Ionic Dynamics of Hydroxylammonium Ionic Liquids: A Classical Molecular Dynamics Simulation Study

We report the structure and dynamics of four ionic liquids (ILs), 2-hydroxyethylammonium formate, bis-(2-hydroxyethyl) ammonium formate, tris-(2-hydroxyethyl) ammonium formate (THEF), and 2-hydroxyethylammonium lactate, employing classical molecular dynamics simulations. The dynamics of ILs are represented by studying mean squared displacements (MSDs), velocity autocorrelation functions (VACFs), and current autocorrelation functions (CACFs). Diffusion coefficients calculated from the VACFs are higher than those obtained from MSDs. The diffusion coefficients calculated from both the methods (MSDs and VACFs) were averaged to calculate the uncorrelated ionic conductivities (ICs). ICs from these two methods agree with the experimental trend. The correlated and uncorrelated ICs were calculated by four methods and compared with experiments. The difference between CACF and center of mass VACF accounts for the correlated motion present in the ILs. The addition of hydroxyalkyl chains on cations causes the dynamics to become slow. The number of hydroxyl groups present on the cations affects the dynamics of ILs studied. A tris-(2-hydroxyethyl) ammonium cation has lower diffusion than any other ions because of the higher molecular weight and number of hydroxyl groups on the cation. We explored the dynamics of hydrogen bonding by calculating the continuous and intermittent hydrogen bond autocorrelation functions. Radial distribution functions between the functional groups of cations and anions reveal the structural arrangement in ILs. The coordination numbers decrease with the increase in the bulkiness of cations due to steric hindrance. Spatial distribution functions of anions around cations show that anions occupy the space around the ammonium hydrogen atoms of the cations. Ion-pair and ion-cage dynamics show that THEF has slower dynamics than the other three ILs and is consistent with MSDs. The inverse of ion-pair and ion-cage lifetimes shows a linear relationship with ICs.