Effect of rough nanoscale slits on methane adsorption behaviour in coal: insights from molecular simulations

Rough nanoscale slits are prevalent in coal matrices and significantly influence methane adsorption. However, due to the nanoscale dimensions of these structures, quantifying the effect of slit roughness experimentally remains challenging. In this study, grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were employed to investigate methane adsorption within rough nanoscale slits. The adsorption configurations, density distributions, and adsorption isotherms were systematically examined under various slit widths and roughness conditions. Based on the Langmuir–Freundlich model, an empirical equation was developed to characterise the maximum adsorption capacity. The results demonstrate that methane adsorption is jointly governed by slit width and roughness. Generally, increasing roughness enhances methane adsorption, but the underlying mechanisms exhibit a gradual transition around 2 nm. For narrow slits (<2 nm), the increased adsorption capacity is primarily driven by the overlap of potential energy fields, whereas for wider slits (≥2 nm), it is mainly attributed to the expansion of the adsorbed-phase volume. The potential energy distribution was further analysed to elucidate the adsorption stability of methane. These results offer mechanistic insights into methane adsorption within rough nanoslits, providing a fundamental basis for understanding interfacial adsorption under nanoconfinement.