Binding of aromatic anions to cetylpyridinium aggregates either adsorbed at silica/water, alumina/water, titanium dioxide/water interfaces or in solution

The influence of the counterions on the adsorption proper ties 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 B-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.