The adsorption of water by active carbons, in relation to their chemical and structural properties

An interesting letter has recently been published in this total micropore volume of the solid, the thermal expansion Journal by Lodewyckx and Vansant [1], introducing the coefficient of the adsorptive and its molar volume in the idea of an affinity coefficient for water with respect to liquid state. G(1 1 1/n) is the tabulated ‘Gamma’ function, benzene, the usual reference in Dubinin’s theory. This which takes values between 0.89 and 0.92 when n varies concept is valid for carbons with relatively low oxygen from 1.5 to 5. This means that the enthalpy of immersion contents, where the water adsorption isotherm is of type V depends essentially on the characteristic energy E 5 bE . o and corresponds to a Dubinin-Astakhov equation, as Since active carbons also possess an external (nonshown earlier [2,3]. microporous) surface S , the experimental enthalpy of e We wish to show that Lodewyckx’s idea can be included immersion, Dh (J /g) is i exp in a more general approach based essentially on the Dh (J /g) 5 Dh (J /g) 1 h S (3) i exp i mi i e comparison of the enthalpies of immersion of carbons into water and benzene and taking into account the chemistry where h , a negative quantity, represents the wetting of the i of the surface through an excess enthalpy of immersion. surface. For benzene and water at 293 K, 2 h corresponds i Our approach follows two recent studies dealing with the 2 to 0.114 and 0.030 J /m [3–5] and the last term of Eq. (3) interaction of water [4] and of methanol and ethanol [5] is only a fraction of the total enthalpy of immersion. with active carbons containing variable amounts of oxygen From Eqs. (2) and (3), it follows that the enthalpy of and basic groups. The starting point is the DA equation immersion of a given adsorptive can be calculated from the [3,6] parameters of the DA isotherm. A good agreement has