Interaction between adsorption and condensation processes in a pore - Relation between condensation pressure and pore width

The pressures at which capillary condensation takes place and at which the condensate vaporizes depend on the adsorption properties at the pore wails surface as well as on the width of the pore. An equation that expresses quantitatively these relations is used for the calculation of the capillary condensation of nitrogen at 77.3 K in porous adsorbents assuming that adsorption at the pore walls obeys the BET equation. The condensation pressure is plotted against the pore width for various values of the constant c of the BET equation for the two branches of the hysteresis loop. The condensation pressures obtained are higher than those predicted by the Kelvin equation with zero contact angle. The width of the hysteresis loop, and the deviation from the value given by the Kelvin equation with zero contact angle, increase with decrease of the pore width and decrease of the parameter c of the BET equation.