Study of Intermicellar Interactions and Micellar Sizes in Ionic Micelle Solutions by Comparing Collective Diffusion and Self-Diffusion Coefficients

Dynamic light scattering and pulsed gradient spin−echo NMR measurements were performed on D 2 O solutions of sodium glycocholate (NaGC). The micelle self-diffusion and collective diffusion coefficients were measured for 0.08 M NaGC solutions at different added NaCl molarities (0.1−0.8 M). Moreover, a study as a function of NaGC concentration at 0.1 M NaCl was performed. Both the surfactant and the NaCl concentration dependencies of the diffusion coefficients were interpreted on the basis of the DLVO theory of colloidal stability. The experimental data were analyzed by considering both the effect of micellar growth and interaction potential variation. The micelle ionization degree α was supposed to be constant over the range of 0.03−0.10 M NaGC and 0.1 M NaCl, whereas no restriction was imposed on its NaCl concentration dependence. The NaGC critical micelle concentrations at all of the studied NaCl molarities were inferred from surface tension measurements on NaGC H 2 O solutions. The best-fit micellar hydrodynamic radii and aggregation numbers point out that very slight micellar growth is induced by increasing the added electrolyte and/or the surfactant concentration. Moreover, the α value slightly decreases by increasing the NaCl molarity. An interpretation model of monodisperse spherical particles with a hard-core interaction shell of suitable thickness was also used to interpret the data at low ionic strength (from 0.1 to 0.4 M NaCl). Satisfactory agreement between the two interpretation models is obtained.