Molecular self-diffusion in micellar and discrete cubic phases of an ionic surfactant with mixed monovalent/polymeric counterions

The molecular self-diffusion in concentrated aqueous mixtures based on an oppositely charged po ymer-surfactant pair has been investigated by NMR pulsed field gradient techniques. The investigated structures were an ordered cubic Pm3n phase (ca. 50 wt % water) and a disordered micellar phase, both containing cetyltrimethylammonium (CTA(+)) micelles with mixed acetate (Ac-) and polyacrylate (PA(-)) counterions. In the cubic phase, CTA(+) gave broad lines in the H-1 NMR spectrum, which made it possible to use the pulsed field gradient (PFG) spin echo (SE) technique to measure the PA(-) diffusion. The observed diffusion coefficients for the four different molecular species covered 4 orders of magnitude with the rate of diffusion decreasing in the order water > Ac- > PA(-) > CTA(+). For all species, the diffusion coefficients were largely insensitive to the counterion composition. A model considering obstruction effects and binding to the micelles could account for the reduced diffusion of Ac- and CTA(+) compared to dilute reference solutions. For water, the reduction of the diffusion was mainly ue to obstruction effects. The PA(-) diffusion was much more retarded than the Ac- diffusion in the cubic phase; moreover, the molecular weight dependence of the diffusion was dramatically enhanced for the polydisperse polyion. Both effects are attributed to electrostatic attractions and obstruction effects experienced by polyions diffusing in a concentrated matrix of stationary micelles