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The adsorption behavior of ionic liquids based on the 1-alkyl-3-methylimidazolium cation with different anions to the air–water interface was investigated through surface tension measurements. From the thermodynamic analysis of the surface tension data, a physical model for the structure of the adsorbed film is proposed: the HMIM + and BF 4 − ions occupy the same level of the surface due to ion-pairing in the HMIMBF 4 system, whereas the Br − ions are mainly distributed in the diffuse electrical double layer below the top surface in the DeMIMBr system. In the former case, the entropy change associated with adsorption increases with increasing concentration when the surface density is close to saturation because the hydrated water is released during the ion-pairing of HMIM + and BF 4 − . Furthermore, the structure of the adsorbed film is almost the same as a neat ionic liquid surface; this is probably because both the anions and the cations are highly concentrated in the adsorbed film.

The adsorbed film of 1-decyl-3-methylimidazolium bromide (DeMIMB) at the air/water interface was investigated employing the surface tension measurement and the x-ray absorption fine structure method under the total reflection condition (TRXAFS). From the surface tension measurement, the surface excess concentrations of ions were determined. From the XAFS measurement, two solvation states of bromide ion were found in the adsorbed film, which were assigned to be "free-Br" and "bound-Br". The hydration number of the former was estimated to be 6 while that of the latter was estimated to be 4. The results based on the XAFS analysis provided significant information on the formation of domains in the adsorbed film; the most conceivable situation is that the adsorbed molecules are definitely not homogeneously dispersed, but domains (islands or clusters) are dispersed in the adsorbed film. A regular and rather tight stacking of immidazolium rings may be formed in the domains.

The miscibility and distribution of Br(-) and BF(4)(-) ions of imidazolium ionic liquid mixtures, 1-hexyl-3-methylimidazolium bromide (HMIMBr) + 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)), at the air/water surface were investigated by surface tensiometry and the total-reflection XAFS (TRXAFS) method. Tensiometry showed that the surface density of BF(4)(-) was much larger than that of Br(-), the adsorbed films of the HMIMBr-HMIMBF(4) mixture were greatly enriched in BF(4)(-) at all surface tensions, and the excess Gibbs energy of adsorption was positive. However, TRXAFS revealed that the Br ions were all in the free-Br state solvated by six water molecules in the mixed adsorbed film. Entropy-originated nonideal mixing, where a kind of segregation of the counterion distribution takes place in the interfacial region, was suggested in the mixture.