Effect of anionic–nonionic-mixed surfactant micelles on solubilization of PAHs

Abstract

The solubility enhancement of phenanthrene (Phe), fluoranthene (FLT), and benzo[a]pyrene (BaP) in water by the single anionic surfactant sodium dodecyl sulfate (SDS), single nonionic surfactant Triton X-100 (TX100), and mixed surfactant solution (SDS-TX100) with varying proportions and the synergism mechanism were investigated in detail. Solubilization of phenanthrene was greatly enhanced either by SDS or by TX100. The sequence of solubilization for different mixed-surfactant solutions or surfactant solutions was SDS-TX100 (1:9, liquid mass ratio) > TX100 > SDS-TX100 (2:8) > SDS-TX100 (3:7) > SDS-TX100 (4:6) > SDS-TX100 (1:1) > SDS-TX100 (7:3) > SDS when the concentrations were above their critical micelle concentration (CMC). The composite surfactant SDS-TX100 (1:9) had better solubilization capacity than any single surfactant, which is attributed to the lower CMC of the mixed surfactant solution and the increase of the molar solubilization ratio (MSR) of the solution. The linear relationship between the solubility values of three targets polycyclic aromatic hydrocarbons (PAHs) and the polarities of the microenvironments of pyrene with SDS-TX100 mixtures has been demonstrated, with a linear regression coefficient of 0.7331, 0.7658, and 0.8669, respectively. The optimum mixed micelles of the anionic and nonionic surfactants (SDS-TX100; 1:9) had lower polarity and higher aggregation number. This indicated that more PAHs were transferred into the mixed micelles of the anionic and nonionic surfactants. Implications: This study investigated the synergism effect of SDS-TX100 surfactants on the solubilization of PAHs. More PAHs were transferred into mixed micelles of SDS-TX100 (1:9), because the polarity of the mixed micelles was lower and their aggregation number (Nm) was larger. A strong positive linear relationship between the solubility values of PAH and the polarities of the microenvironments of pyrene has been demonstrated. The Nm of the SDS-TX100 mixtures ranged from 37 to 90, more than that of TX100. The Nm increased so that the water molecule that was infiltrated into the palisade layer of micelles was forced to move into the aqueous solutions.