Using a Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of Imidazolium-Based Ionic Liquids/Water Mixtures. 2. EXAFS Spectroscopy

Extended X-ray absorption fine structure (EXAFS) spectroscopy is employed, in conjunction with molecular dynamics (MD) simulations, to investigate the interaction of water with the BC ion in an imidazolium-based ionic liquid (IL). 1-Butyl-3-methylimidazolium bromide/water mixtures with molar ratios ranging from 1:3 to 1:200 have been analyzed, and a clear picture of the structural arrangements of the water molecules inside the IL has been obtained from the synergic interpretation of the EXAFS and MD data. At the lowest investigated water content, the presence of water is mainly detected around the BC anion. Upon increasing the water fraction, more water molecules enter the BC first-coordination shell but always in a lower number than what is needed to saturate the inner sphere. This suggests that interactions existence of tight ion pairs has been evidenced, even when water is needed to saturate the inner sphere. This suggests that interactions also exist between water and the imidazolium cation. The existence of tight ion pairs has been evidenced, even when water is present in the mixtures in great excess.