Gibbs Ensemble Monte Carlo for Reactive Force Fields to Determine the Vapor-Liquid Equilibrium of CO2 and H2O

Abstract

Absorption and reactive properties of fluids in porous media are key to the design and improvement of numerous energy related applications. Molecular simulations of these systems require accurate force fields that capture the involved chemical reactions and have the ability to describe the vapor-liquid equilibrium (VLE). Two new reactive force fields (ReaxFF) for CO₂ and H₂O are developed, which are capable of not only modeling bond breaking and formation in reactive environments but also predicting their VLEs at saturation conditions. These new force fields include extra terms (ReaxFF-lg) to improve the long-range interactions between the molecules. For validation, we have developed a new Gibbs ensemble Monte Carlo (GEMC-ReaxFF) approach to predict the VLE. Computed VLE data show good agreement with National Institute of Standards and Technology reference data as well as existing nonreactive force fields. This validation proves the applicability of the GEMC-ReaxFF method to test new reactive force fields, and simultaneously it proves the applicability to extend newly developed ReaxFF force fields to other more complex reactive systems.

Figure 1

Examples of liquid and gas boxes in GEMC simulations. Besides thermalization, the two boxes can exchange molecules and volume.

Figure 2

Comparison of different energy contributions regarding the dissociation of two parallel CO₂ molecules. The gray lines represent the total energy. The blue line represents the Lennard-Jones energy contribution for the TraPPE force field and the van der Waals energy for the ReaxFF force fields. The red line represents the Coulomb energy contribution for the TraPPE force field and the summation of the Coulomb and polarization energy in the ReaxFF force fields. The orange line represents the DFT-D reference.

Figure 3

Average density distributions of the simulation boxes of (a) CO₂ and (b) H₂O after MD simulation. The red solid and dashed lines are the simulation results and the experimental coexistence densities^, for the lower temperatures, respectively. The blue solid and dashed lines are the simulation results and the experimental coexistence densities for the higher temperatures, respectively. The top boxes are representations of the final configurations at 280 and 580 K for CO₂ and H₂O, respectively.

Figure 4

GEMC simulation of H₂O at 580 and 600 K. The solid lines represent the densities of the two boxes simulated by the GEMC–ReaxFF, with red at 580 K and blue at 600 K. The dashed lines represent experimental coexistence data.

Figure 5

VLEs for (a) CO₂ and (b) H₂O. The black lines represent the NIST reference data. The red lines represent the (a) TraPPE force field and (b) TIP4P/2005 force field. The blue lines represent the predicted values by the new ReaxFF-lg force fields. The orange results represent the predicted by the original (a, b) ReaxFF. The asterisks are the computed critical points using eqs 8 and 9