Dielectric relaxation and solvation dynamics in a prototypical ionic liquid + dipolar protic liquid mixture: 1-butyl-3-methylimidazolium tetrafluoroborate + water

Dielectric and solvation data on mixtures of 1-butyl-3-methylimidazilium tetrafluoroborate ([Im₄₁][BF₄]) + water are reported and used to examine the utility of dielectric solvation models. Dielectric permittivity and loss spectra (25 C) were recorded over the frequency range 200 MHz to 89 GHz at 17 compositions and fit to a 4-Debye form. Dynamic Stokes shift measurements on the solute coumarin 153 (C153), made by combining fluorescence upconversion (80 fs resolution) and time-correlated single photon counting data (20 ns range), were used to determine the solvation response at 7 compositions (20.5 C). All properties measured here were found to depend upon mixture composition in a simple continuous manner, especially when viewed in terms of volume fraction. Solvation response functions predicted by a simple dielectric continuum model are similar to but ∼7-fold faster than the spectral response functions measured with C153. The solvation data are in better agreement with the recently published predictions of a semimolecular model of Biswas and co-workers [J. Phys. Chem. B 2011, 115, 4011], but these latter predictions are systematically slow by a factor of ∼3.