The binding of SDS to pluronic F127 was studied using a SDS surfactant selective electrode via electromotive force, isothermal titration calorimetry, and light scattering. At a concentration of 0.5% w/v the block copolymer F127 exists as an equilibrium mixture of micelles and monomers at 35 °C. When SDS is gradually added to this solution, binding of SDS to the F127 micelles takes place even at the lowest measured SDS concentration (1 × 10-5 mol dm-3). Initially F127/SDS mixed micelles are formed and the size of these micelles remains constant until ∼5 × 10-5 mol dm-3 of SDS has been reached. At a total SDS concentration of 5 × 10-5 mol dm-3 there are ∼6 SDS monomers in an aggregate containing 69 F127 monomers. Further addition of SDS results in a dramatic breakdown of the F127 rich mixed micelles into smaller aggregates. During this process SDS continually binds to the F127 micelles forming mixed micelles which simultaneously break down to smaller aggregates and also become richer in SDS. This process continues until the monomer SDS concentration reaches a value of 3.5 × 10-4 mol dm-3 which is the thermodynamic condition for the onset of the binding of SDS micelles on unassociated F127 oligomers. This occurs until the total SDS concentration reaches 1 × 10-3 mol dm-3 which precedes the point where all the F127/SDS mixed aggregates are broken down (3 × 10-3 mol dm-3). Therefore in the SDS concentration region 1 × 10-3 to 3 × 10-3 mol dm-3 two simultaneous binding processes take place (1) SDS forms mixed micelles with F127 and these micelles break down to smaller aggregates, and (2) SDS micelles bind to unassociated F127 oligomers. Further addition of SDS in excess of 3 × 10-3 mol dm-3 results in the formation of more and larger SDS micelles bound to individual F127 monomers until all the unassociated F127 oligomers available for binding become fully saturated with bound SDS micelles at ∼0.1 mol dm-3 added SDS. At this limiting point the complexes contain, on average, one SDS micelle per F127 monomer. No F127 aggregates appear to exist when the SDS concentration reaches or exceeds 3 × 10-3 mol dm-3.

It has been established that sodium dodecyl sulfate (SDS) binds to the micelles and monomers of the block copolymer F127. SDS binds to the monomeric unassociated F127 in the form of polymer/bound SDS micellar complexes. SDS binds to F127 micelles first forming mixed micelles, which dissociate into smaller mixed aggregates and then to single F127 unassociated monomers. A third interaction of SDS, which involves promotion of F127 micelles at concentrations up to 3 °C below the critical micellar temperature of pure F127, was identified and is investigated in the present work. The formation of such SDS-induced mixed micelles was monitored using differential scanning calorimetry, light scattering, isothermal titration calorimetry, and a SDS selective electrode for electromotive force measurements. These investigations have shown how the different binding and aggregation processes between SDS and F127 involving induced micellization, growth of mixed micelles, breakdown of mixed micelles, and binding of SDS to monomeric F127 can be identified and characterized.

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.