(52d) Development and Evaluation of Molecular Models for the Calculation of Thermodynamic and Transport Properties of H2O+CO2+NaCl System

Understanding the thermo-physical properties of the H₂O+CO₂+NaCl mixture is of great importance to geochemistry and design of CO₂ geologic sequestration. Thermodynamic and transport properties of binary H₂O+NaCl and H₂O+CO₂ mixtures were studied over a wide range of temperature and pressure conditions using a series of fixed point charge non-polarizable and Drude oscillator based polarizable molecular models. For the H₂O+NaCl mixture, the mean ionic activity coefficient, salt solubility, vapor pressure, interfacial tension, and viscosity were obtained as functions of temperature, pressure and salt concentration. Among the studied models, the BK3 [1,2] set of polarizable water and ions force fields give reasonable prediction for all properties of interest, and the explicit inclusion of polarization in molecular models was found to be crucial for accurate description of the binary H₂O+NaCl mixture [3]. For the H₂O+CO₂ mixture, the phase equilibrium compositions and diffusion coefficients were obtained from ambient conditions to elevated temperatures and pressures. Since prior simulation studies [4,5] showed that non-polarizable molecular models have significant inaccuracy for the calculation of H₂O compositions in CO₂ rich phase, we focused on the performance of several polarizable H₂O and CO₂ models. In particular, the BK3, GCPM [6] and SWM4-NDP [7] H₂O models were studied. TraPPE [8] and a recently developed Gaussian charge polarizable models [9] were used to represent CO₂. The cross interactions between H₂O and CO₂ models were optimized to equilibrium phase compositions of the binary H₂O+CO₂ mixture. The polarizable models yield more accurate calculation of phase compositions compared to non-polarizable ones, however, the representation of H₂O compositions in CO₂ rich phase was still unsatisfactory, especially at high pressures. H₂O molecules were found to form small clusters in CO₂ rich environment, which may indicate the H₂O composition in CO₂ rich phase may be strongly affected by the H₂O-H₂O interactions instead of the CO₂-H₂O cross interactions. A new polarizable model was developed for H₂O, and its performance with respect to the calculation of H₂O-CO₂ phase equilibria and diffusion coefficients was investigated. The proposed new polarizable H₂O model is extended to study the H₂O-CO₂-NaCl mixture.

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[9] Jiang, H.; Moultos, O. A.; Economou, I. G.; Panagiotopoulos, A. Z. J. Phys. Chem. B 2016, 120, 984.