Gibbs monolayer of ionic liquid + H2O mixtures studied by surface tension measurement and sum-frequency generation spectroscopy

Room-temperature ionic liquid (RIL) + H2O mixture system was investigated using surface tension measurement and surface specific IR–visible sum-frequency generation (SFG) spectroscopy. The surface tension measured as a function of a bulk mole fraction of the ionic liquid indicated that the liquid surface even at very low bulk concentration of the ionic liquid is covered mostly by the cations. A slight increase of surface tension from 0.016 to 0.05 bulk mole fraction suggested that the anions start to appear at the surface in this concentration range. After 0.05 bulk mole fraction, the cations and the anions both occupy the surface, and the chemical composition and the surface density are expected to change little with the further addition of the ionic liquid. The result from the SFG measured at different concentrations is in agreement with the above findings. This result and the similar studies on methanol + water mixtures suggested that SFG is mainly sensitive to the surfaces excess molecules at the air/mixture interface.