Isothermal titration calorimetry studies of binding interactions between polyethylene glycol and ionic surfactants

The binding behaviors between poly(ethylene glycol) (PEG) and sodium dodecy l sulfate (SDS) were examined by isothermal titration calorimetry (ITC) tec hnique. The binding interactions between PEG and SDS are dependent on the m olecular weights of PEG. At low molecular weight (MW < 400 Daltons), SDS do es not bind to PEG chains. As PEG molecular weight increases from 900 to 14 50 Daltons, an endothermic peak, which is attributed to the formation of SD S/PEG aggregation complex by the polymer-induced surfactant micellization p rocess is observed. SDS micelles of lower aggregation number adsorb on the PEG backbones and the PEG segments are solubilized in the hydrophobic core of SIDS micelles. When the molecular weight exceeds 3350 Daltons, an endoth ermic peak followed by an exothermic peak is observed. The exothermic curve is associated with the re-hydration of PEG segments from the SDS micellar core to form SDS/PEG aggregation complexes through ion-dipole association, where the re-hydrated PEG backbones are bound to the outer surface of the S IDS hydrophilic headgroups. The binding behaviors are controlled by the equ ilibrium between polymer-induced micellization at low SDS concentrations an d ion-dipole association at high SDS concentrations. After the polymer satu ration concentration, C-2, free SIDS micelles are formed at Cr,. Increasing the polymer concentrations causes C-2 and C-m to increase, however, the cr itical aggregation concentration (CAC) is independent of polymer concentrat ions. Polymer molecular weights strongly influence C-m and C-2, but only ma rginally on the CAC.