INVESTIGATION OF MIXED MICELLES OF DODECYLDIMETHYLAMINE OXIDE AND SODIUM DODECYL-SULFATE BY SANS - SHAPE, SIZE, CHARGE, AND INTERACTION

Mixed solutions of two surfactants, dodecyldimethylamine oxide (C(12)D AO) and sodium dodecyl sulfate (SDS), were investigated by the small-a ngle neutron scattering (SANS) method. The SANS intensities were analy zed by using the prolate ellipsoidal model for the intraparticle struc ture factor and the rescaled mean spherical approximation (RMSA) for t he interparticle structure factor. Prolate ellipsoidal micelles with a xial ratios of 1.3-4.5 were formed in the mixed solutions. Average num bers of hydrated D2O per surfactant on micelles were 6-9. At equimolar mixing, micellar sizes became maximum and the D2O hydration minimum. The formation of ion pairs between protonated C(12)DAO and SDS is an o rigin of such nonideal behavior. Electric charge numbers per surfactan t in micelles increased with adding SDS, indicating the constant charg e number of 0.13-0.15 per SDS. The addition of NaCl shielded the elect ric charge, resulting in the reduction of the repulsive interaction be tween micelles.