Spherical dodecyltrimethylammonium bromide micelles in the limit region of transition to rod-like micelles. A light scattering study

Dynamic and static light scattering measurements were performed on dodecyltrimethylammonium bromide aqueous solutions in the presence of sodium bromide at different concentrations. The power law dependence was applied to the diffusion coefficient D vs. c(b) (c, surfactant concentration) and the decrease of the parameter b with increasing salt concentration was observed. The dependencies of D and b on the salt concentration are decreasing and show a plateau at intermediate salt concentrations. The Debye plot from the static light scattering measurements shows the presence of stable spherical micelles in the solution at low salt concentration with molecular weight about 23 000 and aggregation number 65-80. The logarithmic plot of the molecular weight vs. the ionic strength shows two linear regions with an intercept at the salt concentration 1.9 M. However, the values of the molecular weight and the aggregation number are too small to allow the assumption of a rod structure.