Multilamellar surfactant vesicle

Figure 5. Coalescence phenomena a few hours after completion of the processing [(a) transmittance and (b) reflectance]. Multilamellar surfactant vesicle development phenomena twelve hours after the completion of the processing [(c) transmittance and (d) reflectance].

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

Colloidal liquid aphrons are a kind of emulsion in which micrometre-size dispersed droplets have an unusually thick stabilizing film and exist clustered together as opposed to either separated, nearly spherical droplets. The stabilizing aqueous film, sometimes called a soapy shell, is thought to have inner and outer surfactant monolayers. Taking this a step further, vesicles are droplets characterized by the presence at their surface of a lipid bimolecular film (bilayer) or series of concentric bilayers. A vesicle can be single or multilamellar and stabilized by natural or synthetic surfactants. Vesicles made from lipid or fat (e.g. phospholipid) bilayers are called liposomes (or, sometimes,/jofyso/wes). 

Other examples of bilayer structures already mentioned are the sponge phase and bicontinuous cubic phases. The sponge phase has been most studied for nonionic surfactants and is related to common microemulsions. Bilayers may also easily close on themselves to form discrete entities including unilamellar vesicles and multilamellar liposomes. Vesicles are of interest because of the division into inner and outer aqueous domains separated by the bilayer. Vesicles and liposomes are normally not thermodynamically stable (although there are exceptions) and tend to phase separate into a lamellar phase and a dilute aqueous solution. Lipid bilayers are important constituents of living organisms and form membranes, which act as barriers between different compartments. Certain surfactants and lipids may form reversed vesicles, i. e. vesicles with inner and outer oleic domains separated by a (reversed) amphiphile bilayer the bilayer may or may not contain some water. 

As indicated by Puig et al. (35). surfactant retention and attendant pressure buildup in the rock can be greatly reduced if the surfactant dispersion is converted into the liquid crystalline state. Unilameller vesicles are preferred in the field work rather than the multilamellar... 

Meso- and (+ )-azobis[6-(6-cyanododecanoic acid)] were synthesized by Porter et al. (1983) as an amphipathic free radical initiator that could deliver the radical center to a bilayer structure controllably for the study of free radical processes in membranes. The decomposition pathways of the diazenes are illustrated in Fig. 36. When the initiator was decomposed in a DPPC multilamellar vesicle matrix, the diazenes showed stereo-retention yielding unprecedented diastereomeric excesses, as high as 70%, in the recombination of the radicals to form meso- and (+ )-succinodinitriles (Brittain et al., 1984). When the methyl esters of the diazene surfactants were decomposed in a chlorobenzene solution, poor diastereoselectivity was observed, diastereomeric excesses of 2.6% and 7.4% for meso- and ( )-isomers respectively, which is typical of free radical processes in isotropic media (Greene et al, 1970). 

Closed, spherical, single-bilayer, 300- to 600-A-diameter surfactant and/or phospholipid aggregates dispersed in aqueous solutions. Ultrasonic dispersal of multilamellar vesicles (MLVs) or employing procedures such as French Press filtration result in SUV formation. 

Experiments by Muller et al. [17] on the lamellar phase of a lyotropic system (an LMW surfactant) under shear suggest that multilamellar vesicles develop via an intermediate state for which one finds a distribution of director orientations in the plane perpendicular to the flow direction. These results are compatible with an undulation instability of the type proposed here, since undulations lead to such a distribution of director orientations. Furthermore, Noirez [25] found in shear experiment on a smectic A liquid crystalline polymer in a cone-plate geometry that the layer thickness reduces slightly with increasing shear. This result is compatible with the model presented here as well. 

Hoffmann, H., Thunig, C., Schmiedel, R andMunkert, U. (1994) Surfactant systems with charged multilamellar vesicles and their rheological properties. Langmuir, 10(11), 3972-81. 

Liposomes occur in nature, but can also be easily synthesized in the laboratory. Depending on the preparation method used, whioh influenoes their size — in relation to the number of bilayer shells — and physical properties, liposomes are olassified as small unilamellar vesicles (SUVs, 25-50 nm), large unilamellar vesioles (LUVs, 100 nm to 1 pm), giant unilamellar vesicles (GUVs, 1.0-200 pm) multilamellar vesioles (MLVs, 0.1-15 pm), and multi-vesicular vesicles (MWs, 1.6-10.5 pm) the last consists of several small vesicles. Bicelles, which contain surfactant molecules in the lipid bilayer, constitute a special type of liposome. 

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. 

A vesicle is a droplet characterized by the presence at its surface of a lipid bimolecular film (bilayer) or series of concentric bilayets. A vesicle can be single or multilamellar and stabilized by natural or synthetic surfactants. Multilamellar vesicles are also termed liposomes. 

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. 

Vesicles are closed bilayers that can be observed in two forms. At low surfactant concentration, the vesicles are unilamellar and behave like a colloidal suspension of polydisperse particles. At more concentrated surfactant solutions, small multilayered vesicles are formed [134], Multilamellar vesicles (known also as spherulites) have also been observed in the lamellar phases of surfactant-brine (or even pure water-alcohol) systems [218]. The surfactant may be SDS [218,223] or DDAB (didodecyldimethylammonium bromide) [224]. In alcohol-containing systems the bilayer structural transformations are controlled by the alcohol/surfactant ratio [134].Thus, in many SDS-brine (or water)-alcohol systems, a vesicle (L4) phase is located between the micellar phase and the lamellar (L ) phase. At fixed surfactant concentration, the sequence of phases L4 -La-L3 (in water) is obtained by increasing the alcohol content, and the sequence L2 -La-L3 (in oil) is obtained by decreasing the alcohol content [ 134]. 

A polyethyleneoxide-Z)-polydimethylsiloxane-polyethyleneoxide surfactant, (EO)i5-(DMS)i5-(EO)i5, was studied with freeze-fracture transmission electron microscopy and pulsed-field gradient nuclear magnetic resonance speetroseopy, in order to establish the effeet of glyeerol on the permeability of vesiele membranes. Small vesicles with diameters of less than 25 run and multilamellar vesicles with diameters larger than 250 nm were observed in pure water, which were modified when water was gradually replaced with glycerol [47]. 

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