Spectroscopic and MD Study of Dynamic and Structural Heterogeneities in Ionic Liquids

The structure of ionic liquids (ILs) surrounding solute dyes and the effects of solvent structure on solute diffusion have been investigated using molecular dynamics (MD) and the experimental tools of confocal and fluorescence correlation spectroscopies. Although confocal microscopy and simulations show that the local environment around solutes in ILs is heterogeneous and that the structural heterogeneity is rather long-lived, the local polarity and the diffusion constant were found to be uncorrelated. Moreover, the complex diffusion observed experimentally is not due to the structural heterogeneity of the IL but rather due to the dynamic heterogeneity arising from the viscous glassy nature of the IL environment. MD simulations show that the degree of dynamic heterogeneity depends on the first nonvanishing electric multipole moment of the solute. The dynamics of a cationic solute are the least heterogeneous, whereas those of a solute without an electric multipole moment are the most heterogeneous. This indicates that the length scale over which the solute-solvent interactions occur, and thus the number of solvent degrees of freedom that couple to the solute, are the key factors governing the dynamic heterogeneity of the solute.