Isothermal titration calorimetric studies on interactions of ionic surfactant and poly(oxypropylene)-poly(oxyethylene)-poly(oxypropylene) triblock copolymers in aqueous solutions

Isothermal titration calorimetry was used to investigate the binding characteristics of sodium dodecyl sulfate (SDS) and PEP-type JP and E represent poly(oxypropylene) and poly(oxyethylene), respectively] triblock copolymers in aqueous solution. Beyond the critical aggregation concentration (CAC), PEP/SDS aggregation complexes are formed through the polymer-induced micellization process. SDS monomers first bind to the PPO segments followed by binding to the PEO segments. The polymer chains are dehydrated and solubilized in the hydrophobic core of SDS micelles containing an aggregation number smaller than that of free SIDS micelles in water. From the contribution to the Gibbs energy, it is found that the formation of polymer/SDS aggregation complex is an entropic-driven process. The CAC is independent of the molecular weight of polymer, is weakly dependent on the polymer concentrations, and is strongly dependent on polymer composition. An increase in the length of PPO segments results in the reduction in the CAC. At the saturation concentration C-2, the polymer chains are saturated with SDS micelles, where the polymer chains are bound to the surface of SDS micelles through ion-dipole associations. C-2 is sensitive to polymer concentration and shifts to higher values with increasing polymer concentrations. A physical model describing the interactions between SDS and the copolymers is proposed.