Behavior of cationic surfactants micellar solution solubilizing an endocrine disruptor bisphenol A

The interactions between bisphenol A (BPA) and cationic surfactant micelles have been studied by interfacial tension (γ), conductivity (κ) and pyrene fluorescence intensity (I1/I3) measurements. The following cationic surfactants have been investigated: decyltrimethylammonium bromide (DeTAB), dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB) and hexadecyltrimethylammonium bromide (HTAB). As the BPA concentration increases, the critical micelle concentration (cmc) and the maximum surface excess (Γmax) of surfactant decreases. Both the effects are more pronounced at smaller concentration of BPA, and also for the surfactant with a longer hydrocarbon tail. The degree of micelle dissociation (α) tends to decrease with an increase of hydrocarbon tail length of surfactant molecule, except for the shortest chain DeTAB. Considering the ionic association constant (KA), however, the KA value increases with the increase of hydrocarbon chain including DeTAB. That is to say, the anomalously small α value of DeTAB could be caused by its large cmc. As for the dependence of α value on BPA, the α value is increased by BPA addition in all the surfactants. The standard free energies of micellization View the MathML source and of interfacial adsorption View the MathML source have also been determined. Those values indicate that BPA stabilizes the surfactant molecules in the micelle and at the air/water interface. These stabilizations are larger at the interface than in the micelle, which appears more significantly with the surfactant of longer hydrocarbon tail. From the fluorescence measurements, the I1/I3 ratio shows almost no change with the increase of BPA concentration above the cmc. This means that the environmental polarity of pyrene is not affected by BPA in the micelle.