Dicyanamide Anion Reports on Water Induced Local Structural and Dynamic Heterogeneity in Ionic Liquid Mixtures
Clara A. Tibbetts, Autumn B. Wyatt, B. M. Luther, Anthony K. Rappé, Amber T. Krummel
Abstract
The effects of limited amounts (under 21.6% χ Water ) of water on 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF 4 ) and 1-butyl-3-methylimidazolium dicyanamide (BmimDCA) room-temperature ionic liquid (RTIL) mixtures were characterized by tracking changes in the linear and two-dimensional infrared (2D IR) vibrational features of the dicyanamide anion (DCA). Peak shifts with increasing water suggest the formation of water-associated and nonwater-associated DCA populations. Further results showed clear differences in the dynamic behavior of these different populations of DCA at low (defined here as below 2.5% χ Water ), mid (defined here as between 2.5% χ Water and 9.6% χ Water ), and high (defined here as between 11.6% χ Water and 21.6% χ Water ) range water concentrations. Vibrational relaxation is accelerated with increasing water content for water-associated populations of DCA, indicating water facilitates population relaxation, possibly through the provision of additional bath modes. Conversely, spectral diffusion of water-associated populations slowed dramatically with increasing water, suggesting that water drives the formation of distinct and noninterchangeable or very slowly interchangeable local solvent environments.