Molecular dynamics simulation on the density distribution and multilayer adsorption of methane in nanopores

The adsorption behavior of methane (CH4) in nanopores affects its spatial density distribution, which is essential for the shale gas extraction. While the average density of CH4 in nanopores has been commonly utilized in practice, the density distribution and the mechanisms of multilayer adsorption remain unclear. In this study, molecular dynamics simulations were conducted to investigate the formation of adsorption layers in nanopores. The effects of pressure, pressure gradient, pore width, and temperature on adsorption were examined. As CH4 pressure increases from 1 to 80 MPa, the adsorption layer transitions from one layer to three, resulting in multilayer adsorption. Although the increased pressure enhances the interactions between CH4 molecules, the force exerted by the pore walls on the CH4 molecules remains unchanged. When the repulsive force from the preceding adsorption layer exceeds the attractive force from the pore walls, a minimum methane density is reached, leading to the formation of a new adsorption layer. Following the application of the methane pressure gradient, it was observed that the carbon (C) atoms are displaced from their adsorption sites to regions of higher potential energy, reducing the peak density value. Additionally, the pore width has a minimal effect on the density distribution, as it does not alter the force exerted on the C atoms. Furthermore, temperature can increase the thermal motion of CH4 molecules, resulting in a more uniform spatial density distribution. Finally, a model was proposed to predict the spatial density distribution of CH4 in nanopores, accounting for multilayer adsorption.