Selective solubilization of polycyclic aromatic hydrocarbons from multicomponent nonaqueous-phase liquids into nonionic surfactant micelles

This research investigates the equilibrium solubilization behavior of naphthalene and phenanthrene from multicomponent nonaqueous-phase liquids (NAPLs) by five different polyoxyethylene nonionic surfactants. The overall goal of the study was to achieve an improved understanding of surfactant-aided dissolution of polycyclic aromatic hydrocarbons (PAHs) from multicomponent NAPLs in the context of surfactant-enhanced remediation of contaminated sites. The extent of solubilization of the PAHs in the surfactant micelles increased linearly with the PAH mole fraction in the NAPL. The solubilization extent and micelle-water equilibrium partition coefficient of the PAHs increased with the size of the polar shell region of the micelles rather than the size of the hydrophobic core of the micelle. The presence of both PAHs in the shell region of the micelles was confirmed by H-1 NMR analysis. This is an important observation because it is commonly assumed that in multi-solute systems the solutes with relatively greater hydrophobicity are solubilized only in the micellar core. A comparison of the 1H NMR spectra of pure surfactant solutions and solutions contacted with various NAPLs demonstrated that the distribution of PAHs between the shell and the core changed with the concentration of PAHs in the micelles and in the NAPL. Competitive solubilization of the PAHs was observed when both PAHs were present in the NAPL. For example, in surfactant solutions of Brij 35 and Tween 80, the solubilization of phenanthrene was decreased in the presence of naphthalene as compared to systems that contained phenanthrene as the only solute. In contrast, with micellar solutions of Tergitol NP-10 and Triton X-100, phenanthrene solubilization was enhanced in the presence of naphthalene. The activity coefficients of the PAHs in the micellar phase were generally found to increase with PAH concentrations in the micelle.