On the Dynamic Interaction of n-Butane with Imidazolium-Based Ionic Liquids.

The impact of a reactant from the gas phase on the surface of a liquid and its transfer through this gas/liquid interface are crucial for various concepts applying ionic liquids (ILs) in catalysis. We investigated the first step of the adsorption dynamics of n-butane on a series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ILs ([Cn C1 Im][Tf2 N]; n=1, 2, 3, 8). Using a supersonic molecular beam in ultra-high vacuum, the trapping of n-butane on the frozen ILs was determined as a function of surface temperature, between 90 and 125 K. On the C8 - and C3 -ILs, n-butane adsorbs at 90 K with an initial trapping probability of ≈0.89. The adsorption energy increases with increasing length of the IL alkyl chain, whereas the ionic headgroups seem to interact only weakly with n-butane. The absence of adsorption on the C1 - and C2 -ILs is attributed to a too short residence time on the IL surface to form nuclei for condensation even at 90 K.