Microstructure studies in surfactant systems

Microstructures in two mixed surfactants systems that show novel transformations as a function of composition and temperature have been explored. Indirect imaging methods like Small Angle Neutron Scattering (SANS) along with direct imaging methods like Cryogenic Transmission Electron Microscopy (Cryo-TEM) and recently developed Freeze Fracture Direct Imaging (FFDI) have been used extensively to characterize and identify the changes in the microstructures in the mixed surfactant systems as a function of composition, temperature and external force fields. As water is added to a solution of an anionic surfactant, AOT (2 ethylhexyl sodium sulfosuccinate) and a ziwtterionic surfactant, lecithin (phosphatidylcholine) in isooctane, the microstructure evolves from a water-in-oil microemulsion to a bicontinuous highly viscous gel phase. SANS experiments show that the gel consists of highly ordered aqueous and organic nanochannels. Depending upon the composition and temperature these ordered phases display either hexagonal or lamellar symmetries. FFDI has been used extensively to image the hexagonal and lamellar microstructures of the gel phase and shows that the structures at high water contents are multilamellar vesicles, and not planar sheets. It has also been observed that the application of shear orients the cylinders in the hexagonal phase in the direction of flow and reduces the inter-cylinder spacing. The lamellar polycrystalline phase does not show any preferential alignment after application of shear, confirming that this phase consists of multilamellar vesicles. When the mixed phase (hexagonal + lamellar) is subjected to shear, the hexagonal component aligns, while the lamellar portion remains unaffected. When an aligned hexagonal phase is heated past the hexagonal to lamellar phase transition temperature, cylinders merge in the (1, 0) direction to form planar sheets parallel to the walls of a Couette cell. Cryogenic-TEM has been used extensively to map the transformation of cetyl trimethyl ammonium bromide (CTAB) micelles to vesicles and to tubules and double layered vesicle structures as a range of phenolic derivatives are doped into the system. The addition of hydrotopes as well as more insoluble dopants to micelles to drive structural transitions represents an interesting approach to controlling functionality in complex fluids