Organization of surfactant micelles adsorbed on a polymer molecule in water : a neutron scattering study

Abstract

Complex particles are formed in water by the spontaneous association of poly(ethylene oxide) macromolecules (PEO) with the small amphiphilic molecules of sodium dodecylsulfate (SDS). As a result, the phase diagram of dilute PEO + SDS solutions shows 2 singularities : a critical SDS concentration for the formation of the aggregates, and a stoichiometry. Small angle neutron scattering experiments on the stoichiometric aggregates are reported; the contrast variation method is used to determine separately the configuration of the polymer and that of the amphiphile in a PEO + SDS aggregate. The following structure is obtained : (i) each aggregate contains a single PEO macromolecule whose radius of gyration is comparable to that of a « free » PEO coil; (ii) the SDS molecules of the aggregate are clustered in subunits which are small spheres, 20 A in radius, similar to the micelles formed by pure SDS solutions at low ionic strengths; (iii) these subunits are adsorbed on the PEO strands; a distance between neighbouring subunits within the aggregate has been measured. Moreover, the contrast variation experiment shows that all the aggregates present in stoichiometric PEO + SDS solutions share the same composition