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

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][BF(4)], and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF(6)]. 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 degrees from normal, with the methyl group oriented 32-35 degrees 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 degrees from normal. The orientation of the [BMIM][PF(6)] ionic liquid was sensitive to trace amounts of water and had to be evacuated to <3 x 10(-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 degrees from normal for [BMIM][BF(4)], while the ring for [BMIM][PF(6)] was tilted 38-58 degrees from normal. This suggests the orientation of the molecule is influenced by the size of the anion.

Spotlight on ionic liquids

“…26 Similar gauche defect reduction was seen in SFG studies of short-to long-chain 1-alkyl-3-methylimidazolium salts at the SiO 2 interface. 116,117 The structure of the IL-vacuum interface for ͓emim͔ + / NO 3 − was reported by Yan et al 119 Surface tension and thickness of IL-vapor interfaces have been reported from both simulations 120 and experiment. 121 By accounting for both capillary waves and zero-point motions, the underlying structure of the IL surface has been determined by comparing MD simulations with x-ray reflectivity experiments.…”

“…A wide variety of experimental, theoretical, and simulation methods have been applied to the study of ILs at interfaces. [107][108][109][110][111][112][113][114][115][116][117][118][119][120][121][122] Santos and Baldelli 112 have used interfacial sumfrequency generation ͑SFG͒ to show that at IL interfaces with gases or vacuum, alkyl chains on either the cations or the anions extend from the surface into the gas or vacuum. Rutherford backscattering spectra for the ͓bmim͔ + / ͓PF 6 ͔ − -vacuum interface also show the cation butyl group protruding from the bulk liquid into vacuum.…”

Solvation of Sodium Chloride in the 1-Butyl-3-methyl-imidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid: A Molecular Dynamics Study

“…These results are consistent with surface spectroscopic and diffraction data. [103][104][105][106][107] Interfaces with crystals are known to induce structural order of liquids, 108 and in the case of ILs, spectroscopy results have been reported at the quartz, 109 silice, 110 and platinum charged interfaces, 111 revealing specific ordering of the imidazolium cations. 112 At the surface of a platinum electrode, the cation's ring orients rather parallel to the surface in response to the negative surface charge.…”

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…When the silver electrode was negatively charged some imidazolium ring vibrational modes and some N-CH 3 vibrations were enhanced, suggesting that the imidazolium rings were parallel to the surface [81]. This seems to be the case also in other systems such as [C 4 6 ], 1-methylimidazole, and 1-butylimidazole, studied by Raman spectroscopy, and with depolarization ratios recorded from 2700 to 3300 cm À1 [82]. Mixed systems of organic molecules and ionic liquids that form separate phases (by thermomorphic phase separation) have been also studied by Raman spectroscopy [83].…”