Specific interactions between salicylate derivatives and cetylpyridinium chloride both at the silica/water interface and in solution - A calorimetric investigation

The enthalpy of solution of sodium salicylate and sodium 4-aminosalicylate in aqueous solution in the presence of cetylpyridinium chloride (CPC) either adsorbed at solid/water interfaces or in pure micellar solutions has been investigated at 25 degrees C using a calorimetric method. The solid dispersions used were non-porous hydrophilic mineral oxides: silica, alumina and titanium dioxide above their isoelectric point. It is shown that the enthalpy of binding of sodium salicylate and of sodium 4-aminosalicylate to cationic surfactants is systematically more exothermic when the organic anions interact with the adsorbed CPC aggregates at the solid/water interfaces than with CPC micelles in solution. Furthermore, the enthalpy of counterion binding of the anionic ions to the head-group of the cationic surfactant adsorbed on silica was shown to decrease (become less exothermic) for both organic salts as the pH increases above the isoelectric point. These observations an discussed in relation to the difference of counterion binding depending upon the structure of surfactant aggregates which are favored in bulk solutions or at solid/water interfaces. A model where the degree of counterion binding of the inner surfactant layer is pH dependent while the outer layer is not explains qualitatively the effects observed in the present investigation. It is also shown that the specific viscoelastic properties induced by sodium salicylate in cationic surfactant aqueous solutions do not seem to play a significant role at a solid/water interface. (C) 1998 Elsevier Science B.V. All rights reserved.