Multilamellar micelles

Above a critical concentration certain amphiphiles may aggregate into membranous micelles, which are globular structures with a nonpolar interior and with a polar surface that interfaces the aqueous environment. At still higher concentrations micelles may fuse into multilamellar structures composed of bilayers. [138]... 

Some surfactants self-assemble into closed bilayers called vesicles (or liposomes when formed from phospholipids). Vesicles are often spherical but can take other shapes and can be unilamellar or multilamellar. In contradistinction to micelles, vesicles may not be thermodynamically stable. Another important difference between vesicles and micelles is that vesicles have an inside that encloses some of the aqueous phase and an outside. The existence of a critical vesiculation concentration, above which some surfactants would form vesicles, is sometimes mentioned. This is probably incorrect. At very low concentrations, surfactants always start forming micelles that may turn into vesicles ct higher concentrations. Given in Fig. I are schematic representations of a micelle and a vesicle, both o f spherical shape. 

One of the several shapes that micelles can take is laminar. Since the ends of such micelles have their lyophobic portions exposed to the surrounding solvent, they can curve upwards to form spherical structures called vesicles. Vesicles are spherical and have one or more surfactant bilayers surrounding an internal pocket of liquid. Multilamellar vesicles have concentric spheres of unilamellar vesicles, each separated from one another by a layer of solvent [2, 3] (Figure 14.1). The bUayers are quite thin ( 10nm) and are stabilized by molecules such as phospholipids, cholesterol or other surfactants (Figure 14.2). Vesicles made from phospholipid bilayers are called liposomes. Liposomes can be made by dispersing phospholipids (such as lecithin) into water and then agitating with ultrasound. 

C-NMR studies have complemented the above H-NMR results (Sears, 1975 Ribeiro and Dennis, 1976). The phospholipid in mixed micelles also exhibit similar Ti values to the phospholipid in the unsonicated multilamellar vesicles, corresponding to similar fast segmental motions on the phospholipid fatty acid chains in all of these systems. However, the phospholipid in mixed micelles very clearly exhibits narrower lines (longer T relaxation times) than in the bilayer systems. 

Bg , 103 S i pictures of emulsified fatty aeid, (wi Tween 20), multilamellar liposome of aqueous lecithin and tnglycerides, and inverted micelles... 

In immiscible liquid mixtures treated with ultrasound, emulsions and micelles are formed. The influence of the power and the precision of the ultrasonic field lead to regular micelles with interesting characteristics. This is the situation in the case of phospholipids, where multilamellar into multilamellar systems can... 

Amphiphiles with two chains per charged head group, such as didodecyldi-methylammonium cation, dicetylphosphate anion, and the lecithin of biomembranes tend to form bilayers, not micelles, in which the chains are presumably more ordered than those in simple micelles. Such layers can exist as flat sheets, as in multilamellar structures (21) or as the curved surface of a spherical micelle (22). N.m.r. evidence... 

Excited-state Phenomena.—Czarniecki and Breslow provide an experimental test for structure and organization in micelles and vesicles based on inter-molecular hydrogen-atom abstraction by benzophenone triplet-state. This leads to functionalization at a particular site in the chain, which may be identified by degradation and mass spectrometry. With didodecyl phosphate and (97) or (98), the specificity is as shown (Figure 9) for sonicated surfactant, presumed to be in vesicular form. This shows a much altered specificity from the corresponding reaction in micelles. When sonication is carried out in a sodium borate buffer, a technique considered to provide multilamellar layers (opaque dispersion ) then the terminal selectivity is completely lost and C(6) is the most favoured site for attack. 

Amphiphilic molecules have a tendency to organize themselves to form different types of morphologies such as bilayer, multilamellar, vesicles, spherical and cylindrical micelles, and so on in bulk solution and are well understood experimentally (Gruner 1989) or by simulation. However, the... 

Gebricki and Hicks systematically studied vesicular fatty carboxylic acids in 1973 (17). At pH values greater than 10, the fatty acid surfactant is fully ionized and micelles are formed. As the pH is reduced into the region 7.5 to 9, partial protonation of the anion at the surface occurs and there is a transition from micellar to vesicular structures. These vesicles are much less stable than phospholipid vesicles for example, oleic acid vesicles are stable for only a few hours, while capric acid vesicles apparently can last for a few days. The difference is thought to be due to oleic acid being unsaturated and liable to autooxidation. Fatty acid vesicles made from micelles tend to be polydisperse and are multilamellar in structure. They are, however, easy to prepare and relatively inexpensive. 

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