Comparing the void space and long-range structure of an ionic liquid with a neutral mixture of similar sized molecules

Abstract

We study the void space and the structure of a coarse-grained molecular model of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [C4mim][PF6] and the corresponding model of an uncharged liquid consisting of the same molecules but with zero charges as a function of temperature between 300 K and 600 K using molecular dynamics simulations. Long-distance radial correlation functions for representative sites and for interstitial spheres were calculated for the individual subsystems representing anions and cations. The geometry of the local structural arrangements was investigated using the technique of Delaunay simplices. As expected, the ionic liquid demonstrates a noticeable regularity with respect to the alternation of anions and cations, which is present in all studied distance-ranges. The structure of uncharged subsystems, however, demonstrates features typical for simple liquids, which are governed by geometrical laws of the dense disordered packing of impenetrable particles. A prominent structural feature of the studied ionic liquid is that the “mutually closest fours” of anions form configurations of pronounced tetrahedral shape with a cation being located in between them. Correspondingly, the cations (here represented by the position of their central site) also form tetrahedral configurations around an anion. In the uncharged system, such mutual arrangements are found to be absent. The analysis of the models for different temperatures demonstrates that the principal differences between the structures of the charged and uncharged subsystems remain present up to 600 K. Quite interestingly, the distribution of void spaces, as represented by the radii of interstitial spheres, is found to be consistently, albeit slightly broader for the charged model liquid.