Molecular insights into the kinetic
effect of seawater ions and
its coupling with the clay surface effect on CH4 hydrate
formation help to understand the complex formation process of natural
gas hydrate resources in marine sediments. Molecular dynamics simulations
are performed to explore CH4 hydrate formation from a homogeneous
salty solution containing seawater ions (Na+, K+, and Ca2+, in the form of salts NaCl, KCl, and CaCl2) in the kaolinite Janus double-nanopore and outside bulk
phase, by analyzing water order parameter, aqueous CH4 concentration,
flows of CH4 and seawater ions, and formation of hydrate-cage
clusters. The kaolinite double-nanopore consists of a hydrophilic
nanopore with gibbsite surfaces and a hydrophobic nanopore with siloxane
surfaces. Simulation results show that in fresh water, the gibbsite
surfaces of hydrophilic nanopore do not inhibit hydrate formation,
while the siloxane surfaces of hydrophobic nanopore are adverse to
hydrate formation due to formation of a large surface nanobubble.
By contrast, hydrate formation in saline water is affected by the
complex coupled effects of seawater ions and kaolinite surfaces. In
the hydrophilic nanopore, most seawater ions quickly adsorb to the
gibbsite surfaces and exert very weak effect on hydrate formation,
whereas in the hydrophobic nanopore, seawater ions show a certain
inhibitory effect on hydrate formation, as few ions adsorb to the
siloxane surfaces. Interestingly, more hydrate solids form in saline
water than in fresh water, as hydrate solids in the outside bulk solution
grow into the throat of the hydrophobic nanopore and decompose the
surface nanobubble. Additionally, transport of CH4 molecules
and seawater ions between the kaolinite double-nanopore and the outside
bulk solution further affects hydrate formation in the systems.