(330f) Simulation Studies of Tetrabutyl-Ammonium Bromide Semi-Clathrate Hydrates

Semi-clathrate hydrates are a class of inclusion compounds where organic ionic co-hosts (e.g., quaternary ammonium salts) stabilize the hydrate cages at temperatures and pressures approaching ambient conditions. These co-hosts are integrated into the clathrate hydrogen-bonding framework, taking the place of one or more water molecules. The broader stability range of these mixed clathrates opens up the possibility of storing hydrogen at lower pressures and higher temperatures for use in fuel cell vehicles. To date, however, there has been little molecular level studies of gas storage in these inclusion compounds. We investigate the stability of semi-clathrate hydrates of tetrabutyl-ammonium bromide (TBAB) and water with methane or hydrogen guests using molecular dynamics simulations. Simulations were performed at varying temperatures and pressures for guest-TBAB ratios of 0, 0.5, 1, 1.5 and 2. Our studies show that while the unit cell volume decreases with the decreasing guest occupancy, the cage shape is preserved. Free energy calculations of guest deletion confirm the preferential stability at higher guest/TBAB ratios and suggest that the conventional van der Waals-Platteeuw model can be used to explain the phase behavior of these inclusion compounds.