Structure of the Room-Temperature Ionic Liquid/SiO2 Interface Studied by Sum-Frequency Vibrational Spectroscopy

In the study presented here, the structure of the RTIL/SiO 2 interface has been examined by sum-frequency vibrational spectroscopy (SFVS). A series of hydrophobic RTILs composed of 1-alkyl-3-methylimidazolium (C n mim, n = 6, 8 and 10) and bis(perfluoromethylsulfonyl)imide (BMSI) and bis(perfluoroethylsulfonyl)imide (BETI) anions have been examined. SFVS was used to determine the orientation of the cation as well as the structure of water at the RTIL/SiO 2 interface. The alkyl chain of the imidazolium cation was determined to be nearly normal to the surface for all the RTILs examined. The conformational order of the alkyl chain on the cation was also determined and was found to be dependent on the length of the alkyl chain, with longer chains being more ordered with few gauche defects. The tilt of the imidazolium ring with respect to the surface normal was also determined and was found to be ∼28° for the BMSI salts with a reduction in the tilt angle to ∼18° for the BETI RTILs. The water at the surface associated with the RTIL was determined to be hydrogen-bonded either singly or doubly to the anions at the SiO 2 interface. In addition, the nature of the hydrogen-bonding was found to be dependent on the amount of bulk water contained in the RTIL.