Interaction between bisphenol A and micelles of ionic surfactants

Interactions between bisphenol A (BPA) and ionic surfactants—cationic hexadecyltrimethylammonium bromide (HTAB) and anionic sodium dodecylsulfate (SDS)—were studied by measuring interfacial tensions and the intensities of pyrene fluorescence. The critical micellar concentrations (CMC) decreased with an increase in the BPA concentration, and the degree of that decrease was greater in HTAB than in SDS. In those micelles, BPA interacted more strongly with HTAB than with SDS. Conversely, BPA adsorbed on the air-water interface cooperatively with the surfactants, even though almost no adsorption of BPA itself was observed. This cooperative adsorption was more enhanced with HTAB than with SDS. Thus, BPA worked on the surfactants to stabilize the micelles and interfacial adsorption. The stability gained by the addition of BPA was greater on the interface than in the micelle. This was evidence of decreased hydrophilicity of the head group of the ionic surfactant, which interacted with BPA, because this decrease acted on the surface activities of the surfactants more directly than on the micelle stabilities. Pyrene fluorescence measurements yielded identical results for the effect on micelle stabilities. It is noteworthy that the fluorescence intensity of peak 1, l ₁, decreased with an increase in BPA concentrations at constant concentrations of surfactant greater than the CMC, but the peak ratio, l ₁/l ₃, remained almost unchanged. This fact was also related to the interaction of BPA with the hydrophilic head groups in the surfactant micelle.