Prediction of the univariant two-phase coexistence line of the tetrahydrofuran hydrate from computer simulation

In this work, the univariant two-phase coexistence line of the tetrahydrofuran (THF) hydrate is determined from 100 to 1000 bar by molecular dynamics simulations. This study is carried out by putting in contact a THF hydrate phase with a stoichiometric aqueous solution phase. Following the direct coexistence technique, the pressure is fixed, and the coexistence line is determined by analyzing if the hydrate phase grows or melts at different values of temperature. Water is described using the well-known TIP4P/Ice model. We have used two different models of THF based on the transferable parameters for phase equilibria-united atom approach (TraPPE-UA), the original (flexible) TraPPe-UA model and a rigid and planar version of it. Overall, at high pressures, small differences are observed in the results obtained by both models. However, large differences are observed in the computational efforts required by the simulations performed using both models, being the rigid and planar version much faster than the original one. The effect of the unlike dispersive interactions between the water and THF molecules is also analyzed at 250 bar using the rigid and planar THF model. In particular, we modify the Berthelot combining rule via a parameter ξO-THF that controls the unlike water-THF dispersive interactions. We analyze the effect on the dissociation temperature of the hydrate when ξO-THF is modified from 1.0 (original Berthelot combining rule) to 1.4 (modified Berthelot combining rule). We use the optimized value ξO-THF = 1.4 and the rigid THF model in a transferable way to predict the dissociation temperatures at other pressures. We find excellent agreement between computer simulation predictions and experimental data taken from the literature.