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 cetylpyri dinium salicylate, cetylpyridinium 4-aminosalicylate, cetylpyridinium 5-ami nosalicylate, and cetylpyridinium chloride has been investigated at silica/ water interfaces using adsorption isotherm determinations and electrophoret ic and surface tension measurements. Earlier results are confirmed which sh owed that replacing the chloride ion by the salicylate ion results in a B-f old increase in the plateau value of the surfactant ion. It is suggested th at this is the consequence of a close packing of the cationic surfactants w hen associated to the salicylate ion at either the solid/water or air/water interface. The results are compatible with the formation of a surfactant m onolayer with the headgroups facing the solid surface. The binding constant s of the organic anions to the cationic headgroups have been evaluated usin g a Langmuir type isotherm and were shown to be larger for adsorbed aggrega tes as compared to free micelles by a factor of about 3. Although the bindi ng 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 s alts. It is suggested that the unusually large adsorption of these surfacta nts 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 inte rfaces do not contradict these conclusions. The case of 2,4,6-trichlorophen ol either as a neutral molecule or as an ionized counterion species in asso ciation with the cetylpyridinium ion was also investigated in solution and at the silica/water interface.