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

From analytical and isopiestic vapor pressure techniques, a substantial amo unt of experimental data on the extent of adsorption (Gamma 1/2) of solute (surfactants, fatty acids, inorganic salts) at liquid as well as solid surf aces suspended in aqueous media at different bulk solute activities (a(2) a re obtained for calculation of free energy of adsorption. Values of ((Gamma 1/2) change with increase of solute activity. Using all these experimental values in the integrated forms of the Gibbs adsorption equations for multi component solutions, the standard free energy change Delta G degrees in kJ m(-2) unit have been evaluated for different solid-liquid systems for the c hange of bulk solute activity from zero to unity in the rational scale. For surfactants, Delta G degrees is negative since (Gamma 1/2) is positive whe reas for inorganic salts Delta G degrees is positive since (Gamma 1/2) is n egative. All values of Delta G degrees, 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 affinitie s of solutes and solvents for different types of solid surfaces. Also using the Gibbs adsorption equation in an alternative form, the excess free ener gies of hydration of different solid surfaces in a different thermodynamic scale may be evaluated. (C) 1999 Elsevier Science B.V. All rights reserved.