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Vesicles from Binary Systems of Symmetric Double-Tailed Surfactants/Water Determined by Cryo-TEM

The phenomena of self-association of amphiphilic molecules play an important role in many aspects: surfactants, either in water or in oil / water mixtures, cluster together into diverse aggregates including micelles, vesicles, liposomes, bicontinuous phases and liquid crystals. The effects of vesicle formation in the aqueous solution seem to be of a special interest for the pharmaceutical and other chemical industries. Phase diagrams of the binary symmetric double-tailed alkylbenzenesulphonate (C11, C13, C15) surfactant / water systems, as well as the structure of phases, were determined using crossed polarizers, 2H NMR-spectroscopy, polarized light micro-scopy, Nomarski-optics light microscopy, and cryo-transmission electron microscopy (cryo-TEM). The isotropic phases, lamellar phase, and a high vis-cosity solution, presenting an intermediate between isotropic phase and mesophase, which refers to the beginning of the formation of mesophase, were found. The isotropic phases at higher concentrations are formed mostly at higher temperatures. Two isotropic phases, one of them pseudo-isotropic, were observed at higher surfactant weight percentages. One of the optically isotropic phases contains a lamellar dispersion of vesicles in solution. The 2H NMR spectroscopic pattern of a broadened maximum amounting to 13 Hz, presented in Figure 1, may indicate a dispersion of lamellar vesicles in solution, while patterns of the samples taken in isotropic region show broadening of an isotropic maximum. While many possibilities result in the same types of spectra, the right answer was given by electron micrographs of the phases. The formation of lamellar bilayers or multilayers was found, exhibiting either flat lamellae, lamellae bent into tubules, or vesicles and embedded vesicles of various shapes, sizes and level of encapsulation. Vesicles, liposomes, and microtubules of a wide range of sizes, shapes and levels of encapsulation appear in the cryo-TEM images, as it is represented in Figure 2. Specimens for direct-imaging cryo-TEM were prepared under controlled conditions of relative humidity of about 100% and temperature of 23 °C in the flow-thru controlled environment vitrification system (FT-CEVS), as described by Bellare et al.1 and Talmon2: thin liquid films (ca. 0.2 micrometer thick), spread over holes in a carbon film, supported on 200 mesh electron microscope copper grids, were quenched into liquid ethane at its freezing point, and transferred into a Gatan cooling holder in its 'work-station'. The holder was then inserted into the transmission electron microscope, a Philips CM12, and the examined specimens at -170 °C using an acceleration voltage of 100 kV. -Ultra-rapid cooling and the precautions taken prior to quenching assured that the structure observed in these cryo-specimens are indeed those found in the original system studied by this technique, as recently shown, for example, by Danino et al.3,4 and Regev et al.5. Recent reviews of the technique and its applications were given by Almgren et al.6 and by Talmon2. Figure 1 2H NMR powder spectrum of 7STBS (12%) / water system at 40 ¤C. Figure 2 Cryo-TEM images of vitrified specimens of 2% C15 sample showing different types of vesicular or liposomal aggregates: single-bilayer vesicles encapsulated in large vesicles, liposomes with many levels of encapsulation, very large single-bilayer vesicles flattened in the specimen, and long tubular vesicles. Bar = 100 nm. References: (1) Bellare, J.R.; Davis, H.T.; Scriven, L.E.; Talmon, Y., J. Electron Microsc. Tech.. 1988, 10, 87. (2) Talmon, Y., Ber. Bunsen-Ges. Phys. Chemie 1996, 100, 364. (3) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R., Science 1995, 269, 1420. (4) Danino, D.; Talmon, Y.; Zana, R., J. Colloid Interface Sci. 1997, 185, 84. (5) Regev, O.; Ezrahi, S.; Garti, N.; Wachtel, E.; Kaler, E.W.; Khan, A.; Talmon, Y., Langmuir 1996, 12, 668. (6) Almgren, M.; Edwards, K.; Gustafsson, J., Current Opinion in Colloid & Interface Science 1996, 1, 270.

Cryo-Transmission electron microscopy; Vesicles

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