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Double-chained cationic surfactants typified by dodecyldimethylammonium bromide are insoluble in water, forming lamellar liquid crystal phases. They form vesicles only on prolonged sonication. If the halide ion is replaced by a hydroxide, the resulting surfactants are highly soluble and form spontaneously a clear solution which appears to comprise a mixture of small micelles and fairly monodisperse vesicles. The distribution of particle size changes with added base or with partial titration with acid (HBr, HC1, HF) which can sometimes yield vesicles with an initially unsymmetric distribution of anions. Evidence for these structures from quasi-elastic light scattering (QELS) and viscosity measurements and an account of their extraordinary properties are presented. Statement of the ProblemPractically all studies1V2 on vesicles exhibit one common feature. The vesicles are not formed spontaneously and are unstable. This is because the starting compounds (in the biological domain typically phospholipids, in the chemical domain typically dialkyldimethylammonium halides and similar surfactants) are highly insoluble, forming liquid crystalline states. Vesicles are usually prepared by sonication, are not generally monodisperse, are almost certainly metastable, and gradually degrade over a period of days, weeks, or months, to the lamellar state from which they emerged. The equilibrium statistical mechanics of self-assembly of dilute surfactant solutions is by now well-established.3-6 Within this theoretical framework which includes metastable states single-walled vesicles are several possible allowed aggregates-spherical micelles, globular, oblate, prolate ellipsoids, cylindrical micelles, vesicles, liposomes, multilamellar bilayers, and inverse micellar structures. Which aggregates form is determined primarily by geometric packing of amphiphiles, hydrocarbon chain stiffness, and the hydrophobic-hydrophilic balance. Intramolecular interactions complicate the self-assembly process somewhat. There is no reason to believe that single-walled vesicles are-in general-in any way more unstable or disallowed than are normal micelles. Broadly pea king^,^ necessary (geometric) conditions for formation of aggregates are the following: (1) spherical micelles, u/al < 1/3; (2) globular or cylindrical micelles, 1 / 3 < u / a l < lI2; (3) vesicles or bilayers, 1 / 2 < u / a l 51. Here u is the volume per hydrocarbon chain, or of the hydrophobic region of the surfactant, a is the head-group area, and 1 is an optimal hydrocarbon chain length related to the maximum extended length. (These conditions assume that these aggregates have a fluid-oil-like interior.) By changing chain length or chain stiffness (e.g., by varying temperature), hydrocarbon volume, or head-group area, one can in principle dictate which type of aggregate will form under prescribed conditions. Thus, e.g., for ionic surfactants, decrease in repulsive head-group interactions induced by added salt will decrease a, increase ulal, and cause a

It is shown that n-butane, condensed in autoclave, can readily dissolve the anionic surfactant AOT (bis-2-ethylhexylsodium sulphosuccinate) and that this solution sohibilizes water up to Wo = molar ratio/water-AOT).The microemulsion can be transformed into a gel by sohibilizing gelatin in the water microphase. These microemulsion gels are characterized by a very high viscosity (several hundred poise). However, the partial vapour pressure of the n-butane component in the gel is as high as in the liquid state. Infrared studies show that part of the water in the gel is still flee.Stability diagrams of this system and ~H-NMR studies to assess the state of water are presented. Analogously to reverse micelles from liquid hydrocarbons, the chemical shift of water protons is shifted to higher fields with respect to bulk water. This effect increases with decreasing temperature and w0. At low w0 values (< 10), a part of the water in the micelles does not freeze even at -35 ~ Due to the rapid evaporation of n-butane from the gel, gelatin films can be prepared simply by casting. Differential scanning calorimetry studies of the gels (from n-butane as well as from isooctane and other hydrocarbons) show that the films have a thermal stability comparable with, and sometimes higher than, gelatin films prepared by conventional methods. Preliminary permeability studies of these films with solutions of amino acid derivatives and with gas such as nitrogen and helium are presented.