Binding characteristics of hydrophobic ethoxylated urethane (HEUR) and an anionic surfactant: Microcalorimetry and laser light scattering studies

The binding behaviors of an anionic surfactant, sodium dodecyl sulfate (SDS), to an associative polymer, hydrophobic ethoxylated urethane (HEUR) were studied by isothermal titration calorimetric (ITC) and laser light scattering (LLS) techniques. Static and dynamic light scattering results suggest that HEUR polymers form flowerlike micelles with an average aggregation number of about 20. Isothermal titration calorimetric studies of SDS and HEUR solutions were conducted by varying both the concentrations of SDS and HEUR. For the titration of SDS into HEUR solutions, the thermograms display an endothermic peak after the critical aggregation concentration (cac), followed by an exothermic peak that eventually merges with the SDS dilution curve. The endothermic and exothermic peaks are interpreted by different binding mechanisms at different surfactant concentrations. Although the values of cac are independent of HEUR concentrations, the saturation concentration C-2 is strongly dependent on HEUR concentrations. With increasing HEUR concentrations, C-2 increases proportionately. Dynamic light scattering results corroborate with the findings from ITC studies. The apparent hydrodynamic radii increase with increasing SDS concentrations for cac < CDSDS< C-2, and remained constant at C-SDS > C-2. However, the thermograms for titration of HEUR into SDS micellar solutions differ from those of titrating SDS into HEUR, suggesting that different binding mechanism must be in operation. The binding enthalpy changes increase With SDS concentration and decrease with HEUR concentration when titrating HEUR into SDS solutions.