DETERMINATION OF THE COMPOSITION OF MIXED HYDROGENATED AND FLUORINATED MICELLES BY SMALL-ANGLE NEUTRON-SCATTERING

The compositions of mixed micelles formed in aqueous solutions of sodi um dodecanoate and sodium perfluorooctanoate at different total surfac tant concentrations and at three (0.33, 0.53, and 0.73) sodium perfluo rooctanoate mole fractions were determined by the small angle neutron scattering technique coupled with the external contrast method. At eac h concentration, measurements were performed as functions of the solve nt H2O/D2O composition in order to determine the micellar neutron scat tering densities. The method described has allowed, at least for the c ases under study, a direct determination of the micellar compositions, which, in the past, had been particularly difficult and the object of considerable debate. At all concentrations considered and within the experimental error, one kind of mixed micelle was always observed; the se micelles were always richer in the component present in solution in greater proportion. An overall qualitative agreement between the pres ent results and literature predictions based on regular solution theor y was found, although a significant difference was noticeable at large sodium perfluorooctanoate concentrations, suggesting that further tes ts of the theory are required in order to include subtle interaction e ffects due to differences in the chemical nature of the surfactants.