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.

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The influence of the counterions on the adsorption properties of cetylpyridinium salicylate, cetylpyridinium 4-aminosalicylate, cetylpyridinium 5-aminosalicylate, and cetylpyridinium chloride has been investigated at silica/water interfaces using adsorption isotherm determinations and electrophoretic and surface tension measurements. Earlier results are confirmed which showed that replacing the chloride ion by the salicylate ion results in a 6-fold increase in the plateau value of the surfactant ion. It is suggested that this is the consequence of a close packing of the cationic surfactants when associated to the salicylate ion at either the solid/water or air/water interface. The results are compatible with the formation of a surfactant monolayer with the headgroups facing the solid surface. The binding constants of the organic anions to the cationic headgroups have been evaluated using a Langmuir type isotherm and were shown to be larger for adsorbed aggregates as compared to free micelles by a factor of about 3. Although the binding constant is larger by a factor of 10 for the salicylate ion as compared to the aminosalicylate derivatives, the surfactant adsorption isotherms and the electrophoretic mobilities are similar for the three cetylpyridinium salts. It is suggested that the unusually large adsorption of these surfactants at the silica/water interface is related to the surface stacking of the aromatic counterions which favors the close packing of the surfactant ions. Results obtained at the titanium dioxide/water and the alumina/water interfaces do not contradict these conclusions. The case of 2,4,6-trichlorophenol either as a neutral molecule or as an ionized counterion species in association with the cetylpyridinium ion was also investigated in solution and at the silica/water interface.