A generalized scale for free energy of adsorption from Gibbs adsorption equation

Abstract

From analytical and isopiestic vapor pressure techniques, a substantial amount of experimental data on the extent of adsorption Γ 2 1 of solute (surfactants, fatty acids, inorganic salts) at liquid as well as solid surfaces suspended in aqueous media at different bulk solute activities ( a 2) are obtained for calculation of free energy of adsorption. Values of Γ 2 1 change with increase of solute activity. Using all these experimental values in the integrated forms of the Gibbs adsorption equations for multicomponent solutions, the standard free energy change Δ G o in kJ m −2 unit have been evaluated for different solid–liquid systems for the change of bulk solute activity from zero to unity in the rational scale. For surfactants, Δ G o is negative since Γ 2 1 is positive whereas for inorganic salts Δ G o is positive since Γ 2 1 is negative. All values of Δ G o, positive or negative refer to unit solute activity in this generalized scale of free energy of adsorption and hence the order of their values expresses the relative measure of affinities of solutes and solvents for different types of solid surfaces. Also using the Gibbs adsorption equation in an alternative form, the excess free energies of hydration of different solid surfaces in a different thermodynamic scale may be evaluated.