Microstructures and Interaction Analyses of Phosphonium-Based Ionic Liquids: A Simulation Study

Abstract

Simulation study of eight kinds of phosphonium-based ionic liquids (ILs) is reported in this work. Force fields for two kinds of alkoxyphosphonium ILs are proposed through systematic method and validated by the experimental densities. The study was conducted by molecular dynamics simulations. A connection between the intermolecular energy, divided into the electrostic force and the van der Waals force, and the experimental viscosities was found. Radial distribution functions (RDFs) were analyzed to probe the local organization of the liquids. First-shell coordination numbers are also reported by integral of RDFs from zero to the first minimum. In order to compare the interaction strength and position between ions for different kinds of ILs, the relative density distributions along with the distance between cation and anion are proposed. Hydrogen bond numbers were investigated to depict the microinteraction. We found that, although there are six anions in the first solvation shell of [P2,2,2,5](+), only one hydrogen bond could be found. Along with increase in the length of alkyl chain, the hydrogen bond number becomes less, and no hydrogen bond interaction is found for 20% of the ions in [P4,4,4,14][Tf2N] In order to depict the effect of carbon chain length on the structure, the space distribution functions were also computed and compared.