A FIELD-DEPENDENT H-2 NUCLEAR MAGNETIC-RELAXATION STUDY OF THE AGGREGATION BEHAVIOR IN MICELLAR SOLUTIONS OF CTAB AND SDS

Micellar solutions of cetyltrimethylammonium bromide (CTAB) in water a nd of sodium dodecyl sulfate (SDS) in 0.22 m NaCl were investigated wi th field dependent H-2 nuclear magnetic relaxation. The full theoretic al model for the molecular dynamics of aggregated surfactant molecules recently developed by Halle (J. Chem. Phys. 1991, 94, 3150.) was appl ied, making it possible to evaluate the relaxation data in terms of ag gregate sizes and shapes. Average aggregation numbers were determined within a simple model for the polydispersity of the solutions. For bot h systems it was found that nonspherical aggregates were formed at con centrations well below the second cmc. Above the second cmc, the aggre gates were found to be rod-shaped and the average aggregation number i ncreased rapidly with the concentration.