A generalized scale of free energy of excess adsorption of solute and absolute composition of the interfacial phase

Moles of a surfactant (Gamma(2)(1)) absorbed per unit area of the solid-liquid interface estimated analytically from the difference of the solute molality in the bulk phase before and after adsorption have been quantitatively related to the absolute compositions Deltan(1) and Deltan(2) of the solvent and solute forming the inhomogeneous surface phase in contact with the bulk phase of homogeneous composition. By use of isopiestic experiments, negative values of Gamma(2)(1) for the adsorption of inorganic salts onto a solid-liquid interface have been calculated in the same manner. From the linear plot of Gamma(2)(1) versus the ratio of the bulk mole fractions of the solute and solvent, values of Deltan(1) and Deltan(2) have been evaluated under a limited range of concentrations. For the adsorption of the surfactant and the inorganic salt respectively onto the fluid interface, Gamma(2)(1) values have been evaluated from the surface tension concentration data using the Gibbs adsorption equation. Gamma(2)(1) based on the arbitrary placement of the Gibbs dividing plane near the fluid interface is quantitatively related to the composition of the inhomogeneous surface phase. Also, the Gibbs equation for multicomponent solutions has been appropriately expressed in terms of a suitably derived coefficient m. Integrating the Gibbs adsorption equation for a multicomponent system, the standard free energy change, DeltaGdegrees, per unit of surface area as a result of the maximum adsorption Gamma(2)(m) of the surfactant at fluid interfaces due to the change of the activity alpha(2) of the surfactant in the bulk from zero to unity have been calculated. A similar procedure has been followed for the calculation of DeltaGdegrees for the surfactant adsorption at solid-liquid interfaces using thermodynamically derived equations. DeltaGdegrees values for surfactant adsorption for all such systems are found to be negative. General expressions of DeltaGdegrees for negative adsorption of the salt on fluid and solid-liquid interfaces respectively have also been derived on thermodynamic grounds. DeltaGdegrees for all such systems are positive due to the excess spontaneous hydration of the interfacial phase in the presence of inorganic salt. Negative and positive values of DeltaGdegrees for excess surfactant and salt adsorption respectively have been discussed in light of a generalized scale of free energy of adsorption.