Molecular modelling of ionic liquids: Physical properties of species with extremely long aliphatic chains from a near-optimal regime

Abstract

Basic issues of molecular modelling of ionic liquids with fixed-charge force fields are validated from extensive free energy calculations, component-specific force-field refitting, and box-size dependence of various properties in our previous works. It is found that the combination of scaled RESP charges and GAFF2 parameter set provides an accurate yet efficient option for atomistic simulations of ionic liquids derivatives. Based on hundreds of solvation and partition free energies accumulated for six typical room-temperature ionic solvents, the scaling parameter 0.8 is found to be a near-optimal solution universally applicable to most ionic liquids, due to its ability of simultaneously reproducing the experimental solvation, partition and bulk properties with small errors. In this work, we examine whether the previously obtained general guidelines on fixed-charge modelling of ionic liquids can be directly extended to unexplored species with structural features significantly different from the ions considered in our previous benchmark set. Specifically, the two ionic liquids under investigation involve a cation with extremely long aliphatic chains, which differs significantly from previous cations from the imidazolium family. The anions forming the two ionic liquids are also significantly different from those in our previous works. Numerical results of solvation thermodynamics of a spectrum of structurally diverse solutes in the two anhydrous ionic solvents, the water-ionic-liquids partition and the mass density using the recommended near-optimal fixed-charge modelling regime are in good agreement with experimental references, thus validating the applicability of the general guidelines proposed in our previous works.