Structural and dynamical properties of ionic liquids: a molecular dynamics study employing DL_POLY 4

Molecular dynamics simulations are often used to study the structures and dynamics of ionic liquids. Here, we have simulated three ionic liquids, trihexyl(tetradecyl)-phosphonium chloride [P66614][Cl], 1-butyl-3-methylimidazolium acecate [BMIm][Oac] and 1-ethyl-3-methylimidazolium dicyanamide, [EMIm][DCA] in a comparison of two force fields, GAFF and CL&PFF. In most cases, the resulting theoretical densities agree with experimental values within a 2% error. Diffusive properties were characterised by mean squared displacements to show the significant effect of the alkyl chain on the movement of the [P66614] cation. Activation energies of diffusion were calculated from linear Arrhenius plots which agree with previous studies. Simulations of the dynamical behaviour show retention of short and medium-range structure of the ionic liquids with temperature. However, although with increasing temperature more high energy local configurations become accessible, they are observed less frequently as energy barriers are more easily overcome, resulting in more ordered time-averaged structures.