On the evaporation, bonding, and adsorbate capture of an ionic liquid on Au(111)

Abstract

Temperature programmed desorption has been used to measure the bonding of 1-methyl-3-ethylimidiazolium bis[(trifluoromethyl)sulfonyl]imide, [C₂C₁Im][Tf₂N], to Au(111), and the bonding of acetone within the ionic liquid (IL) at both the liquid–vacuum and liquid–Au(111) interfaces. Multilayer evaporation the ionic liquid occurred with an activation energy of 126 ± 5 kJ mol⁻¹ and a pre-exponential term of 10^16±1 s⁻¹, the evaporation mechanism being the direct emission into the vacuum of an ion pair from within the liquid surface. [C₂C₁Im][Tf₂N] chemisorbed to Au(111) had an activation energy for desorption which varied from 158 to 132 kJ mol⁻¹ for a coverage change of 0 → θ → 1.0. The chemisorbed layer was thought to comprise a single layer of co-planar ions at θ = 0.5 with dipoles parallel to the surface, and a bi-layer at θ = 1 with dipoles perpendicular to the surface. The acetone experiments consisted of a layer of porous ionic liquid glass deposited on top of a layer of solid acetone at 100 K. During desorption acetone was captured within the ionic under-layer of the glass with an activation energy for subsequent desorption that dropped from 54 to 43 kJ mol⁻¹ for 0 → θ → 1.0. This variation in energy is thought to be due to a range of chemical environments within the ionic under-layer.