Correlation of Macroscopic Surface Tension and Microscopic Surface Composition of Binary Ionic Liquid Mixtures with Common Cations and Anions of Different Size

We investigated the surface tension and surface composition of binary ionic liquid (IL) mixtures of ILs sharing the same cation. As model system, binary mixtures of 1-ethyl-3-methylimidazolium acetate ([C₂C₁Im][OAc], molar volume: 154.4 cm³·mol^–1 at 293 K) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂C₁Im][Tf₂N], molar volume: 256.2 cm³·mol^–1 at 293 K), with very different surface tensions, and with anions of very different chemical structure and size were studied over the whole composition range. The surface tension was obtained by pendant-drop (PD) measurements in the presence of 0.1 MPa argon between 294 and 323 K, and the surface composition was determined by angle-resolved photoelectron spectroscopy (ARXPS) in ultrahigh vacuum at 293 K. The ARXPS results reveal a strong preferential enrichment of [C₂C₁Im][Tf₂N] at the vacuum–liquid interface of the binary mixtures, which is more pronounced at lower [C₂C₁Im][Tf₂N] bulk contents. This microscopic behavior is reflected in the macroscopic surface tensions, which are significantly lower than calculated assuming a linear mixing behavior based on the bulk composition. A previously developed prediction model to correlate the surface tension with the molar surface composition yields deviations of more than 5% from the measured values, which we attribute to the strongly different sizes of the anions. By accounting for the surface areas occupied by the ILs, we present an improved new model which describes the experimental data very well within 1.4%.