Self-association of sodium cholate with poly(ethylene oxide) cooperatively induced by sodium dodecyl sulfate

The properties of mixtures of poly(ethylene oxide) (PEO) and mixed micelles formed from sodium cholate (NaC) and sodium dodecyl sulfate (SDS) in tris/HCl buffer solutions, pH 9.00, were investigated by means of electrical conductivity and steady-state fluorescence of pyrene. The I1/I3 pyrene ratio-surfactant concentration plots were used as a technique to follow the behavior of the mixed micelles in the presence of PEO. This technique was able of distinguishing the micropolarity of the mixed micelles from the polymer–NaC–SDS complexes. It distinguishes also, above ≈0.8 NaC molar fractions, the presence of NaC–SDS mixed aggregates of multiple binding sides with different polarities. The analyses of the parameters, critical aggregation concentration (cac), critical micellar concentration (cmc) and the slopes obtained from specific conductivity versus surfactant concentration plots, in conjunction with the fluorescence measurements, permitted the identification of a cooperative process of surfactant binding on the polymer at high SDS molar fractions only. The results lead to the conclusion that, as the mixed micelles become richer in bile salt surfactant, the tendency of losing the reactivity with the polymer increases and the most likely consequence is the lack of any cooperative binding process.