Influence of Molecular Structure on the Size, Shape, and Nanostructure of Nonionic CnEm Surfactant Micelles

Nonionic alkyl ethoxylates (CnEm) have been extensively studied for their adsorption, aggregation, and solubilization individually and in small groups. In this work, we report a more systematic study of the effects of alkyl chain (tail) and ethoxylate (head) length on the size, shape, and extent of intermixing within the CnEm micelles in aqueous solution. Data from small angle neutron scattering (SANS) and nuclear magnetic resonance (NMR) were combined to undertake the structural characterization of micelles formed from the two separate series of surfactants CnE6 (n = 10, 12, 14) and C12Em (m = 5, 6, 8, 10, 12). The micellar core volume (V-core) could be well determined with reasonable accuracy and linked to the hydrophilic-lipophilic balance (HLB) of the surfactant, with a sharp size and shape transition occurring around HLB = 12.5. NOESY NMR results revealed protrusions of the terminal methylene groups into the ethoxylate shell, thus providing direct experimental evidence for the phenomenon of roughness or intermixing of the core-shell interface. These detailed studies are compared with previous investigations on this model surfactant system.