Probing translational and rotational dynamics in hydrophilic/hydrophobic anion based imidazolium ionic liquid–water mixtures

Abstract

In this investigation, we examine the effect of water concentration and temperature on the dynamical properties of [Hmim][Cl] and [Hmim][NTf₂] ionic liquids (ILs). The dynamical properties such as translational diffusion coefficients, ion-pair lifetimes, and rotational correlation times are calculated using molecular dynamics simulations. The simulations predict that water concentration also significantly impacts the magnitude of dynamical properties. At low, intermediate and high water concentrations, the following trend in diffusion coefficients is seen: Cl⁻ > Hmim⁺; Cl⁻ > NTf₂⁻; Hmim⁺ ([Hmim][Cl]) > Hmim⁺ ([Hmim] [NTf₂]). At ultra-low water concentrations of [Hmim][Cl] IL, several bridge like configurations form between water molecules and Cl⁻ anions, which are supported by a complex distribution of water clusters. The effect of an increase in the water concentration leads to a decrease in ion-pair lifetimes between the Hmim⁺ cations and Cl⁻/NTf₂⁻ anions, which strongly correlates with the trends observed from the diffusion coefficients. A biexponential function was found to be the best fit for the RACF at neat/ultra-low water concentrations of [Hmim][Cl] and [Hmim][NTf₂] ILs, whereas a single exponential function was sufficient to fit the RACF at low, intermediate and high water concentrations. The rotational relaxation time of the Hmim⁺ cations is larger in neat [Hmim][Cl] compared to that in neat [Hmim][NTf₂] with an opposite trend seen with hydration. The rotational correlation time of water molecules is larger in [Hmim][Cl] compared to that in [Hmim][NTf₂] at low and intermediate water concentrations, with similar correlation times observed at high water concentrations.