The absence of cooperative binding in mixtures of sodium cholate and poly(ethylene oxide) as indicated by surface tension, steady-state fluorescence and electrical conductivity measurements

The properties of mixtures of poly(ethylene oxide) (PEO) and sodium cholate (NaC) in tris–HCl buffer solutions, pH 9.00, varying the sodium cholate concentration up to 0.08 M and PEO concentration up to 0.227 M (≈1 wt%), were investigated by means of electrical conductivity and surface tension measurements. In order to provide consistent support in the discussion of the conductivity and surface profile of the PEO–NaC mixtures, parallel experiments were performed for the highly studied polymer–surfactant interaction known as the PEO–sodium dodecyl sulfate (SDS) system. The I1/I3 pyrene ratio–surfactant concentration plots were used as an additional technique to follow the behavior of sodium cholate in the presence of PEO. Parameters in the conductivity and surface tension profiles, the critical aggregation concentration (cac), the breakpoint for the saturation of the polymer by the surfactants (psp) and other characteristics, such as the slopes of conductivity plots, the effect of polymer concentration on the cac and psp parameters and the amount of the associated surfactant per monomol of PEO were all investigated. The results led to the conclusion that no evidence of a cooperative process of binding on the polymer exists.