The Structure of Nonionic Micelles in Less Polar Solvents

Small angle neutron scattering, SANS, has been used to investigate the structure and geometry of nonionic micelles of monododecyl octaethylene glycol C12EO8, monododecyl hexaethylene glycol C12EO6, and monohexadecyl octaethylene glycol C16EO8 in less polar solvents: mixed solvents of water and glycerol, sorbitol, and ethylene glycol. For C12EO8 in glycerol/water solvent mixtures the addition of glycerol results in an increase in the micelle aggregation number and a decrease in the ethylene oxide (EO) headgroup hydration. The increase in micelle size is associated with the dehydration of the ethylene oxide headgroup and the consequent closer proximity of the lower consolute boundary. This follows from the observation that the same trends are seen at fixed glycerol content and increasing temperature where the intermicellar interactions become attractive. For C12EO6 micelles in water/ethylene glycol mixtures similar trends are observed, and the same conclusions may be drawn despite an increase in the clouding temperature. This can be explained by a greater dehydration of the solvent ethylene glycols such that the rate of change of solvent quality for the micelles with temperature is suppressed. A combination of SANS and Couette shear flow alignment has been used to investigate the structure of C16EO8 micelles in water/sorbitol and water/glycerol mixtures. The addition of sorbitol or glycerol results in less anisotropically shaped micelles. The resulting micelles are more labile, and show evidence of flexibility and shear induced transformations or extreme polydispersity. The temperature dependence of the micelle rod length of C16EO8/water/glycerol is similar to that previously observed for C16EO6 and C16EO8 micelles in water and for the mixed micelles of C16EO6/C16TAB.