Isothermal titration calorimetric studies on the interaction between sodium dodecyl sulfate and polyethylene glycols of different molecular weights and chain architectures

The binding interaction between polyethylene glycols (PEG) and sodium dodecyl sulfate (SDS) was examined by isothermal titration calorimetric (ITC) technique. For PEG with molecular weight of 7000–8000 Da, unusual ITC profile attributed to the structural reorganization of SDS/PEG aggregates was observed. The basic binding segment for PEG was found to be close to an averaged molecular weight of ∼4000 Da. When the PEG molecular weight is less than 20000 Da, structural re-organization of SDS/PEG aggregates occurs at a critical PEG molecular weight of (4000 × N), where N is an integer. For binding interaction between SDS and PEG mixtures, CAC and C2 are dependent on the average molecular weight; however, the ITC profiles of SDS/PEG mixtures are different from the mono-dispersed PEG system of identical molecular weights. In addition, PEG chain architecture and configuration have a dominant influence on the binding isotherms, where the contribution of steric constraint, excluded volume effect and enhanced electrostatic interaction, to the enthalpy during structural-reorganization is evident for three, four and eight arm PEGs of identical molecular weights. Such constraints from either critical chain length or chain architecture affect the formation of SDS/PEG necklace-like aggregates, which gives rise to an additional endothermic contribution to the ITC thermogram.