Grand canonical Monte Carlo simulation study of cyclohexane, oxane, 1,4-dioxane, and 1,3,5-trioxane confined in carbon slit pore

Abstract

A grand canonical Monte Carlo (GCMC) simulation has been conducted to investigate the adsorption of cyclohexane, oxane, 1,4-dioxane, and 1,3,5-trioxane into a carbon slit pores with widths of 0.8, 1.0, and 1.2nm at 298K. Particular emphasis has been paid to the effect of molecular size and shape on the intermolecular structure of the adsorbate confined in the carbon slit pore. Simulations showed that cyclohexane and oxane form a disordered single layer structure at saturation, while 1,4-dioxane and 1,3,5-trioxane form a bilayer structure in the 0.8nm wide pore. Among the four adsorbates, 1,4-dioxane forms the most regular packing structure in the 0.8nm wide pore at saturation; most 1,4-dioxane molecules are parallel to the pore surface and form a square lattice packing structure, which may be associated with the square molecular shape. In addition, 1,4-dioxane molecules are oriented so that the OO axes in the molecule are alternately arrayed vertically and horizontally to avoid Coulomb repulsion between intermolecular oxygen atoms with negative charges. In addition, this packing structure is formed via two transitions (a gas to a liquid-like state and a liquid-like state to a solid-like state), as determined from analysis of the isothermal compressibility and 2D occupancy maps. These transitions are manifested as a step and a sub-step in the adsorption isotherm. On the other hand, triangular 1,3,5-trioxane forms a hexagonal lattice packing structure in the 0.8nm wide pore at saturation; however, no definite sub-step appeared in the adsorption isotherm. The pore size dependence of the packing structures of these molecules is also discussed.