Thermodynamics of adsorption of surfactants at solid-liquid interface

Abstract

Adsorption of surfactants at interfaces is determined by a number of forces involving electrostatic attraction, covalent bonding, hydrogen bonding, hydrophobic bonding and solvation and desolvation of various species. The extent and type of forces involved vary depending on the adsorbate and the adsorbent and also the composition and other characteristics of the solvent and dissolved components in it. The influence of such forces on the adsorption behavior is reviewed here from a thermodynamics point of view. Experimental results from micro calorimetric study of adsorbed layers of different surfactant systems at solid-liquid interface are also presented. It was found that the entropy effect was more pronounced for the adsorption of anionic-nonionic surfactant mixtures than for the anionic surfactant alone. High surface activity of the nonionics and its hydrophobic interaction with adsorbed anionic surfactants are considered to be the main mechanisms for the marked entropy effect in mixture adsorption. Calorimetric investigations of the effect of the position of sulfonate and methyl groups on the aromatic ring of isomerically pure alkylxylene sulfonates on their adsorption at solid/liquid interface showed the position of the sulfonate with respect to the alkyl chain to be more critical than that of the methyl groups in determining the adsorption behavior.