Monolayer vesicles

The use of ordered supramolecular assemblies, such as micelles, monolayers, vesicles, inverted micelles, and lyotropic liquid crystalline systems, allows for the controlled nucleation of inorganic materials on molecular templates with well-defined structure and surface chemistry. Poly(propyleneimine) dendrimers modified with long aliphatic chains are a new class of amphiphiles which display a variety of aggregation states due to their conformational flexibility [38]. In the presence of octadecylamine, poly(propyleneimine) dendrimers modified with long alkyl chains self-assemble to form remarkably rigid and well-defined aggregates. When the aggregate dispersion was injected into a supersaturated... 

During the end of the 20th century, a surge in the development of significantly advanced techniques has advanced nanoscience and technology in the development of self-assembly structures—micelles, monolayers, vesicles—biomolecules, biosensors, and surface and colloidal chemistry. In fact, the current literature indicates that there is no end to this trend regarding the vast expansion in the sensitivity and level of information. 

J.H. Fuhrhop, U. Liman, H.H. David, Sealing and Opening Porous Monolayer Vesicle Membranes , Angew. Chem. Int. Ed., 24,339 (1985)... 

Figure 4.1 Models of a 2D layer within a monolayer vesicle membrane made of bolaam-phiphiles la-c with identical head groups a) with regular spacings between head groups, b) with small domains. The distances given between the head groups relate to a vesicle circumference with an inner diameter of 25 nm, an outer diameter o/30 nm and a membrane thickness of 1.5 nm.

Figure 4.5 A small part of a monolayered vesicle membrane made by a one-sided precipitation of the given bipyridinium bolaamphiphile 2 with perchlorate.

The immisdbility of CF2- and CHi-chains was also utilized for the preparation of unsymmetric vesicle membranes. The hydrophobic parts of bolaamphiphile 5 with fatty acid and fluorocarbon sulfonate halves do not mix and all the fluorosulfonate halves were on the outer side of the monolayered vesicle membrane (Figure 4.7). The sulfonate head group was once more localized by the metachromatic effect, the hydrophobic parts with F- and H-substituted spin labels. 

Figure 4.24 A monensin-based, dianionic bolaamphiphile assembles to form hydrated ion pores in monolayered vesicle membranes. They can be reversibly closed by a,(xi-diamino bolaamphiphiles. ...

Figure 4.28 a) Disentanglement of the polymer main chain from the membrane surface by hydrophilic spacers, often ethylene glycol oligomers, is the best means to synkinetize vesicles from amphiphilic polymers. Ordering of the amphiphiles must not be disturbed by their covalent connections, b) Folding of the ionene polymer spontaneously yields monolayer vesicle membranes. ... 

What remains to be done is to integrate the ultrathin assemblies into society. This is being tried in many application labs throughout the world which work on LB surface monolayers. One may hope that they succeed in developing useful and stable devices. If this expectation is fulfilled, the whole field will flourish since (cast) monolayers, vesicles, micellar fibres and porous microcrystals can easily be combined and integrated. Much more is to be expected from building workable reaction systems or new materials than from mimicry of biological systems which has already been abandoned by most supra-molecular chemists. 

Surfactants having two alkyl chains can pack in a similar manner to the phospholipids (see Box 6.4 for examples). Vesicle formation by the dialkyldimethylammonium cationic surfactants has been studied extensively. As with liposomes, sonication of the turbid solution formed when the surfactant is dispersed in water leads ultimately to the formation of optically transparent solutions which may contain single-compartment vesicles. For example, sonication of dioctadecyldimethyl-ammonium chloride for 30 s gives a turbid solution containing bilayer vesicles of 250-450 nm diameter, while sonication for 15 min produces a clear solution containing monolayer vesicles of diameter 100-150 nm. The main use of such systems has been as membrane models rather than as drug delivery vehicles because of the toxicity of ionic surfactants. 

Surfactant aggregates (microemulsions, micelles, monolayers, vesicles, and liquid crystals) are recently the subject of extensive basic and applied research, because of their inherently interesting chemistry, as well as their diverse technical applications in such fields as petroleum, agriculture, pharmaceuticals, and detergents. Some of the important systems which these aggregates may model are enzyme catalysis, membrane transport, and drug delivery. More practical uses for them are enhanced tertiary oil recovery, emulsion polymerization, and solubilization and detoxification of pesticides and other toxic organic chemicals. 

Spherical vesicles (see Sec. 2.5.4) are made by the same kind of amphiphiles that form micelles. Highly soluble amphiphiles (e.g., sodium salts of fatty acids or soaps) form micelles badly soluble amphiphiles (e.g., free fatty acids) give vesicles or crystallize. Amphiphilic monomers with two or three long alkyl chains are often totally water insoluble as monomers but dissolve well as vesicular assemblies. Vesicles usually collapse upon drying (Fig. 1.5.8a), but one isolable monolayer vesicle made of rigid carotenoid bolaamphiphiles has also been reported (Fig. 5.5). Hydrogen bond chains convert spherical vesicles to tubules. Such tubules can again be isolated in the dry form and can be stored. They are particularly stable if monolayer membranes are used (Fig. 1.5.8b). 

Figure 2.5.14 Asymmetrical monolayered vesicle membranes have been obtained from the two bolaamphiphiles shown, (a) All large headgroups are on the outer surface, all small headgroups at the inside of the vesicle. This phenomenon is by no means universal. It has to be tested for each individual asymmetrical bolaamphiphile. In most cases there is only a small difference in the localization of different headgroups. In (b) the metachromatic effect of polyanions on methylene blue aggregation on the sulfonated membrane outside is indicated (see text above).

Synkinetic assemblies of quinones in bulk water include monolayer vesicles, where a negatively charged large quinone carboxylate is localized outside and a fatty acid carboxylate group inside. Upon addition of sodium borohydride exactly 50% of the quinone is reduced to the hydroquinone, because the borohydride anion cannot pass the membrane to react inside quinone headgroups (Scheme 7.2.6) (Fuhrhop, 1990). 

Fuhrhop. J.-H., Mathieu, J. (1983). An unsymmetric monolayer vesicle membrane, J. Chem. Soc., Chem. Commun., p. 144. 

The potential applications of bolaamphiphiles include the formation of monolayer vesicles for drug/gene delivery, ultra thin monolayer membranes, inclusion of functionalities into membranes, and disruption of biological membranes [59a]. 

Membrane composition also has profound effects on the cation complexation activity of lasalocid within vesicle membranes In pure DMPC monolayer vesicles,... 

With the advent of polymersomes and vesicles of other nonconventionar (i.e., not phospholipid-like) amphiphiles, numerous examples of monolayer vesicles in water have been reported. Typically, monolayer vesicles are prepared from small molecules with a hydrophobic core and two hydrophilic head groups (bolaform amphiphiles, see below) or from triblock-copolymers with two hydrophilic terminal blocks. The molecule must have a cylindrical or rectangular shape, so that it can arrange into a monolayer. 

Small amphiphiles need not necessarily have a phospholipid-like structure with two tails and one head group. For example, bolaform (or bipolar) amphiphiles are amphiphilic molecules that contain two head groups separated by an extended hydrophobic chain. Bolaform amphiphiles form monolayer vesicles in which each amphiphile extends across the monolayer membrane, exposing both head groups to water and sheltering the hydrophobic chain from water. However, also these types of... 

Recently, there has been a fascinating report on reverse vesicles formed by hydrogen-bonded tri-ammonium cyclo-phanes and hexa-ammonium capsules. Using a range of methods, it was shown that these cationic macrocycles and cages can form reverse monolayer vesicles in chloroform and dichloromethane due to a combination of polar interactions inside the monolayer and exposure of the alkyl substituents to the solvent. 

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