Vesicles transformation into micelles

Fig. 67 Schematic of phase behaviour for blend of novolac epoxy resin with nearly symmetric poly(methyl acrylate-co-glycidylmelhacrylate)-0-polyisoprene. Ordered L can be swollen with up to about 30% of resin before macroscopic phase separation occurs, producing heterogeneous morphologies containing various amounts of L, C, worm-like micelles and pristine epoxy. At lower concentrations, disordered worm-like micelles transform into vesicles in dilute limit. According to [201]. Copyright 2003 Wiley...

The basic nanoreactor in particle formation is a reverse micelle in most cases, with a generally accepted spherical shape. The particles generated from these micelles transforming into W/O microemulsions are also often spherical in shape. However, other surfactant architectures may also yield particles. Thus, vesicles have been instrumental in the formation of particles in many cases (not discussed in this book) [98] similarly, cylindrical micelles could also generate elongated nanoparticles with the required manipulations in the system. Unfortunately clear-cut evidence on this offshoot procedure of synthesis, i.e. rodlike particle formation from rod-like micelles is apparently not so extensively available. 

The lamellar phases on the surface of the emulsion particles are mainly composed of monoacylglycerides, lyso-phospholipids and ionized fatty acids. When the phases have reached a certain size, they will desorb from the emulsion surface and form multi-lamellar vesicles, which are transformed into uni-lamellar vesicles upon increased incorporation of bile salts (Rigler et al., 1986). Upon further incorporation of bile salts, the ratio of lipid amphiphiles to bile salts will decrease to 1 or lower, whereby the uni-lamellar vesicles are transformed to mixed micelles (Staggers et al., 1990). These events are presented in Figure 4. 

The reverse micelles get transformed into a liquid crystalline phase or vesicle dispersion, when it comes in contact with the aqueous body fluids. This reduces the rate of release of the solubilized drugs. ... 

Bernheim-Groswasser. A. Zana, R. Talmon, Y. Micro- 30. structures in aqueous solutions of mixed dimeric surfactants Vesicle transformation into networks of thread-like micelles. J. Phys. Chem.. B 2000. 104. 12192-12201. and references therein. 

When mixed micelles are enriched in lecithin, the particles become bigger and the solutions manifest the Tyndall phenomena[8] mixed micelles are transformed into vesicles so that the solution shows a new phase (liquid crystals). In this manner a solution resembling bile can present different phases in relation to the molar percentage composition of the three components (Ch, Lee, and BS) and in particular solid (Ch crystals), liquid (micelles), and liquid crystalline phases may be present[12]. Cholesterol is solubilized in micellar and liquid-crystalline phases and this possibility has been demonstrated for some hydrophilic bile acids such as ursodeoxycholic acid[ll]. 

In principle, what has just been stated for surfactant micelles also holds for the larger and more complex self-assembhes that surfactants and amphiphific block copolymers can form microemulsion droplets, vesicles, and mesophases. The lifetimes of these assembhes are much longer than for micelles, mainly when they involve block copolymers. Nevertheless, exchanges and other processes can also take place. Vesicles and lyotropic mesophases can be considered as permanent objects. However, vesicles can be transformed into micelles, and vice versa. Likewise, a lyotropic mesophase may be transformed into another mesophase or in a micellar solution by an appropriate change brought to the system. The kinetics of these transformations is of basic as well as of practical interest. 

Lipid-surfactant mixtures have gained much interest in context with the solubilization of membranes and with the problem of reconstituting membrane proteins into artificial membrane systems such as unilamellar vesicles. For the solubilization of membranes, a sufficiently high concentration of an aqueous micellar solution has to be added to the membrane suspension, so that the bilayers are transformed into mixed micelles containing surfactant, membrane lipids, and membrane proteins. This solubilization process is quite complicated and the necessary amounts of surfactant for complete solubilization depends on the nature of the surfactant, the type of membrane, and the total concentration of lipid and surfactant. 

Addition of surfactant molecules to lipid vesicles first leads to a partitioning of the surfactant into the lipid bilayers, an equilibrium is established, the partition coefficient depending on the nature of the lipids in the membrane and the chemical stmcture of the surfactant. Further increase of the total surfactant concentration leads finally to a saturation of the bilayers with surfactant molecules and first mixed micelles of a different surfactant to lipid ratio appear. Over a certain concentration range mixed bilayers and mixed micelles coexist until finally all bilayers have been transformed into micelles. 

Still further increase of surfactant concentration leads to a disappearance of vesicles till finally above the line called all vesicles have been transformed into micelles. The two lines enclosing the coexistence region are described by the equation... 

When the partition dynamics are rapid, the solute distribution in vesicles will obey the same laws as the distributions in micelles. However, when the transmembrane diffusion time of molecules entrapped within the aqueous vesicle core or incorporated into the hydrocarbon phases exceeds the characteristic time of their chemical transformation in chemical reactions, then their partitioning is set by their initial statistical distribution rather than their migration dynamics. In this case also, a Poisson law is appropriate to approximate their distribution among vesicles. This follows because the volume of the inner aqueous phase generally exceeds 10 A, and the maximiun number of molecules that can be entrapped inside the vesicle is correspondingly large. 

Al-Jamal KT, Sakthivel T, Florence AT. Solubilisation and transformation of amphipathic lipid dendron vesicles (dendrisomes) into mixed micelles. Colloids Surf A Hiysicochem Eng Asp 2005 268 52-59. 

We have mentioned before the possibility of combining chemical evolution with self-replication. In principle, chemical evolution can be associated to self-reproducing micelles or vesicles. There are in principle two ways to conceive this in this case on the one hand, the surfactants of the self-reproducing vesicles could be chemically transformed during their reproduction cycles into compounds which may give rise to more efficient cell-like compartments. This possibility has been discussed theoretically some time ago. On the other hand, the supramolecular structure can help and determine the evolution of internalized compounds—i.e. permitting certain reactions and avoiding others thanks to the semipermeable character of the membrane. As already mentioned, studies of this type with vesicles still remain to be initiated. 

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