We applied isothermal titration calorimetry (ITC) and surface tension (ST) and electromotive force (emf) measurements using a coated wire sodium dodecyl sulfate membrane-selective electrode to measure the mixed micellar composition of various mixtures of the triblock copolymer EO97PO69EO97, a nonionic surfactant code-named Pluronic F127, with sodium dodecyl sulfate (SDS). In the region where mixed micelles are formed, the interaction between the two surfactants showed synergistic behavior and interaction parameters β, which characterize the nonideal interaction in the mixed micelles, could be calculated over a range of mole ratios. For several compositions, the critical micelle concentrations of the mixed micelles were determined using ITC and ST measurements. In addition, small-angle neutron scattering (SANS) experiments were carried out in order to investigate the structure and provide additional information about the composition of the mixed micelles, taking advantage of contrast variation between SDS-h12 and SDS-d12. Mixed F127/SDS aggregates could be confirmed, and from an examination of the results of all methods the mixed F127/SDS system can be explained in considerable detail.

The interactions of sodium dodecyl sulfate (SDS) with the triblock copolymer L64 (EO13-PO30-EO13) and hexaethylene glycol mono-n-dodecyl ether (C12EO6) were studied using electromotive force, isothermal titration microcalorimetry, differential scanning microcalorimetry, and surface tension measurements. In certain regions of binding, mixed micelles are formed, and here we could evaluate an interaction parameter using regular solution theory. The mixed micelles of L64 with both SDS and C12EO6 exhibit synergy. When L64 is present in its nonassociated state, it forms polymer/micellar SDS complexes at SDS concentrations above the critical aggregation concentration (cac). The cac is well below the critical micellar concentration (cmc) of pure SDS, and a model suggesting how bound micelles are formed at the cac in the presence of a polymer is described. The interaction of nonassociated L64 with C12EO6 is a very rare example of strong binding between a nonionic surfactant and a nonionic polymer, and C12EO6/L64 mixed micelles are formed. We also carried out small angle neutron scattering measurement to determine the structure of the monomeric polymer/micellar SDS complex, as well as the mixed L64/C12EO6 aggregates. In these experiments, contrast matching was achieved by using the h and d forms of SDS, as well as C12EO6. During the early stages of the formation of polymer-bound SDS micelles, SDS aggregates with aggregation numbers of approximately 20 were found and such complexes contain 4-6 bound L64 monomers. The L64/C12EO6 data confirmed the existence of mixed micelles, and structural information involving the composition of the mixed micelle and the aggregation numbers were evaluated.