Competitive solubilization of low-molecular-weight polycyclic aromatic hydrocarbons mixtures in single and binary surfactant micelles

• Naphthalene and phenanthrene/fluorene had synergistic effect in surfactant micelle.
• Cosolubilization of phenanthrene and fluorene showed inhibitive effect.
• LMW PAHs could solubilize in the micellar shell and core regions.
• Synergism and competition related with amounts of PAHs in micellar shell region.
• Cosolubilization effects were consistent in single and mixed micelle.

The present study aims to assess mutual interaction among low-molecular-weight polycyclic aromatic hydrocarbons (LMW PAHs), predominant contaminants at petroleum contaminated sites, in nonionic, anionic and their mixed micelles. Cosolubilization effects were evaluated by the molar solubilization ratio, the micelle-water partition coefficient and the deviation ratio. Two conditions existed: (i) naphthalene/phenanthrene and naphthalene/fluorene, with significant different hydrophobicities, showed synergistic effect, a new finding compared to previous observations that no more than one solute’s solubility would increase when two solubilizates coexisted, e.g. solubility of naphthalene and phenanthrene in Triton X-100 was increased by 20.8% and 38.5% when cosolubilized, respectively and (ii) phenanthrene/fluorene, with similar hydrophobicity, exhibited inhibitive effect due to competition for the same solubilization sites, i.e. solubility of phenanthrene and fluorene in Triton X-100 was decreased by 36.1% and 6.5% when cosolubilized, respectively. Solubility of PAHs with higher log Kow gained larger enhancement. Synergism was stronger in bisolutes solubilization system where more PAHs could solubilize in the micellar shell region. Solubility inhibition was more intense to PAHs with lower log Kow. The cosolubilization trends were consistent in single and mixed surfactant systems, whereas the enhancing and suppressive extent, on the whole, were weakened and strengthened with increasing mole ratio of anionic surfactant in the mixed micelles because of reduction in micelle size. This study suggested the significance of considering multi-solutes’ interaction to estimate solubilization power of surfactants for selective separation of LMW PAHs from water and soil.

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