Prediction of the solvation and structural properties of ionic liquids in water by two-dimensional correlation spectroscopy

Two-dimensional vibrational spectroscopy is applied to investigate the dilution process of 1-ethyl-3-methyl imidazolium tetrafluoroborate ([Emim][BF4]) in water. With increasing water content in ionic liquid (IL)/water mixtures, the C-H stretching vibration of the imidazolium cation showed systematic blue-shifts, which reflect the weakening of the cohesion between the cation and anion of ILs. The two-dimensional IR results reveal that the ILs sense quite different environments during the whole dilution process. First, the three-dimensional network structure of pure ILs was destroyed gradually into ionic clusters, then the clusters were further dissociated into ionic pairs surrounded by water molecules, and finally the latter became the dominant form in bulk water. Within the concentration range we investigated (0.02 < X-D2O < 0.90), the intruding water does not dissociate ILs into ions completely. The structure evolution of ILs can be clearly detected and visualized in the asynchronous spectrum.