Molecular Dynamics Study On The Formation Mechanism And Control Of Gas Hydrate Under Confinement

Gas hydrate refers to the host and guest structure formed by gas molecules and water molecules.Combustible ice,that plays a vital role in solving global energy problems,is an ice-like clathrate hydrate formed by methane molecules and water molecules under high pressure and low temperature.However,the mining of combustible ice is difficult and expensive because it often lies in the deep sea.Therefore,the study of combustible ice,as one kind of gas hydrates,is of far-reaching significance.In the past few decades,new nanostructures have been discovered in nanoscale confined(gas)aqueous solutions,such as polygonal ice nanotubes,spiral ice nanotubes,or double-layered ice structures confined in high pressure slits.At present,a large number of scholars all over the world have found that new low-dimensional methane hydrates confined in nanoscale can form even under room temperature and pressure by computer simulation.In 2014,Chinese scholars even found the existence of various forms of quasi-one-dimensional hydrogen hydrate through molecular dynamic simulation,indicating that the potential of gas hydrate in clean energy has become even greater.In addition,gas separation,purification and enrichment can also be realized by the different priorities of different gas molecules in the mixed gas aqueous solution to form quasi-one-dimensional gas hydrate.We systematically investigate the effects of an axial electric field on the formation and decomposition of quasi-one-dimensional nitrogen gas hydrates within a single-walled carbon nanotube(SWNT)by using molecular dynamics(MD)simulations.We find that the nitrogen hydrate in the SWNT undergoes a series of structure phase transitions with increasing electric field.Corresponding to the structure transition,the nitrogen gas releases from the carbon nanotube in the electric field range of 1 V/nm to 2 V/nm.However,nitrogen molecules are trapped as guest molecules,forming a molecule wire,in the ice nanotube when the electric field is less than 1 V/nm or larger than 2 V/nm.Our simulations indicate that the nanotube is an excellent tiny gas tank that can be used to trap gas molecules and control their release triggered sensitively by electric signals.The key to this phenomenon is the change in orientations of water dipoles induced by the electric field,which leads to the structural change in the hydrogen-bonding network and the change in the diffusion coefficient of the water molecules.Our findings here may help understanding the mechanism of the electrorelease of gas from hydrates confined in the nanoscale space.