INFLUENCE OF THE POLAR HEAD SIZE ON THE DYNAMICS OF MICELLIZED SURFACTANTS

The dynamics of micellized quaternary ammonium surfactants has been investigated by means of the frequency dependence of the C-13 longitudinal relaxation rates. Trimethyl-, and tributyldodecylammonium bromides were used to systematically vary the bulk of the polar head and examine its influence on the internal and overall motions of the surfactants. The relaxation data were analyzed by means of the two-step model and of a rotational jump model relevant to the trans-gauche isomerizations about the bonds. In the case of the trimethyldodecylammonium bromide an excellent agreement is obtained between the order parameters derived from these two models. A marked discrepancy is noticed for the triethyl- and tributyldodecylammonium bromides. For the latter, the order parameters obtained by the two-step model at the vicinity of the nitrogen are well above the values expected for methylene groups. The jump model shows that this behavior results from a slowing of the internal motions beyond the domain of validity of the two-step model.