Microscopic configurations of methanol molecules in graphitic slit micropores: a computer simulation study

We report a detailed computer simulation study of methanol adsorption in graphitic slit-like micropores to investigate the effects of temperature and pore size on the adsorptive capacity and the configurations of methanol molecules in a confined space. Simulation results show that in the temperature range studied (273-422 K) the amount adsorbed increases gradually with pressure in 0.65 and 0.8 nm pores (where only one molecular layer can be accommodated), while for pores having widths greater than 1.0 nm the adsorption isotherms exhibit a sharp jump at low temperatures which becomes gradual as temperature is increased above the critical pore temperature, which increases with pore width. For a given pore size, the pressure at which a large uptake of adsorption occurs, increases and the excess amount adsorbed, decreases with temperature. The interaction between adsorbate molecules and a pore was studied via the solvation pressure, which exhibits oscillations with pore size. The peaks of this oscillation correspond to pores that have an integer number of layers of methanol molecules. At low loadings snapshots showed methanol molecules in isolated clusters of four or five molecules which maximise the hydrogen bonding within each cluster, in the same way as they do on an open surface. At high loadings, the isolated cluster configuration changes to molecular chains in small pores (0.65 and 0.8 nm), which become more distorted by inter-layer interactions in larger pores. The positions of the first peaks of the O-O and O-H radial distributions for the confined methanol are the same as those for bulk liquid for all pore sizes. However, for confined methanol in pores with an integer number of molecular layers, the amplitude of the first peaks of the O-O and O-H radial distributions are higher than for the bulk liquid, and the positions of the second peaks are slightly shifted to the left. In the incommensurate pores the amplitude of the first peaks and the positions of the second peaks of the O-O and O-H radial distributions are similar to those of liquid methanol. Our simulation results agree well with the experimental results of Ohkubo et al.