Some observations on the effect of the trivalent counterion Al3+ to the self-assembly of sodium dodecyl sulphate in water

The phase behavior and phase structure of the SDS (sodium dodecyl sulfate)–Al(NO3)3–H2O system have been studied by combined NMR, SAXS, cryo-TEM and optical polarization microscopy methods at 298 K (25 °C) and 311 K (38 °C). At 298 K, the micellar solution phase of the binary system SDS–water is not capable of solubilising substantial amount of Al(NO3)3 salt, and at a certain substoichiometric molar ratio Al(NO3)3/SDS, the solution phase coexists with a precipitate. When the concentration of the salt exceeds 5 wt.%, the precipitate is completely solubilised giving rise to a concentrated micellar phase that dominates the phase diagram. At high concentration of Al(NO3)3 (30–40 wt.%), a vesicle phase is formed at low (maximum 10 wt.%) SDS content. On warming to 311 K, the vesicle region expanded, by converting the hydrated surfactant crystals to a lamellar liquid crystalline phase, around the concentrated micellar phase, with a narrow width towards the SDS–H2O axis (up to 43.5 wt.% SDS and 3 wt.% Al(NO3)3). The lamellar phase coexists with both hexagonal and micellar phases with appropriate multiphase regions. The optical- and cryo-transmission electron microscopy images show the presence of unilamellar vesicles and the liquid crystalline phases are characterized by 2H NMR quadrupolar splitting and following the liquid crystalline texture by optical microscopy techniques. The NMR self-diffusion measurements, together with the cryo-TEM micrographs, identified the long worm-like micelles in the redissolution area.