Micelles, aqueous, and similar assemblies

Micellar catalysis in organic reactions kinetic and mechanistic implications, 8,271 Micelles, aqueous, and similar assemblies, organic reactivity in, 22,213... 

Organic Reactivity in Aqueous Micelles and Similar Assemblies... 

C. A. Bunton, G. SaveUi, Organic reactivity in aqueous micelles and similar assemblies, Adv. Phys. Org. Ghent., 1986, 22, 213-309. 

Another type of self-assembly mode is based on looser molecular interactions, where one of the main binding forces comes from hydrophobic interactions in aqueous media. Amphiphihc molecules (amphiphiles) that have a hydrophihc part and a hydrophobic part form various assembhes in water and on water. The simplest example of this kind of assembly is a micelle, where amphiphiles seh-assemble in order to expose their hydrophilic part to water and shield the other part from water due to hydrophobic interactions. A similar mechanism also leads to the formation of other assembhes, such as hpid bilayers. These molecules form spherical assembhes and/or two-dimensional membranes that are composed of countless numbers of molecules. These assembhes are usually very flexible. When external signals are applied to them, they respond flexibly while maintaining their fundamental organization and shape. This research held was initiated by the work of Bangham in 1964. It was found that dispersions of hpid molecules extracted from cells in water spontaneously form cell-like assembhes (liposomes). In 1977, Kunitake and Okahata demonstrated the formation of similar assembhes from various arti-flcial amphiphiles. The latter finding showed that natural lipids and artificial amphiphiles are not fundamentally different. 

Names given to differentiate between supramolecular mono- and bilayer assemblies should be clear and relate to structure in a simple and general way. Micelle (Latin mic(a), grain -t- ella, diminutive suffix) indicates nothing but the ultimate smallness of these assemblies. The word microemulsion (Greek micro, small Latin emulsus, milked out) evokes a milk-like aqueous system containing (fat) droplets similar to those found in milk but much smaller. Both names produce, to a first approximation, an appropriate impression of the real systems. 

Photoredox reactions at organized assemblies such as micelles and microemulsions provide a convenient approach for modeling life-sustaining processes. Micelles are spontaneously formed in solutions in the presence of surfactants above a certain critical concentration. In aqueous solutions, the hydrophobic tails of the surfactant form aggregates with the polar head facing toward the aqueous environment, as depicted in Fig. 9. The hydrophobic core in micelles is amorphous and exhibits properties similar to a liquid hydrocarbon. The polar heads are also randomly oriented, generating an electrical double layer around the micelle structure. In this respect, surface properties of micelles can be somewhat correlated with the polarized ITIES. The structure of micelles is in dynamic equilibrium, in which monomers are exchanged between bulk solution and the assembly. 

We have discussed the self-assembly of nonionic surfactants that occurs in RTILs. Overall, the self-assembly properties in RTILs are largely similar to the aqueous medium. Notable differences between the aqueous and nonaqueous systems are sometimes seen when nonionic surfactants form micelles or lyotropic liquid crystals at certain compositions and temperatures, and this mainly results from the different affinity of the nonionic surfactants with the liquids. In other words, it may be possible to expect the formation of micelles or lyotropic liquid crystals to a certain degree by considering the solvophobic or solvophilic nature of the nonionic surfactants in the RTILs. An interesting feature of RTILs is their self-assembly in bulk liquids and at interfaces. This feature also makes a significant impact on the self-assembly of nonionic surfactants in RTILs. Particularly, we have demonstrated the importance of this feature when nonionic surfactants adsorb at solid/RTIL interfaces. We believe that the self-assembled structures of amphiphilic molecules with RTILs are of great interest not only from academic but also from industrial standpoints. One of the potential applications based on such self-assembled structures should be high-performance ion-conductive electrolytes as a new device system with nanolevel order [50]. 

Amphiphilic homopolymers containing both hydrophilic (carboxylic acid units) and lipophilic (benzyl moieties) functionalities in each repeat unit have been reported to assembly [180]. These polymers are soluble in both aqueous and organic solvents, where they assemble into micelle-like or inverse miceUe-Uke structures. All these solutions were optically clear. The hydrophilic carboxyhc acid unit and the hydrophobic benzyl moiety are placed on the opposite sides of the polymer backbone in solvents of different polarity. The observed solubUity characterishcs are the result of formation of a micehe-like structure in water, in which the hydrophihc carboxylate groups are exposed to the bulk solvent and the hydrophobic benzyl substituents are tucked in the interior of an assembly (Scheme 7). Similarly, an inverted micelle-like structure would be expected in apolar solvents, in which the functional group placements are reversed. 

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