Solubilization of arenes and n-alkylbenzenes into ionic surfactant micelles

The solubilization of benzene, naphthalene, anthracene and pyrene in aqueous micellar dodecylammonium trifluoroacetate solution (DAPA) and of n-alkylbenzenes (benzene to n-pentylbenzene) in 1-dodecanesulfonic acid (DSA) solution were measured. The maximum additive concentrations (MAC) of naphthalene, anthracene, and pyrene and the equilibrium concentrations of the benzenes were determined spectrophotometrically at 298.2, 303.2 and 208.2 K for DAPA and at an additional temperature of 293.2 K for DSA. The first stepwise association constants ((K) over bar(1)) between solubilizate monomer and vacant micelle were evaluated from the MAC and the equilibrium concentration, and were found to increase with the hydrophobicity of the solubilizate molecules. Standard enthalpy and entropy changes of the solubilization were obtained from the temperature dependence of the (K) over bar(1) values, and the location of these solubilizates in the micelle was discussed thermodynamically, with the spectra of the solubilizates in solvents of varying dielectric constants also being used to compare with the spectra in their solubilized state. The standard entropy changes are negative for the polycyclic aromatic compounds, and positive for the n-alkylbenzenes. This fact indicates that structured water molecules around the alkyl chains play an important role in their solubilization into micelles, although quite the same association takes place between micelle and solubilizate for both kinds of solubilizates.