Kinetic bilayer trapping

Probe diffusion rates observed from parachutes or hollow polymer spheres may be indistinguishable since the probe could be bound in either a polymer bead or polymer shell with similar release characteristics. For example, only hydrophobic probes could be trapped in the polymerized vesicles synthesized by Nakache et al. Here, trapping refers to a decrease in the rate of probe release after vesicle polymerization. The trans-membrane diffusion rates of hydrophilic probes should decrease following polymerization if a polymer shell is successfully formed in the surfactant bilayer. Nakache et al. only observed a decrease in the trans-membrane diffusion rate of hydrophobic probes. This is important since the hydrophobic probe may be released from both hollow polymer spheres and polymer latices with similar release kinetics. Again, caution should be taken, as already shown by German et al. [20] in the case of the fluorescence... 

A number of kinetic studies were carried out to elucidate the interaction between a-tocopherol and ascorbic acid in liposomes. By using either water-soluble or oil-soluble diazo radical initiators, the effect of radicals can be compared when they are produced either in the phospholipid bilayer or in the aqueous phase. Oxidation of a soybean phosphatidylcholine liposome in the presence of radicals produced in the water phase with a water-soluble radical initiator [2,2 -azobis(2-amidinopropane) dihydrochloride, (AAPH)], showed an induction period with both a-tocopherol (vitamin E) and ascorbic acid (vitamin C) (Figure 10.9). With a mixture of vitamin E and vitamin C, the length of the induction was close to the sum of the individual induction periods. This result indicates an additive effect in suppressing oxidation by both vitamin E and vitamin C. Vitamin C apparently traps radicals in the water phase. When oxidation was induced by radicals produced in the lipid phase with an oil-soluble radical initiator [2,2 -azobis(2,4-dimethylvaleronitrile), (AMVN)] incorporated into the membrane, ascorbic acid alone had no effect, while a-tocopherol had a greater effect because it is lipophilic. However, the mixture... 

The small fraction of single free chains together with slow exchange kinetics essentially freezes the exchange process. A sufficiently fast exchange of amphiphiles is, however, necessary to sustain thermodynamic equilibrium. Therefore vesicles, once formed, are in a metastable, trapped, or quenched thermodynamic state. The number of amphiphiles and therefore their bilayer area is essentially constant on timescales of most experiments. An extreme case is glassy polymers with very slow lateral mobility which can even impede shape changes of polymer vesicles ( frozen vesicles). 

Dispersion of silicates in water soluble polymers need not result in kinetically trapped systems, and such is the behavior of PEO/Na MMT hybrids. The structure of these polymer/inorganic hybrids is well known (Fig. 8.3), studied extensively both experimentally, as well as by molecular simulations, and is markedly independent of the filler loading. When enough PEO exists in the composite an intercalated stmcture is formed (with d-spacings distributed around 1.7 nm, which corresponds to a PEO bilayer of about 0.8 nm thickness). For composites with extremely small amounts of PEO ( polymer-starved composites at montmorillonite loadings of i mt > 90%), an intercalated monolayer of PEO can also be observed, with an intercalated d-spacing of about 1.37 nm. These latter structures are of no interest to this present work. For the... 

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