Molecular Dynamics Study on the Equilibrium and Kinetic Properties of Tetrahydrofuran Clathrate Hydrates

Tetrahydrofuran (THF) is an effective promoter of methane hydrates, and itself with water can form clathrate hydrates even without the presence of methane gas. In this work, the stability limit and kinetic properties of THF hydrates were simulated using molecular dynamics (MD) simulations. The change in dissociation temperature of THF hydrates with pressure and concentration of THF in the aqueous phase were well reproduced with MD simulations. The rate of growth of THF hydrates is found to exhibit a maximum value when the liquid-phase THF concentration is about 0.30.8 times (depending on temperature) of the THF concentration in the hydrate phase. The existence of some optimal growth concentration explains the preferred lateral growth in experiments. The maximum growth rate is a result of two competing effects: the adsorption of THF molecules to the growing interface, which is the limiting step at low THF concentrations, and the desorption/rearrangement of THF molecules at the interface, limiting step at high THF concentrations. The large cages of structure II (sII) hydrate are fully occupied by THF molecules, regardless of the THF concentration in the aqueous phase, implying a strong stabilization effect of THF molecules to the cage structures of sII hydrates.