The Horvath–Kawazoe method revisited

A modified Horvath–Kawazoe (HK) pore size analysis method is presented in this paper. The new HK model is generalized so as to account for the effect of the temperature and the solid–fluid interaction potential strength on the adsorbed fluid density within a slit-shaped pore. Two representations of the local adsorbate density profile are considered in the modified HK model, an ‘unweighted’ version similar to the original HK method in which a uniform density profile is assumed; and a ‘weighted’ version in which a structured density profile is adopted. The pore filling correlations predicted by the two HK models are compared with density functional theory (DFT) results calculated using the same solid–fluid potential and potential parameters. It is found that the pore filling correlation of the unweighted HK model agrees surprisingly well with the DFT correlation for argon and nitrogen adsorption at 77 K. Interestingly, the weighted HK pore filling correlation is in poorer agreement with DFT results, even though the weighted HK model more realistically represents the local density profile than does the unweighted HK approach. As in the original HK method, the modified HK models do not describe the pore wall wetting that occurs prior to capillary condensation in mesopores. In this respect, they remain inferior to DFT or molecular simulation adsorption models. The modified HK method is, however, much more computationally efficient than either DFT or molecular simulation, and it can be conveniently implemented on a spreadsheet for interpretation of adsorbent PSDs in gas–solid systems for which DFT model isotherms are not available.