sub>(111) surface, resulting in a first layer of anions directly in contact with BaF<sub>2</sub>(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 BaF<sub>2</sub>(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 BaF<sub>2</sub>(111) surface, suggesting a more complicated structure of the double layer at the interface than a simple Helmholtz-type model.