Molecular Dynamics Simulation Study on the Nucleation and Growth of CO ₂ Hydrate in Layered Double Hydroxide

Systematic molecular dynamics simulations have been conducted to investigate the nucleation and growth of CO2 hydrate from CO2/water two-phase systems in the slit-nanopore of layered double hydroxide (LDH) with the chemical formula of Mg5Al(OH)12Cl. The simulation results show that the positively charged surfaces of LDH sheets adsorb Cl– ions and water molecules so strongly that almost all of the CO2 molecules are repelled from the interfacial region. Compared with the reported CO2 hydrate formation in bulk solution without any surface, the presence of LDH seems to not alter the formation mechanism of CO2 hydrate, including nucleation mechanism, cage-type preference, and hydrate crystallinity. High aqueous CO2 concentration is required for CO2 hydrate nucleation, and only one of the six simulation runs develops a local aqueous region with high gas density, and hydrate nucleation occurs. In the slit-nanopore of LDH, hydrate nucleation initiates from the bulk regions, rather than near the surface of LDH, and then, the hydrate grows toward the surfaces and the CO2 nanobubble. At the later simulation stage, CO2 hydrate growth significantly slows down due to the rapid depletion of free water and the increased mass transfer barrier for CO2 to diffuse from the nanobubble into water, as the nanobubble is gradually covered by the growing hydrate solids and separated from water. Several Cl– ions and water molecules near the surface of LDH could form interfacial cages. Most direct contacts of the CO2 hydrate solids with the surface of LDH are separated by the strongly adsorbed interfacial water layer, with only several interfacial cages directly interacting with the surface. These molecular insights help to deepen our understanding of sediment surface effects on CO2 hydrate formation and give some implications on CO2 sequestration via forming CO2 hydrate in marine sediments.