Standard Free Energy of Adsorption at Liquid Interfaces

Abstract

Based on the Gibbs adsorption equation appropriate relation has been determined for the calculation of the change in free energy (ΔG_b ^o) for the positive adsorption of one mole of surfactant or negative adsorption of one mole of inorganic salt at air-liquid or liquid-liquid interface when the bulk concentration of the surfactant or salt altered fron zero to unit mole fraction hypothetically. For this purpose previously measured boundary tension (γ) data for various aqueous concentrations of organic monobasic and dibasic acids and their salts, SDS, DTAB and CPC1 respectively are used to calculate ΔG_b ^o for excess positive adsorption of solute using the Gibbs adsorption equation. Boundary tension data for various aqueous concentrations of inorganic salts like NaCl, LiCl, CaCl₂, A1C1₃, Na₂SO₄ and Al₂(SO₄)₃ are used for calculation of ΔG_b ^o for excess negative adsorption of salts at the interface. Values of ΔG_b ^o, however, for different systems are not comparable since they do not include the effect of maximum packing of adsorbed surfactant or salt and water molecules at the interface It is, therefore, proposed that standard free energy (ΔG^o) of adsorption of solutes for all cases may be taken to be equal to product, ΔG_b ^o. Γ _R,^m where Γ _R,^m stands for the maximum amount of solute in moles transferred as positive or negative excess per square centimeter of liquid surface when the solute concentration in the bulk is altered from zero to unity. Values of ΔG° expressed in calories per square centimeter for positive adsorption of different surfactants and negative adsorption of inorganic salts at air-liquid or liquid -liquid interfaces of various types are comparable in this proposed universal scale of thermodynamics. Air-water, benzene-water, petroleum ether-water, heptane-water, nitrobenzene-water and toluene-water interfaces have been used in all these studies.