Solubilization of pentachlorophenol in micelles and confined surfactant phases

The solubilization of pentachlorophenol (PCP) in cationic surfactant micelles and in surfactant-silica mesostructures is studied. The micelle-water distribution coefficient is derived from both theoretical models and experiments. The sorption isotherm in the mesostructure is determined by the solution depletion method and compared to that of phenol, mono-, di- and tri-chlorophenol. The solubilization or sorption energies of PCP in the various surfactant phases are obtained by microcalorimetry. The interaction between the anionic form of the PCP molecule and the cationic surfactant is so strong that the precipitation of a phenolate form of the surfactant is observed. This alkyltrimethylammonium phenolate form can be considered as a new chemical species that replaces the bromide form of the cationic surfactant. In the case of the non-ionic form of polychlorophenols, the water-micelle distribution coefficients, the solubilization enthalpies in micelles and the sorption enthalpies in the mesostructure increase with increasing hydrophobicity. This latter which is defined by the octanol-water partition coefficient increases itself with the number of chloride molecules in the phenol derivative. On the other hand the affinity for the mesostructure follows a non-monotonous evolution with the same parameter. This could be due to steric effects that would appear with an increase of the phenol derivative molecular size.