Computational Study of the Effect of Functional Groups on Water Adsorption in Mesoporous Carbons: Implications for Gas Adsorption

The adsorption and diffusion of water in realistic CMK-1, CMK-3, and CMK-5 carbon models at 300 K has been studied via grand canonical Monte Carlo and molecular dynamics simulations. The presence of −COOH and −OH functional groups has been found to be crucial to describe the adsorption process, while the models without functional groups are unable to capture the host–guest interactions. Functional groups were attached in cylindrical shells on the outer walls of CMK models in 2 and 4 cylindrical shells to study the effect of their concentration and location on water adsorption. The adsorption isotherm starts at a lower chemical potential, with water adsorbed near functional groups forming small clusters. On increasing chemical potential the water cluster grows and merges to form bigger clusters and completely fills the pores. We also analyzed the isosteric heat, radial distribution functions, hydrogen bond, and cluster size of water molecules. It indicates that the adsorption occurs due to the formation and growth of water clusters. For CMK-1, CMK-3, and CMK-5 models with functional groups, the pore filling happens at lower chemical potential, when compared to the models without functional groups, owing to the presence of active sites which favors the nucleation of water molecules. The effect of functional groups is also remarkable in the diffusion of water inside the pores of CMK-1, CMK-3, and CMK-5 models, lowering the mobility of the adsorbed molecules. The agreement between the results of the models with functional groups and the experimental observations makes the presence of these groups necessary to study the adsorption and diffusion of water in CMK-1, CMK-3, and CMK-5 carbon models.