Molecular insights into CO2 hydrate formation in the presence of hydrophilic and hydrophobic solid surfaces

Microsecond molecular simulations have been performed on CO2 hydrate formation in the slit-nanopores of graphite and hydroxylated-silica surfaces. The simulation results show that different hydrophilic/hydrophobic properties of graphite and silica surfaces exert substantially different effects on CO2 hydrate formation. It is found that hydrate nucleation requires high aqueous CO2 concentration, and the solid surface affects hydrate nucleation mainly by changing the aqueous CO2 concentration in the systems. The hydrophobic graphite surfaces could adsorb CO2 molecules so strongly that the surfaces are almost covered by CO2 molecules, thus, the aqueous CO2 concentration is lowered. On the contrary, a partially cylindrical CO2 nanobubble is adsorbed at the hydrophilic silica surface and results in a high aqueous CO2 concentration. In the slit-nanopores of graphite and silica surfaces, hydrate nucleation starts from the bulk region and then grows towards the surfaces. CO2 hydrate solids interact with the silica surfaces mainly via semi-cages, which are constituted by the strong hydrogen bonds formed between silanols and interfacial water. At the end of the simulation, the hydrophobic graphite surfaces are still covered by the strongly adsorbed CO2 molecules, preventing the formation of ordered interfacial water on the surfaces, which is previously reported to play a critical role in promoting hydrate formation. These molecular insights into the effects of solid surfaces on CO2 hydrate formation are beneficial to CO2 hydrate-based technologies, such as geological CO2 sequestration.