OS6A-5:EXPLORING THE EFFECT OF MINERAL SURFACES ON GAS HYDRATE FORMATION

Shui LIANG
Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, CAS, CHINA

    Mineral surfaces have been recognized to play an important role in the formation and crystal growth of gas hydrates, although the relevant molecular mechanisms remain poorly understood. Here we present our recent molecule dynamics simulation studies of the nucleation and crystal growth processes of gas hydrates in the presence of model silica surfaces. The crystal growth of gas hydrates shows clear preference to the center of the pore spaces. Under appropriate conditions, the hydroxylated silica surfaces can provide favorable location for gas bubbles and serve as a source of gas molecules, which can apparently promote hydrate growth near the solid surfaces. Gas hydrates nucleation also show varied behavior in confined silica nanopores, where the nucleation can initiate from both the silica surfaces and the center of the pore spaces. The local gas concentration of the solution appears to be a critical factor in determining the observed the nucleation and crystal growth preferences. The nucleation can be described as a two step process with initial formation of amorphous hydrate like solids, as that observed in bulk systems. Intermediate half-cage structures were observed at the silica surfaces in the nucleation and crystal growth processes, apparently helping minimize the free energy penalty due to the structural mismatch of the gas hydrate and silica phases. In comparison with crystalline hydrate phases, the amorphous solids apparently better able to connect with the silica surface due to their local structural flexibility. This observation identify potential mechanisms that nucleation might be induced by solids that are lack of structural match with the formed phases. The observed nucleation and crystal growth behavior is apparently consistent with the varied behavior suggested by experimental studies, where the initiation of hydrate formation is reported both from the solid surfaces and pore center. The formation of gas hydrates in porous media and in the presence of other solid surfaces can clearly be a complicated process, which can be further influenced by many factors such as mass transfer and heat transfer. The observations made here reinforce the important roles that a solid surface might play in the nucleation and crystal growth processes of the gas hydrates. Apparently further detailed and more quantitative investigations of the nucleation behavior of gas hydrates in the presence of various solid surfaces are needed for a thorough understanding of the surface impact on the gas hydrate formation processes.