The role of intermolecular interactions in the prediction of the phase equ libria of carbon dioxide hydrates

The direct phase coexistence methodology was used to predict the three-phase equilibrium conditions of carbon dioxide hydrates. Molecular dynamics simulations were performed in the isobaric isothermal ensemble for the determination of the three-phase coexistence temperature (T3) of the carbon dioxide water system, at pressures in the range of 200-5000 bar. The relative importance of the water water and water guest interactions in the prediction of T-3 is investigated. The water water interactions were modeled through the use of TIP4P/Ice and TIP4P/2005 force fields. The TraPPE force field was used for carbon dioxide, and the water guest interactions were probed through the modification of the cross-interaction Lennard-Jones energy parameter between the oxygens of the unlike molecules. It was found that when using the classic Lorentz Berthelot combining rules, both models fail to predict T-3 accurately. In order to rectify this problem, the water guest interaction parameters were optimized, based on the solubility of carbon dioxide in water. In this case, it is shown that the prediction of T-3 is limited only by the accuracy of the water model in predicting the melting temperature of ice. (C) 2015 AIP Publishing LLC.