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The coadsorption of cationic surfactants and several naphthalene derivatives on a porous silica, Sorbsil C30, has been investigated in aqueous solutions as a function of surfactant concentration below and above the critical micelle concentration (cmc) at two pH values. 2-Naphthol and 2-(2-naphthyl)ethanol are not adsorbed onto the silica surface. Adsorption of cetylpyridinium chloride or cetyltrimethylammonium bromide on the silica induces a considerable coadsorption of the neutral molecules. The effect may be described by a partitioning process. Above the cmc, the solutes desorb from the surfactant structures as they are solubilized in the free micelles. Naphthalene, which is strongly adsorbed on the silica surface in the absence of surfactant, is also desorbed as micelles are formed. Comparisons of the micellar solubilization effect as determined from solute desorption or direct micellar solubilization experiments display differences that may be attributed to the retention of some solutes on the adsorbed surfactant structures (hemimicelles or admicelles) in the presence of free micelles. It is shown that for all solutes studied the free energy of coadsorption is larger than the free energy of micellar solubilization.

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