Molecular dynamics simulation of methane in potassium montmorillonite clay hydrates

We present molecular dynamics simulation results for molecular structure, density profile and radial distribution functions of water, cation and methane in hydrated potassium montmorillonite clays. The systems we have studied consist of eight Otay Montmorillonite unit cells forming a clay patch of 21.12 × 18.28 Å2 with a thickness of 6.56 Å, eight potassium interlayer cations, with 0.5, 1.0 and 2.25 CH4 per unit cell, corresponding to 11.5, 11.0 and 9.75 H2O per unit cell. The calculations are conducted in constant (N, V, T) ensembles at T = 300 K and with a layer spacing of 18 Å. Visualization of simulation results in the 1.0 CH4 per unit cell system indicated that the methane molecule is nested inside the hexagonal cavities of the clay surface oxygen due to the fact that the clay surfaces include neutral regions that are hydrophobic. The calculated CH4–O coordination number of around 20 for 0.5 and 1.0 CH4 per unit cell systems is consistent with previous simulations. Our simulations show that water in potassium montmorillonite formed stretched hydrogen bonds with either the clay surface or other water molecules.