Temperature-dependent surface-enrichment effects of imidazolium-based ionic liquids.

We present the first systematic study of the influence of temperature on the degree of surface enrichment of 1-alkyl-3-methylimidazolium-based ionic liquids (ILs). Using angle-resolved X-ray photoelectron spectroscopy, we demonstrate that the degree of surface enrichment strongly decreases with increasing temperature for all the studied ILs. For ILs with the same cation, but different anions, [C8 C1 Im]Br, [C8 C1 Im][TfO] and [C8 C1 Im][Tf2 N], no significant differences of the temperature-induced partial loss of surface enrichment are found. Measurements for [C4 C1 Im][TfO], [C8 C1 Im][TfO] and [C18 C1 Im][TfO] indicate a small effect of the chain length. For [C18 C1 Im][TfO], a continuous decrease of alkyl surface enrichment is found with increasing temperature, with no abrupt changes at the phase-transition temperature from the smectic A to the isotropic phase, indicating that the surface enrichment is not affected by this phase transition.