Micellar aggregation and structure of dodecyl heptaethoxylates (C12E7) with different oxyethylene distributions in aqueous media

Abstract

The micellar aggregation and structure, including aggregation number, hydrophobicity, micelle size, and micelle shape, were studied for four dodecyl heptaethoxylates with different oxyethylene distributions (Pure-7, NRE-7, BRE-7, and xBRE-7) using fluorescence, NMR, dynamic light scattering (DLS), and small angle neutron scattering (SANS). The aggregation number, measured from a steady state fluorescence quenching method, increased with the polydispersity of oxyethylene chain and with concentration. It was shown from the line half-width of proton NMR signals that the intermediate layer becomes wider as the polydispersity of the chain is increased. Changes in the hydrodynamic radius and radius of gyration show that micelles take different shapes with changing oxyethylene distributions and there is a transition, probably from rodlike to spherical, at a specific concentration. At very low concentrations, the fitted core and shell radii of monodisperse C12E7 from SANS experiment's showed a marked change with concentration. The aggregation number estimated from a polydisperse core-shell sphere model showed fairly good agreement with that from steady state fluorescence quenching. However, the fitted results for broad ethoxylates using sphere, prolate, and oblate models were physically meaningless. Two possible explanations are suggested: first, a rough surface of micelles, and-second, polydispersity of micelles in shape and size.