Sum Frequency Generation Study of the Room-Temperature Ionic Liquids/Quartz Interface

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Abstract
The purpose of this investigation is to study the ionic liquid/quartz interface with sum frequency generation vibrational spectroscopy (SFG). SFG spectroscopy was chosen for this study because of its unique ability to yield vibrational spectra of molecules at an interface. Different polarization combinations are used, which probe different susceptibilities, giving SFG the ability to determine molecular orientation at the interface. The ionic liquids used were 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6]. To determine the influence of the molecular structure and charge on orientation at the interface, neutral, 1-methylimidazole, and 1-butylimidazole were also studied. Raman spectra and depolarization ratios were obtained for neat samples of 1-methylimidazole, 1-butylimidazole, and 1-butyl-3-methylimidazolium tetrafluoroborate recorded from 2700 to 3300 cm-1. SFG spectra of the 1-methylimidazole/quartz interface showed both methyl and aromatic C−H vibrations. Orientation calculations determined that the ring of the molecule is tilted 45−68° from normal, with the methyl group oriented 32−35° from normal. The SFG spectra of 1-butylimidazole contain several resonances from the alkyl chain with only one weak resonance from the aromatic ring. Orientation calculations suggest that the ring is lying in the plane of the surface with the methyl group pointing 43−47° from normal. The orientation of the [BMIM][PF6] ionic liquid was sensitive to trace amounts of water and had to be evacuated to -5 Torr for the water to be removed. SFG spectra of both ionic liquids were similar, displaying resonances from the alkyl chain as well as the aromatic ring. Orientation analysis suggests the aromatic ring was tilted 45−90° from normal for [BMIM][BF4], while the ring for [BMIM][PF6] was tilted 38−58° from normal. This suggests the orientation of the molecule is influenced by the size of the anion.

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