ADSORPTION OF POLYACRYLIC-ACID AND SODIUM DODECYLBENZENESULFONATE ON KAOLINITE

Individual adsorption and adsorption from mixtures of polyacrylic acid (PAA) and sodium dodecylbenzenesulfonate (SDBS) on kaolinite at low ionic strength have been found to decrease with increase in pH, The surfactant adsorption isotherms show a typical two-stage process, The results are analyzed by a model in which the first stage of adsorption results in monolayer coverage on the surface of kaolinite and the second stage appears to involve the formation of small surface aggregates of three or four surfactant molecules. The acid-base titration results of kaolinite satisfactorily correlate these adsorption results with the presence of positively protonated aluminol binding sites at the edge surfaces, An anion-exchange mechanism (electrostatic interactions) for negatively charged SDBS and carboxylate groups from PAA is primarily involved. Evidence of a ligand-exchange mechanism, of a more chemical nature, is also obtained from the desorption of PAA by the addition of specifically adsorbed phosphate compounds. Exothermic enthalpies for initial stages of polymer and surfactant adsorptions also support previous adsorption mechanisms, The near-zero or positive enthalpy values at plateau surfactant or polymer adsorption, however, hint at an entropically driven process, In the case of the adsorption from a mixture of PAA and SDBS, the results show the chronological importance of the mode of addition. However, successive or simultaneous additions of both components indicate a competition between similarly charged PAA and SDBS for the same binding sites on the kaolinite surface. (C) 1995 Academic Press, Inc.