Molecular simulation study of water-methanol mixtures in activated carbon pores

We report a theoretical study of the adsorption behavior of water-methanol mixtures in slit activated carbon micropores. The adsorption isotherms are obtained for a pore of width 2 nm at a temperature of 298 K from grand cano nical ensemble Monte Carlo simulations. The water molecules are modeled usi ng the four point transferable intermolecular potential functions (TIP4P) a nd methanol by the optimized potentials for liquid simulations (OPLS). Carb oxyl (COOH) groups are used as active sites on a structured carbon surface. The effect of the relative contributions from dispersion and hydrogen bond ing interactions of adsorbates, and of the chemical activation of adsorbent s on adsorption behavior is investigated. The adsorption of the mixture com ponents in activated carbon pores occurs by continuous filling, without the sharp capillary condensation observed in graphite pores. Water is preferen tially adsorbed over methanol in activated carbon pores for a wide range of pressures, except at lower pressures. The hydrophilic nature of activated carbon pores results in the complexation of both water and methanol molecul es with the active sites on the surfaces, leading to bulklike water behavio r over the entire pore width. Solvation forces are also calculated as a fun ction of pore size. The negative values found for the solvation force for a ll pore sizes reflect the hydrophilic interactions of the mixtures with the activated carbon surfaces. (C) 2000 American Institute of Physics. [S0021- 9606(00)51339-7].