Insights into the surface composition and enrichment effects of ionic liquids and ionic liquid mixtures

Abstract

A systematic study of ionic liquid surfaces by angle resolved X-ray photoelectron spectroscopy (ARXPS) is presented. By reviewing recent and presenting new results for imidazolium-based ionic liquids (ILs), we discuss the impact of chemical differences on surface composition and on surface enrichment effects. (1) For the hydrophilic ethylene glycol (EG) functionalised ILs [Me(EG)MIm][Tf₂N], [Et(EG)₂MIm][Tf₂N] and [Me(EG)₃MIm][Tf₂N], which vary in the number of ethylene glycol units (from 1 to 3), we find that the surface composition of the near-surface region is in excellent agreement with the bulk composition, which is attributed to attractive interactions between the oxygen atoms on the cation to the hydrogen atoms on the imidazolium ring. (2) For [C_nC₁Im][Tf₂N] (where n = 1–16), i.e. ILs with an alkyl chain of increasing length, an enrichment of the aliphatic carbons is observed for longer chains (n > 2), at the expense of the polar cation head groups and the anions in the first molecular layer, both of which are located approximately at the same distance from the outer surface. (3) To study the influence of the anion on the surface enrichment, we investigated ten ILs [C₈C₁Im][X] with the same cation, but very different anions [X]⁻. In all cases, surface enrichment of the cation alkyl chains is found, with the degree of enrichment decreasing with increasing size of the anion, i.e., it is most pronounced for the smallest anions and least pronounced for the largest anions. (4) For the IL mixture [C₂C₁Im][Tf₂N] and [C₁₂C₁Im][Tf₂N] we find a homogeneous distribution in the outermost surface region with no specific enrichment of the [C₁₂C₁Im]⁺ cation.