THERMODYNAMIC RELATIONS FOR THE NONCALORIMETRICAL DETERMINATION OF THE HEAT OF ADSORPTION IN MULTICOMPONENT SYSTEMS

The molar heat of adsorption, DELTAh(ad), of a single solute in ideal dilute solution can be determined indirectly from the temperature depe ndence of the adsorption isotherm by means of the so-called Clausius-C lapeyron equation. The present paper deals with the generalization of this equation to systems with several adsorbing components. The genera lization must allow for the nonstoichiometry of the adsorption process ; that is for the fact that, in general, the amounts of the adsorbing substances exchanged between the bulk phase and the surface phase are not coupled by constant ratios. Instead, these ratios change with the composition of the bulk solution. These changes can be represented by a path P in the space of the bulk solute concentrations. To make the v alue of the differential molar heat of adsorption unique, it must be r elated to this path P, which leads to the definition of a correspondin g heat DELTAh1,P.(ad). With use of this definition the Clausius-Clapey ron equation is generalized on the level of the individual components as well as on the level of the adsorption process as a whole. The latt er approach turns out to be much more fruitful for practical applicati ons. Several examples are discussed in detail.