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.