Impact of an adsorbed phase nonideality in the calculation of the filling pressure of carbon slit-like micropores

The influence of the model of an adsorbed phase on the pore-filling pressure and the pore-size distribution (PSD) curve is analysed for a carbon adsorbent possessing the slit-like micropores. Besides the Henry equation, applied previously in the original Horvath and Kawazoe (HK) approach, and the Langmuir one, studied in the improved HK method of Cheng and Yang, in this study we propose the application of the Dubinin-Astakhov (DA) adsorption equation. The reason is that this equation is more reliable for the description of the mechanism of adsorption in carbon micropores. The correction proposed by us leads to the change in the plot of the micropore filling pressure curve. It is shown that the type of the equation describing the behaviour of the adsorbate slightly modifies the shape of the pore-size distribution curves. Moreover, the improvement assuming the Langmuir equation of state does not predict the larger effective micropore widths than approximately 1.2 nm, contrary to the proposed in this study correction assuming DA-type behaviour of the adsorbed phase. Finally, we explain why the condensation approximation method can be effectively used for the calculation of the PSD of activated carbons where the process of primary micropore filling predominates.