Development of a ReaxFF reactive force field for tetrabutylphosphonium glycinate/CO2 mixtures

Carbon dioxide interacts with the ionic liquid tetrabutylphosphonium glycinate, [P(C4)4][Gly], through both physical and chemical absorption. We present a parametrization of the ReaxFF force field for this system that accounts for both chemical and physical interactions. The parametrization was developed from an extensive training set including periodic density functional theory (DFT) calculations of reaction pathways between CO2 and the anion [Gly](-) in the condensed phase, condensed-phase molecular dynamics (MD) configurations, gas-phase CO2-anion and CO2-cation interactions, and gas-phase cluster calculations for intra-ion interactions. The optimized ReaxFF parameters capture the essential features of both physical and chemical interactions between CO2 and [P(C4)4][Gly] as compared with experiments, van der Waals-corrected DFT calculations, or, in the case of physical interactions, classical force field calculations. The probability distributions of the distance between C (from CO2) and N (from the anion) and the CO2 bend angles calculated from MD simulations with the optimized ReaxFF force field are in good general agreement with those from DFT-based MD simulations. We predict that the density of CO2/[P(C4)4][Gly] mixtures increases with increasing CO2 concentration up to at least 50 mol % CO2. We attribute the significant increase in density to the small effective volume occupied by chemically bound CO2 in the mixture. The predicted increase in density may be tested experimentally.