Effect of pH on the coadsorption of weak acids to silica/ water and weak bases to alumina/water interfaces as induced by ionic surfactants

Surfactants which adsorb at solid/water interfaces may induce the coadsorpt ion of various charged or uncharged molecules. In the present investigation , weak acids (1-naphthoic, Caminosalicylate, 8-nitrophenol, 2,4,6-trichloro phenol) are coadsorbed on silica using cetylpyridinium chloride and weak ba ses (4-toluidine, 1-naphthylamine, 1,10-phenanthroline) are coadsorbed on a lumina with sodium octylbenzenesulfonate. The effect of pH changes on the c oadsorption effect for both types of systems has been studied in a pH range between 3 and 9. The surfactant concentration was kept constant correspond ing to the isotherm region of a low surface coverage. Solute partition coef ficients Pads (for. the undissociated forms) and binding constants Kg (for the completely dissociated forms) have been determined. It is shown that in all cases the solute coadsorption goes through a maximum at a pH value equ al to the pK of the acids or of the bases. It is argued that the initial pH changes induces the dissociation of the solutes and a favorable interactio n with the adsorbed surfactant ions. However as the pH further changes and the silica and alumina surfaces become increasingly ionized, these ionic si tes are responsible of a repulsion effect on the dissociated solute species , hence the coadsorption maximum. A simple thermodynamic model which depend s only upon chemical equilibria and the corresponding constants P-ads, K-B, and pK fits well the experimental data provided Kg is assumed to be a func tion of the solution proton activity.