SHAPE AND SIZE OF MICELLES IN THE SODIUM DODECYL-SULFATE FORMAMIDE SYSTEM

We have investigated the surfactant self-assembly in the sodium dodecy l sulfate (SDS)-formamide system with H-2 NMR relaxation on selectivel y H-2-labeled surfactant Studies performed over a large concentration range at 60-degrees-C (slightly above the Krafft temperature) revealed a moderate increase in the micellar size with increasing surfactant c oncentration. When the relaxation data are analyzed surfactant dynamic s in both the aggregated and nonaggregated states have to be taken int o account. In doing so it was possible to evaluate how the fraction of nonaggregated surfactant molecules varies with the concentration in t he micellar phase. These results were compared with the predictions fr om the Poisson-Boltzmann cell model. The quadrupolar splittings measur ed in the hexagonal phase were similar to those measured previously in the SDS-water system indicating similar molecular packing conditions in the cylindrical aggregates. Below the Krafft temperature a metastab le cubic phase is formed. Self-diffusion measurements demonstrate that the cubic phase is of the micellar type which is also supported by th e relative position of this metastable phase in the phase diagram.