Adsorption, absorption and desorption of gases at liquid surfaces: water on [C8C1Im][BF4] and [C2C1Iml][Tf2N].

The adsorption, absorption and desorption of water vapour at the liquid, and frozen solid, surfaces of two ionic liquids (ILs) is described. Surface kinetics were measured by sticking probability (S), and temperature programmed desorption, using line of sight mass spectrometry in ultra-high vacuum. The two ILs used were 1-octyl-3-methylimidazolium tetrafluoroborate ([C8C1Im][BF4]) and 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, (C2C1Im] [Tf2N]. At room temperature, continuous absorption into the bulk occurred (S approximately = 0.12), which reduced with decreasing temperature, reaching zero when the ILs froze to glassy solids. At still lower temperatures, monolayer, and then multilayer, growth of water ice occurred at the solid surfaces. Adsorption is thought to occur first into a physisorbed state, and then into an ionic underlayer, the activation energy between the physisorbed state and the underlayer state being 48.0 +/- 0.8 kJ mol(-1) and 58 +/- 2.0 kJ mol(-1) for [C2C1Im][Tf2N] and [C8C1Im][BF4], respectively. From the underlayer state, water diffuses into the bulk rather than desorbing back into the gas phase. Adsorption and desorption kinetics were consistent with the existence of an outermost, hydrophobic layer covering the ionic underlayer. For [C8C1Im][BF4] this consists of outwardly orientated octyl chains, while for [C2C1Im][Tf2N] it is probably outward orientated ethyl and CF3 groups. Adsorption on the hydrophobic glassy surface was unusual, with S increasing for increasing coverage. The barrier to surface diffusion of water on the glassy surface of [C8C1Im][BF4] was 13 +/- 12 kJ mol(-1).