Surfactant vesicle-forming

Microscopic Studies of Surfactant Vesicles Formed During Tar Sand Recovery... 

Stability to Lysis. The addition of surfactants eventually leads to lysis of vesicles. In general, the stability of polymer vesicles against lysis is expected to be higher compared to lipid vesicles, because of the reduced entropy of mixing of polymer and surfactant chains which leads to a reduced partition of surfactants in the polymer membrane. When using Triton-X as a nonionic surfactant, it was found that the concentration of surfactant required for dissolution after a fixed amount of time increases nearly linearly with membrane thickness (158). Because of low partition of hydrocarbon surfactants, vesicles formed by siloxane and fiuorinated polymers are expected to have relatively high stability. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

T. E Yen, J. K. Park, K. I. Lee, and Y. Li. Fate of surfactant vesicles surviving from thermophilic, halotolerant, spore forming, Clostridium thermohydrosulfuricum. In E. C. Donaldson, editor. Microbial enhancement of oil recovery recent advances Proceedings of the 1990 International Conference on Microbial Enhancement of Oil Recovery,... 

In order to determine whether these surfactant vesicles were of polymerized vesicle forms, a 25% V/V ethanol (standard grade) was added to the three year old sample solution. Alcohols are known (34) to destroy surfactant vesicles derived from natural phospholipids, however, synthetically prepared polymerized vesicles are stable in as much as 25% (V/V) alcohol addition. Photomicrographs shown in Figures 7c and 7d indicate that these vesicles partially retain their stability (being mesomorphic) and therefore are suspected to be polymerized surfactants. Whether surfactant molecules of these vesicles are single or multipla bonds in tail, or in head groups remains to be seen. 

Lukac S (1984) Thermally induced variations in polarity and microviscosity of phospholipid and surfactant vesicles monitored with a probe forming an intramolecular charge-transfer complex. J Am Chem Soc 106 4386 -392... 

The structure of vesicles formed from a given surfactant depends upon the extent of sonication, and over a period of time vesicles fuse and separation of phases occurs. The ease of fusion depends upon vesicular charge and the extent to which it is neutralized by added electrolyte. 

Finally, surfactants that break down into non-surface active products in a controlled way may find use in speciahzed applications, such as in the biomedical field. For instance, cleavable surfactants that form vesicles or microemulsions can be of interest for drug dehvery, provided the metabolites are nontoxic. 

One counter-argument is presented by the group of Doron Lancet at the Weiz-mann Inshtute, who has been pushing for the notion of an informational content in liposome reproduction when starting from vesicles formed by a mixture of surfactant molecules. Sidebox 7.1, by Doron Lancet, gives an insight into this concept. 

Two popular vesicle-forming surfactants are shown in Figure 9.20, fatty acids and palmitoyl-oleoyl-phosphatidylcholine (POPC). In both cases, the hydrophobic parts are emphasized. Oleate, as for most long-chain fatty acids, forms vesicle spontaneously, on simple addition of its concentrated aqueous or methanol solution into water POPC and other lipids also form hposomes spontaneously when added to water from an alcoholic solution, or by first preparing a lipid him from an organic soluhon (by evaporahon), then adding water and stirring so as to induce a vortex. 

Figure 9.20 The amphiphilic character of vesicle-forming surfactant molecules the combination of oleic acid and oleate forms vesicles in the slightly alkaline pH range.

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. 

The need for increased stabilities and for controllable permeabilities and morphologies led to the development of polymerized surfactant vesicles [55, 158-161]. Vesicle-forming surfactants haw been functionalized by vinyl, methacrylate, diacetylene, isocyano, and styrene groups in their hydrocarbon chains or headgroups. Accordingly, SUVs could be polymerized in their bilayers or across their headgroups. In the latter case, either the outer or both the outer and inner surfaces could be polymerized separately (Fig. 38). Photopolymerization links both surfaces selective polymerization of the external SUV surface is accomplished by the addition of a water-soluble initiator (potassium persulfate, for example) to the vesicle solution. 

Dissymmetrical SUVs can be formed by limiting reactions to the outer surfaces of polymerized surfactant vesicles (Fig. 39). Chemical dissymmetry has been created, for example, in polymerized vesicles prepared from surfactants containing ester-linked viologen moieties in their headgroups. Cleavage of the... 

A related system is that of the lipid-bilayer corked capsule membranes which are formed from ultrathin (about 1 pm thick), spongy, 2.0- to 2.5-mm-diameter, more-or-less spherical nylon bags in which multiple bilayers are immobilized (Fig. 43) [343-345]. They were considered to combine the advantages of mechanical and chemical stabilities of polymeric membranes with the controllable permeabilities of surfactant vesicles. Polymerization of the bilayers, in situ,... 

BLMs can also be formed by the Montal-Mueller method [391,392], In this procedure, the surfactant (or liquid), dissolved in an apolar solvent, is spread on the water surface on both sides of the pinhole so as to form monolayers below the level of the pinhole. Careful injection of an electrolyte solution below the surface raises the water level above the pinhole and brings the monolayers into apposition to form the BLM. An advantage of the Montal-Mueller method is that it permits the formation of solventless (in reality, containing only a few solvent molecules) [387] and dissymmetrical BLMs (i.e. those containing different surfactants in the apposed monolayers). However, the necessity of a rather small pinhole ( > 0.5mm) is a disadvantage of the Montal-Mueller method. BLMs have also been prepared from surfactant vesicles (SUVs) via the Montal-Mueller method [391-399]. SUVs injected into an aqueous solution formed monolayers which, in turn, could be converted into BLMs (Fig. 60). 

The photocatalyst used for these conversions can be generated in situ, by photolysis of a zinc dithiolene salt by preformed catalysts , or by particles supported within surfactant vesicles The idea of employing semiconductor surfaces as environments for the enhanced coupling of radicals had previous support in the photochemical coupling of cyclopentadienyl radicals formed by excitation of the corresponding anions at single crystal electrodes... 

Arunothayanun, P., et al. 2000. The effect of processing variables on the physical characteristics of non-ionic surfactant vesicles (niosomes) formed from a hexadecyl diglycerol ether. Int J Pharm 201 7. 

Generally, vesicles are prepared from double-tailed surfactants, and a simple single-tailed surfactant cannot form vesicles due to its relatively large hy-... 

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