Synergistic solubilization of low-brominated diphenyl ether mixtures in nonionic surfactant micelles

The present study investigated the co-solubilization of three representative low-brominated diphenyl ethers (low-BDEs) in nonionic surfactant micelles. Co-solubilization effects were quantified in terms of molar solubilization ratio (MSR), micelle-water partition coefficient (log Km), extent of synergism/inhibition (RΔMSR), first stepwise association constant (K1), the average number of solute molecules per micelle (SM), excess Gibbs energy (ΔG0 excess), and interaction parameter (ω/RT). The solubilities of low-BDEs during co-solubilization were increased (i.e., synergism) in all of the surfactants tested, relative to their respective solubilities during solubilization. In each surfactant system, synergism was weaker for BDE-15/BDE-28 than that for BDE-15/BDE-47 because BDE-15 and BDE-28 may be partitioned more uniformly in the micelle core than BDE-47, which increased the possibility of their competition. Unexpectedly, BDE-28 and BDE-47 with similar log Kow values demonstrated enhanced co-solubility. This phenomenon was attributed to the positive electrostatic potentials of bromines (computed by Gaussian program) on BDE-28 and BDE-47 which may provide additional attractions to oxygen atoms of the hydrophilic chains. The synergism observed for BDE-15/BDE-28 and BDE-15/BDE-47 was stronger in the Brij35 system than that in Brij58 and Brij78 systems, suggesting that a surfactant with lower aggregation number and/or longer hydrophilic chain length can avoid the competition between solutes for the micelle core. This study may improve the understanding of multi-solutes interactions in surfactant micelles.