Isothermal titration calorimetric and electromotive force studies on binding interactions of hydrophobic ethoxylated urethane and sodium dodecyl sulfate of different molecular masses

The interactions between hydrophobic ethoxylated urethane (HEUR) and sodium dodecyl sulfate (SDS) were examined by isothermal titration calorimetric (ITC) and surfactant-selective electrode (EMF) techniques. During the isothermal titration of SDS into HEUR aqueous solutions, SDS/HEUR complexes are produced via the polymer-induced micellization process at the critical aggregation concentration (CAC). With increasing SDS concentration, the SDS aggregation number continues to increase, and the SDS/HEUR complexes reorganize to form a necklace-like structure through the ion-dipole association. At the saturation concentration C-2, all of the binding interactions between SDS and HEUR are completed. Because of the increase in the hydrophobicity of polymer chains, the CAC value of SDS/HEUR is lower than that of SDS/PEO. The CAC is independent of HEUR concentrations, but C2 Shifts to higher SDS concentrations when the polymer concentration is increased. The values of CAC and C2 are not affected by polymer molecular masses (for MM greater than or equal to 17 500 g/mol) and the size of end-capped hydrophobes. The electromotive force measurements revealed the existence of uncooperative binding prior to the cooperative binding at CAC for the C16H33 end-capped alkyl hydrophobic chains. Both the CAC and C2 obtained from ITC and EMF are identical, and the free SDS monomer concentrations during the binding process could be determined from the EMF measurements. During the isothermal titration of HEUR into SDS micellar solutions, SDS/HEUR complexes are formed through ion-dipole association with a necklace-like structure, where the PEO segments along the HEUR chains are bound to the hydrophilic surface of SDS micelles. With increasing HEUR molecular masses, the apparent binding enthalpies increase, but the size of end-capped alkyl hydrophobic chains does not alter the binding characteristics.