Surface characterization of imidazolium ionic liquids by high-resolution Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy

The surface composition of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM] [PF6]) and 1-butyl-3-methylimidazolium dicyanamide ([BMIM] [DCA]) are studied by high-resolution Rutherford backscattering spectroscopy. Although [BMIM] [PF6] is almost stoichiometric up to the topmost molecular layer, considerable deviation from the theoretical stoichiometry is observed for [BMIM] [DCA] in a surface layer of ∼1.5 nm thickness. Nitrogen is almost completely depleted in this layer while carbon is enhanced. In addition, there are oxygen impurities of ∼3 × 1014 atoms/cm2 in this surface layer. With the help of X-ray photoelectron spectroscopy measurements it is concluded that the surface of [BMIM] [DCA] is covered by ∼1.7 × 1014 molecules/cm2 of esters and/or carboxylic acids. These contaminant molecules have a preferred orientation, i.e. the carbonyl groups are on the surface of [BMIM] [DCA] and the alkyl chains are pointing towards vacuum. The origin of the contamination layer could be the surface segregation of bulk impurities.