Energetics of water adsorption and immersion on carbons

Abstract

The equations of adsorption of water proposed by Dubinin and Serpinsky (DS), Barton et al. and D'Arcy and Watt are being analysed; and the isosteric enthalpy of adsorption and the enthalpy of immersion equations are developed where possible. It is proven that the derivation of the adsorption enthalpy formula from DS equation proposed by Kraehenbuehl, Stoeckli and co-workers is incorrect; the reason is that it neglects the temperature dependence of the maximum adsorption. If one takes this into account, it leads to some decrease in the enthalpy of adsorption with coverage and not-as it was suggested by those authors-to the constant enthalpy of adsorption. The new adsorption equation is being proposed which takes into account the energetic heterogeneity of carbon adsorption centers and postulates the physical adsorption of water. All analysed equations of adsorption can simultaneously generate the observed experimentally water isotherm shapes as well as the heat plots obtained from calorimetry. Moreover, for D'Arcy and Watt and for bimodal DS equations as well the shape of adsorption enthalpy changes with the number of adsorption centers. On the contrary, DS and Barton's equations generate only one type-convex to adsorption axis-of enthalpy of adsorption plot.