Solubilization of polycyclic aromatic hydrocarbons in C16E7 nonionic surfactant solutions

• Nonionic surfactant plays an important role in solubilizing PAHs.
• The string-like micelles grew with increasing incorporation of PAHs.
• The size of micelles depended on the concentration of three kinds of PAHs.
• The micelle remained string-like structure even when they were saturated with PAHs.
• The naphthalene in the micelles was mainly solubilized in palisade region.

Polycyclic aromatic hydrocarbons (PAHs), namely, naphthalene, phenanthrene, and pyrene, were solubilized in nonionic surfactant micelles formed from heptaoxyethylene monohexadecyl ether (C16E7). The sizes of the PAH-incorporated micelles, and the location of the solubilized molecules, were studied using dynamic light scattering, transmission electron microscopy (TEM), and 1H nuclear magnetic resonance (NMR) spectroscopy. The solubilization of the PAHs increased significantly above a C16E7 concentration of 1 mM, which corresponded to the point at which the morphology of the micelles changed from globular to string-like, as demonstrated by TEM imaging. The string-like micelles then grew with increasing incorporation of PAHs (naphthalene, phenanthrene, and pyrene). The 1H NMR chemical shifts of the discrete groups of the C16E7 micelles shifted upfield with increasing naphthalene concentration as a result of the ring current and/or local anisotropic effects of the PAH. The 1H peak of the oxyethylene segment at 3.7 ppm clearly split with increasing naphthalene concentration in the C16E7 solution. The rotating frame nuclear Overhauser and exchange spectroscopy of naphthalene solubilized in C16E7 micelles showed a cross peak between the oxyethylene segment and the 1H naphthalene peaks. The NMR spectral measurements showed that the solubilized naphthalene molecules were located in the palisade region of the micelles rather than in the core.

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