What roles do interlayer cations (K+) and salt ions (Na+ and Cl−) play in methane hydrate formation in illite nanopore?

Interlayer cation (K+) and salt ions (Na+ and Cl−) are commonly found in hydrate-bearing sediments and inevitably affect hydrate formation. Herein, microsecond molecular dynamics simulations are executed to elucidate the influence of the location of interlayer cation and salt ions on CH4 hydrates formation from the homogeneous solution and gas/water two-phase system in the illite nanopores. Simulation results show that interlayer cations and salt ions exhibit various influences on hydrate formation via ion hydration and surface adsorption, i.e. the interlayer cations promote hydrate formation, whereas salt ions exhibit an inhibitory effect in the illite nanopore. In the homogeneous solutions, interlayer cations rapidly adsorb on the illite surfaces and then repel CH4 molecule, thus, increasing the CH4 concentration and promoting hydrate formation therein. In the two-phase systems, the adsorption of interlayer cations on the illite surfaces prevents the nanobubbles from contacting the surfaces, thus, promoting nanobubbles decomposition and favoring hydrate formation; the hydration of ions in the solution hinders nanobubbles decomposition and inhibits hydrate formation. These molecular insights gained regarding the influence of the location of interlayer cations and salt ions on hydrate formation expand the comprehension of the formation mechanisms of natural gas hydrate resources in marine sediments.