Predicting Thermodynamic Stability of Clathrate Hydrates Based on Molecular-Dynamics Simulations and Its Confirmation by Phase-Equilibrium Measurements

Abstract

In order to establish a method to predict the formation of new clathrate hydrates under milder temperature-pressure conditions, we performed a molecular-dynamics (MD)-based free energy calculation, and then hydrate phase equilibrium measurements were done to confirm the prediction. The free energy differences are calculated for the structure-H hydrates each formed with methane and each of the following large-molecule guest substances: 2-methylbutane, 2,3-dimethylbutane, 2,2-dimethylbutane, 2,2,3-trimethybutane, and 2,2,3,3-tetramethylbutane. MD simulations were performed under a constant pressure and temperature with 6120 TIP4P water molecules, 900 OPLS-UA methane molecules, and a multisite modeled 180 OPLS-UA LMGS molecule. The results of the free energy calculation indicated the structure-H hydrate formed with 2,2,3,3-tetramethylbutane is the most stable hydrate although the relevant equilibrium pressure data were not previously reported in the literature. The experimental phase equilibrium measurement...