Phase equilibria in two aqueous chiral surfactant systems

In this study, we present the binary phase diagrams of the pure (R)- and the racemic aqueous sodium-2-methyldecanoate surfactant systems. The latter is a 1.1 mixture of the pure (S)- and pure (R)-form. The systems were investigated using crossed polaroids, polarizing optical microscope and H-2 NMR splittings. Both systems form a micellar phase, followed by a hexagonal, some intermediate phase and a large cubic phase. At even higher concentrations, the H-2 NMR studies showed larger quadrupolar splittings up to 1200 Hz, due to a lamellar phase. The intermediate phase was in both cases very narrow, and extended up to > 23 degrees C in the racemic system and up to 30 degrees C in the pure (R)-system. The similarity between these two binary phase diagrams indicates that the change in molecular packing is so small that it does not drastically affect the phase behavior. The Krafft temperature in the micellar phase in both systems is 1 degrees C, which is very low compared to that of unsubstituted alkanoates with the same chain length.