Successive Adsorption of Cations and Anions of Water-1-Butyl-3-methylimidazolium Methylsulfate Binary Mixtures at the Air-Liquid Interface Studied by Sum Frequency Generation Vibrational Spectroscopy and Surface Tension Measurements

We have investigated the surface behavior of 1-butyl-3-methylimidazolium methylsulfate ([bmim][MS]) aqueous solutions by sum frequency generation vibrational spectroscopy (SFG-VS) and surface tension measurements, including the adsorption of ions and its relationship with surface tension. At very low [bmim] [MS] concentrations, SFG-VS data indicate that with increasing mole fraction of [bmim] [MS], adsorption of cations at the interface rapidly increases, whereas the surface tension the decreases. When cation adsorption to the surface is close to saturation, the change of the surface tension tends to be gradual. When the mole fraction of [bmim][MS] reaches 0.1, anions begin to adsorb to the interface, leading to the changes of the orientation angle of cations and the aggregation behavior of cations and anions at the interface. The previously reported unusual minimum point in the surface tension curve of [bmim] [BF4] aqueous solution suggested to be caused by successive adsorption of cations and anions was not observed for [bmim] [MS] aqueous solution. SFG-VS spectra and the surface tension curve of [bmim] [MS] aqueous solution indicate that anion adsorption does not significantly affect the surface tension. These results provide important information about the surface behavior of ionic liquid aqueous solutions and the effect of adsorption of ions on the surface tension