Surface interactions on silica particles between a cationic surfactant and sodium salicylate

Cetylpyridinium chloride, a cationic surfactant, adsorbs at a silica/water interface, forming double-layer structures which coadsorb salicylate ions below the equilibrium critical micelle concentration (cmc) as the result of strong interactions between the surfactant head-group and the aromatic anion. Above the cmc the salicylate ions are distributed between adsorbed surfactant aggregates, free micelles, and water. Using a simple pseudophase formalism, ion partition coefficients are calculated in dilute solutions as ratios of bound to free salicylate concentrations for the various adsorbed or free surfactant structures. It is shown that the salicylate coadsorption ion partition coefficient decreases as the pH increases above the silica isoelectric point. The partition coefficient is larger for the adsorbed aggregates than for the free micelles at low (acidic) pH but is equal to the latter values at higher (alkaline) pH values. On the basis of a previous investigation on neutral solutes in the same silica/surfactant system, the following interpretation of these observations is suggested. At low pH values, salicylate ions form complexes with the cationic head-groups of both surfactant layers: the inner layer with the head-groups facing the silica surface and the outer-layer facing the bulk of the solution. At higher pH values, salicylate ions are repelled from the inner layer by the high negative surface charge density on the silica particles. As a consequence only the surfactant outer layer complexes the aromatic anions. This configuration is closer to that of free micelles, hence, the decrease of the salicylate partition coefficients and the similar partion coefficient values for the aromatic anion for both types of adsorbed and free surfactant structures. Additional evidence is provided by the behavior of sodium naphthoate and 2-4-6-trichlorophenol in the same chemical systems.