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 charac teristics of sodium dodecyl sulfate (SDS) and PEP-type JP and E represent p oly(oxypropylene) and poly(oxyethylene), respectively] triblock copolymers in aqueous solution. Beyond the critical aggregation concentration (CAC), P EP/SDS aggregation complexes are formed through the polymer-induced micelli zation process. SDS monomers first bind to the PPO segments followed by bin ding to the PEO segments. The polymer chains are dehydrated and solubilized in the hydrophobic core of SDS micelles containing an aggregation number s maller than that of free SIDS micelles in water. From the contribution to t he Gibbs energy, it is found that the formation of polymer/SDS aggregation complex is an entropic-driven process. The CAC is independent of the molecu lar weight of polymer, is weakly dependent on the polymer concentrations, a nd is strongly dependent on polymer composition. An increase in the length of PPO segments results in the reduction in the CAC. At the saturation conc entration C-2, the polymer chains are saturated with SDS micelles, where th e polymer chains are bound to the surface of SDS micelles through ion-dipol e associations. C-2 is sensitive to polymer concentration and shifts to hig her values with increasing polymer concentrations. A physical model describ ing the interactions between SDS and the copolymers is proposed.