Molecular simulation of collection of methane isotherms in carbon material using all-atom and united atom models

Abstract

In this study the performances of all-atom (AA) and united atom (UA) models (fitted and unfitted) of the methane molecule are evaluated for the description of adsorption isotherms on graphite surfaces and in a collection of graphitic slit pores. We simulated collections of isotherms using the grand canonical Monte Carlo method with an all-atom model and adjusted united atom models in graphene layers made up of discrete atoms of carbon. The collections of isotherms are used to determine the pore size distribution (PSD) of four activated carbon samples. We also investigated the sensitivity of the system to the cutoff and solid–fluid standard parameterization. It was found that the simulated AA model isotherm shape on the graphite surface is much more similar to the experimental data than the UA model isotherm. The cutoff had little influence on isotherm and different solid–fluid standard parameterizations change the PSD. We also found that despite presenting similar fitting with the experimental isotherms, the models presented distinct PSDs. The unfitted united atom model (UA1) suggested less plausible PSDs, while the all-atom (AA) and the fitted united atom model (UA2) model provided apparently more realistic estimates of the internal structure of microporous carbons.