On the collective network of ionic liquid/water mixtures. II. Decomposition and interpretation of dielectric spectra

Abstract

This study deals with the dielec. spectra of mixts. of the ionic liq. 1-butyl-3-methyl-imidazolium (BMIM+) tetrafluoroborate with water at three selected mole fractions 0.767 ≤ xH2O ≤ 0.967. The focus lies on the comparison of exptl. and computational data. On the one hand, a computational anal. permits a complete decompn. of spectra, both with respect to dynamical behavior (translation and rotation) as well as to compn. of the mixt. (cation, anion, and water). Thereby, not only the peak assignment in exptl. spectra is enabled but one can also learn more about solvation properties. Of particular importance is the interplay of the dielec. const. and the cond. representing a measure of collective rotational and translational motion. On the other hand, the comparison with exptl. spectra is essential for the validation of the force fields used in simulation. The satisfying agreement between corresponding peaks in the dielec. spectra confirms not only computed dielec. relaxation times but also other collective dynamical properties such as the viscosity. Nevertheless, the detailed fine structure of the cond. regime reveals specific ion-pair effects not covered by the simulation. A possible confinement of dynamical heterogeneity as a consequence of a system size effect is also indicated. (c) 2008 American Institute of Physics.