Neutron scattering and electrophoresis of dodecyldimethylamine oxide micelles

Small-angle neutron scattering (SANS) measurements were carried out at 25 degrees C on dodecyldimethylamine oxide solutions under different conditions. In media with no added salt, the micelle aggregation number remained nearly constant (70-78) over the range of the degree of ionization, alpha(M), between 0 and 0.73 in contrast with the sharp critical micelle concentration increase in a narrow range of alpha(M) from 0.35 to 0.40. This characteristic alpha(M) dependence that deviates markedly from the prediction of the regular solution approach is thus shown to take place without a considerable change with respect to micelle size and shape. The surface electric potential employed to calculate the intermicellar repulsion under the Debye-Huckel approximation was found to be much lower than the actual surface potential determined from hydrogen ion titration. This inconsistency was solved by introducing a rescaled particle size for the hard-sphere interaction part. The surface potential from SANS was rather similar to the zeta potential determined by electrophoretic light scattering. In the range of NaCl concentrations higher than about 0.2 M, micelle growth was observed for both the hemihydrochloride (1:1 complex) and the cationic species and the growth into a cylindrical shape was confirmed.