A variety of applications of surfactant and micellar solutions have been explored over the last century. This will continue throughout this century. Surfactant molecules have a strong tendency to be adsorbed on the surfaces and interfaces, which results in a dramatic decrease in the surface tension of water or the interfacial tension at oil-water interfaces. This is the first key to their applications. On the micelle side, the counterion binding and micellar solubilization phenomena are important. Ionic species in surfactant solution can be highly localized on the very narrow region of the micelle surface through the process of counterion binding. Micellar solubilization literally means solubilizing water-insoluble compounds in aqueous solution and localizing them within a small confined space of the micellar interior.
The oldest application of surfactant solution may be its detergency (Culter and Kissa, 1987). First, decreased interfacial tension at the oil-water interface by the adsorption of surfactant will promote the detachment of oil from the solid substrates (glass, fiber, soil, etc.). Then, the oil will be solubilized within the micelle in nearby solution, which will be eventually removed from the solution, leaving the clean solid substrates behind. The well-known rollup mechanism and the force balance during the change of contact angle at the three phase contact regions are important. Additional major applications include cosmetics and pharmaceutical formulations, delivery vehicles, micellar liquid chromatography, micellar - enhanced ultrafiltration, rheology modifier, tertiary oil recovery, and so on. Readers will find details in excellent references: Scamehorn and Harwell (1989); Morrow (1991).
Micelles in solution can dramatically change the reaction rates of a number of different reactions. Also, micelles in solution can make reactions possible that usually are not possible in aqueous solution or that are possible only under very specific condition such as vigorous stirring. This phenomenon is called micellar catalysis. While traditional catalysis is involved with the change of activation energy of an intermediate reaction complex, micellar catalysis is the result of the dramatic localization of ionic and non- (or less-) polar compounds on the micelle. This is also differentiated from the biological enzyme catalysis that occurs via molecular - specific bonding. Micellar catalysis is a noncovalent and nonmolecular-specific process. It is rather a zone- (or regional-) specific process at the nanometer scale.
Counterion binding and micellar solubilization are again two key points. Also, the extremely high oil-water interfacial area in the surfactant solution greatly contributes to this process. With the size of ordinary micelles and the roughness of their surfaces, the interfacial area of a typical micellar solution can be estimated as ~1,000—2,000cm2 per g of surfactant molecule.
Figure 3.11 shows three typical micellar catalysis processes. First, type (a) is involved only with the counterion binding process. When the ionic organic or inorganic compound (or compound with ionic functional group) is solubilized or dispersed in aqueous solution with low concentration, its chance of reaction is very low or the reaction rate is very slow even though it is reactive. However, when it is solubilized or dispersed in the surfactant solution whose micelle head group has opposite charge, strong charge-charge interaction brings it to the
The name microhetero-geneous system originates here.
surface of the micelle, and localizes it in a confined space. This can increase the reaction rate up to ~100-fold (the figure represents an example ofpolymerization or condensation after counterion binding). Second, type (b) is also involved only with the counterion binding process. But the sequential binding of two or more different reactants on the surface of the micelle makes the more diverse reaction possible. The binding competition between the different ionic species and possible saturation by one reactant can limit the catalytic effect. The third type (c) is the combination of micellar solubilization and counterion binding. Reactant that is water insoluble (or less soluble) is first solubilized into the micelle and the second reactant that is ionic is localized on the surface of the micelle. Two reac-tants that are immiscible can be brought together this way, which makes the reaction possible with a enhanced reaction rate.
Concepts of micellar phase-transfer catalysis and micellar autocatalysis have been well studied based on this scheme (Cordes, 1973; Fendler and Fendler, 1975; Almgren, 1991; Bunton, 1991 ; Rathman, 1996). Biological systems have evolved some vital functions that are strikingly similar to the scheme of this micellar catalysis. Some examples include transport across the membrane and self-reproduction of lipid vesicles for the model growth of cell membranes (Luisi, Walde, and Oberholzer, 1999). It also provides an excellent model system for the study of enzyme activity enhancement in the human body, such as the role of phospholipase A2 (PLA2) in hydrolysis of glycerophospholipids (Tatulian, 2001 ).
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