Surface studies of 1-butyl-3-methylimidazolium based ionic liquids: A sum frequency generation analysis

Sum Frequency Generation spectroscopy, SFG, was used for the surface characterization at the gas-liquid and electrode-liquid interface of the 1-butyl-3-methylimidazolium cation combined with the following anions: Br-, I-, PF6-, BF4-, (CF3 SO2)2N- (imide), SCN-, CH3SO3- (MeSO3), CH3 SO4- (MS) and (CN)2N- (DCN). The results of the gas-liquid interface analysis for the different ionic liquids were similar independent of the anion selected when the ionic liquid was considered dry. The cation orientation was determined by analysis of polarization-dependent SFG spectra. For a compound dried in vacuum to ≤ 2x10-5 Torr, the cation appears to be oriented with the ring laying flat along the surface plane and the butyl chain projecting into the gas phase independently of the anion identity. Furthermore, contamination of the ionic liquid with water affected the surface orientation of the hydrophobic ionic liquids while causing a negligible effect in the hydrophilic ones.; A combination of SFG and cyclic voltammetry (CV) measurements were conducted on the ionic liquid/platinum electrode system. SFG spectra were taken in situ under potential control from 2750-3300 cm-1 (C-H stretching region). Polarization-dependent SFG spectra were used to extract orientation information for the cation at the electrode surface. Results indicated that the cation changed orientation as the electrode potential was changed within the double-layer region. The plane of the imidazolium ring is closer to the electrode at negative surface charge and moves away the electrode at positive surface charge. A model for ions at the surface is presented based on these spectroscopic and electrochemical measurements. Furthermore, the arrangement of ions at the platinum electrode was determined analyzing the Stark effect of CO using SFG, and electrochemical impedance spectroscopy (EIS). The results indicate that CO adsorbed on the Pt electrode has a Stark shift of 24-33 cm-1/V in the ionic liquid. The potential of zero charge (PZC) of the ionic liquid-Pt system is approximately -500 mV (vs Ag wire), with a capacitance of 0.12-0.19 F/m2. In combination, these results indicate that the ions of a neat ionic liquid are organized in a Helmholtz layer at the electrified metal electrode interface.