Composition Dependence of Dynamic Heterogeneity Time- and Length Scales in [Omim][BF4]/Water Binary Mixtures:...

Composition dependence of four-point dynamic susceptibilities, overlap functions, and other dynamic heterogeneity (DH) parameters have been investigated by using all-atom molecular dynamics simulations for aqueous solutions of the ionic liquid (IL), 1-octyl-3-methyl imidazolium tetrafluoroborate ([Omim][BF4]) covering the pure-to-pure range. Upon addition of water in the IL, the DH time scales become faster and the four-point dynamic susceptibility time scale softens. Evidences for jump motions for both water and ions have been found from the simulated single particle displacements that show strong deviation from Gaussian distribution. Estimated dynamic correlation length for water reflects effects of IL, whereas those for ions remain largely insensitive to the mixture composition. Simulated structural aspects and DH time scales provide microscopic explanations to the existing experimental observations from time-resolved fluorescence and Kerr spectroscopic measurements.