Isothermal Titration Calorimetry Studies of Binding Interactions between Polyethylene Glycol and Ionic Surfactants

Abstract

The binding behaviors between poly(ethylene glycol) (PEG) and sodium dodecyl sulfate (SDS) were examined by isothermal titration calorimetry (ITC) technique. The binding interactions between PEG and SDS are dependent on the molecular weights of PEG. At low molecular weight (MW < 400 Daltons), SDS does not bind to PEG chains. As PEG molecular weight increases from 900 to 1450 Daltons, an endothermic peak, which is attributed to the formation of SDS/PEG aggregation complex by the polymer-induced surfactant micellization process is observed. SDS micelles of lower aggregation number adsorb on the PEG backbones and the PEG segments are solubilized in the hydrophobic core of SDS micelles. When the molecular weight exceeds 3350 Daltons, an endothermic 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 SDS hydrophilic headgroups. The binding behaviors are controlled by the equilibrium between polymer-induced micellization at low SDS concentrations and ion−dipole association at high SDS concentrations. After the polymer saturation concentration, C2, free SDS micelles are formed at Cm. Increasing the polymer concentrations causes C2 and Cm to increase, however, the critical aggregation concentration (CAC) is independent of polymer concentrations. Polymer molecular weights strongly influence Cm and C2, but only marginally on the CAC.