(311e) Modeling Chemical and Physical Interactions in CO2/Ionic Liquids With the Reaxff Force Field

Ionic liquids (ILs) are promising sorbents for CO₂ due to their essentially zero vapor pressure and the potential to tune the CO₂-IL interaction.  Both experimental and computational studies of CO₂/IL systems have been carried out. The ILs can interact with CO₂ through one or more of the following: physical absorption, complexation, chemical reaction. It is a challenge for a computational model to be able to accurately account for all three of these types of interactions. Physical interactions can be described with classical potentials very well, but cannot account for the bond formation and breaking events associated with chemical interactions between CO₂ and ILs. Treatment of complexation and chemical reactions usually requires high-level quantum chemical methods that are computationally expensive. An alternative approach is to use a reactive force field that can capture both physical interactions and chemical reactions. We have developed a new parameterization of the ReaxFF reactive force field for tetrabutylphosphonium glycinate ([P(C₄)₄][Gly]) and CO₂. We developed an extensive training set for both the pure [P(C₄)₄][Gly] system and the [P(C₄)₄][Gly]/CO₂ mixture from periodic and gas phase density functional theory (DFT) calculations. We have validated the optimized ReaxFF force field by comparing the energies from DFT and ReaxFF calculations for the structures in the training sets. We also tested the predictability of the optimized force field by comparing the same configurations of ILs with CO₂ not in the training sets. The trends of relative stability of different configurations based on energies from DFT and ReaxFF methods are in good agreement. The optimized ReaxFF force field has been used in large-scale molecular dynamics (MD) simulation. The calculated density is in very good agreement with experiment measured values at 300 K at ambient pressure. The radial distribution function and transport properties (MSD, diffusivity) of pure IL and IL with CO₂ mixture have been calculated and compared with those from classical potential. The reaction between CO₂ and IL has also been monitored and compared with first principles MD results.