Computational study of imidazolium-based ionic solvents with alkyl substituents of different lengths

Structural and dynamical properties of 1-alkyl-3-methylimidazolium hexafluorophosphate (where alkyl=hexyl, octyl, decyl, dodecyl) are studied using molecular dynamics. In this work, we compute radial and angular distribution functions, the distribution of cavities in the liquid and diffusion constants. The power spectrum is obtained from averaged atomic velocity autocorrelation functions. In order to characterize the response of these liquids upon external Coulomb perturbation, a diatomic probe is used in an electronic photo-excitation computational experiment. We find that the radial distribution functions that couple anions and cations are more structured when alkyl substituent chains are longer. We also find that diffusion constants for anions and their corresponding cations are highly correlated. Cavity distributions are nearly identical for all solvents studied. We find that large cavities, if present in the liquid, have very long lifetimes. We observe at least two different time scales for the decay of energy fluctuations in the liquid after a neutral apolar diatomic probe molecule is photo-excited into a dipolar excited state. Fast sub-picosecond relaxation occurs after which correlations decay in approximately 0.1 ns.