EFFECT OF PH AND IONIC-STRENGTH ON THE ADSORPTION OF CETYLPYRIDINIUM CHLORIDE AND THE COADSORPTION OF PHENOXYPROPANOL AT A SILICA/WATER INTERFACE

The adsorption of a cationic surfactant, cetylpyridinium chloride, on silica particles in water and the coadsorption of a hydrophobic solute, 3-phenoxy-1-propanol, on the surfactant-adsorbed aggregates at the solid/water interface have been studied. The effect of pH and of ionic strength on both of these phenomena have been investigated. It is shown that the solute partition coefficient between the adsorbed aggregates and the solution is independent of pH and of ionic strength and is equal to the partition coefficient of the classical micellar solubilization effect for the same solute/surfactant system. The partition coefficient is therefore only dependent upon the amount of surfactant adsorbed on the particles and on the solute hydrophobicity. The enthalpy of solute coadsorption has also been investigated by calorimetry. It is shown that the solute enthalpy of transfer is the same whether from aqueous solution to regular micelles or to adsorbed aggregates at low pH. However at higher pH values (pH 8.5), the enthalpy of transfer is systematically more negative. A discussion of these various observations is presented.