Structure and composition of mixed micelles formed by nonionic block copolymers and ionic surfactants in water determined by small-angle neutron scattering with contrast variation

Hypothesis: Complex fluids comprising polymers and surfactants exhibit interesting properties which depend on the overall composition and solvent quality. The ultimate determinants of the macroscopic properties are the nano-scale association domains. Hence it is important to ascertain the structure and composition of the domains, and how they respond to the overall composition. Experiments: The structure and composition of mixed micelles formed in aqueous solution between poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers (Pluronics or Poloxamers) and the ionic surfactant sodium dodecylsulfate (SDS) are determined from an analysis of small-angle neutron scattering (SANS) intensity data obtained at different contrasts. Different polymers and concentrations have been probed. Findings: The SDS + Pluronic mixed micelles include polymer and some water in the micelle core that is formed primarily by alkyl chains. This is different than what was previously reported, but is consistent with a variety of experimental observations. This is the first report on the structure of SDS + Pluronic P123 (EO₁₉PO₆₉EO₁₉) assemblies. The effects on the mixed micelle structure and composition of the surfactant concentration and the polymer hydrophobicity are discussed here in the context of interactions between the different components.