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

We have investigated the surfactant self-assembly in the sodium dodecyl sulfate (SDS)-formamide system with H-2 NMR relaxation on selectively 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 concentration. When the relaxation data are analyzed surfactant dynamics in both the aggregated and nonaggregated states have to be taken into account. In doing so it was possible to evaluate how the fraction of nonaggregated surfactant molecules varies with the concentration in the micellar phase. These results were compared with the predictions from the Poisson-Boltzmann cell model. The quadrupolar splittings measured 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 metastable cubic phase is formed. Self-diffusion measurements demonstrate that the cubic phase is of the micellar type which is also supported by the relative position of this metastable phase in the phase diagram.