DIRECT IMAGING OF SURFACTANT MICELLES, VESICLES, DISKS, AND RIPPLE PHASE STRUCTURES BY CRYO-TRANSMISSION ELECTRON-MICROSCOPY

Surfactant microstructures in dilute aqueous solutions and dispersions-globular, swollen, and cylindrical or wormlike micelles, discoid and ripple phase structures, and uni- and multilamellar vesicles-can be seen at high resolution by cryo-transmission electron microscopy (cryo-TEM) of thin vitrified sample films. Sample films are prepared within a chamber where temperature and chemical activities of the surrounding vapor are controlled, thereby preventing evaporation and temperature changes that could alter the microstructure in the labile systems. The thin liquid films are quenched by rapidly plunging them into liquid ethane. The resulting vitrified samples are mounted into a cold-stage and transferred into a TEM for direct observation. Monophasic solutions of cetyltrimethylammonium bromide (CTAB) show globular micelles that swell with added toluene or styrene to form swollen micelles. Worm-like micelles form in CTABNaBr solutions. Dilute mixtures of dipalmitoylphosphatidylcholine (DPPC) and diheptanoylphosphatidylcholine (DHPC) show discoid structures above the main transition temperature of DPPC and A and A/2 ripple structures of the Pβ′ phase at temperatures below the main transition temperature. A new model is proposed for the A structure and the ripple structures are shown to exist as single bilayers. Biphasic dispersions of sodium 4-(1′-heptylnonyl)benzenesulfonate (SHBS) show spheroidal and tubular vesicles, and complex encapsulated vesicles and coiled tubules. Vesicle-like microstructures of SHBS persist at 90°C. At the relatively low SHBS concentrations studied there is no evidence of the constant spacing characteristic of the lamellar phase at higher concentrations, suggesting that the structures observed may result from unbinding fluctuations that disrupt lamellar phases. © 1991.