Adsolubilization of 4-nitrophenol at a kaolinite/water interface as a function of pH and surfactant surface coverage

The incorporation of a weak acid, 4-nitrophenol, into a kaolinite/water interface has been achieved by the addition of an adsorbing cationic surfactant, cetylpyridinium chloride. This so-called adsolubilization phenomenon has been studied as a function of surfactant concentration and of pH. In the absence of the surfactant 4-nitrophenol is hardly adsorbed onto the naked kaolinite particles in the whole pH range studied, from 3 to 10. At constant pH, the adsolubilization is at a maximum at a surfactant concentration corresponding to the onset of the saturation plateau. At still higher concentrations, the solute is. distributed between the surface and free aggregates. The influence of the pH on adsolubilization is very much dependent upon surfactant surface coverage. At low surfactant concentration the behavior of 4-nitrophenol is very similar to that observed with the same surfactant at a silica/water interface: the adsolubilization decreases with increasing pH as the result of the repulsive interaction between the deprotonated solute molecule and the negatively charged sites on kaolinite particles. At higher surfactant concentrations increasing the pH induces increased adsolubilization as a consequence of the favorable interaction which takes place between the deprotonated weak acid and the positively charged surface surfactant aggregates. Hence, depending upon the extent of surface coverage, increasing the pH may retain 4-nitrophenol at the kaolinite/water interface or reject it into the bulk micellar solution. (C) 2003 Elsevier Science (USA). All rights reserved.