Reaction in Microemulsions

Microemulsions are transparent dispersions of oil in water (o/w), or water in oil (w/o), generally stabilized by both a surfactant and a cosurfactant, typically a medium chain length alcohol or amine [147]. However surfactantless microemulsions have been reported over limited ranges of solute concentration [148]. 

Although microemulsions are of great industrial importance, it is only in recent years that their role as media for organic and inorganic reactions has attracted attention [148-156]. 

Microemulsion droplets can be considered as akin to micelles, in that there is a lipid-like region with polar or ionic groups in contact with water. There seems to be no real difference between w/o microemulsions and reverse micelles in apolar solvents. In both systems the interior is water surrounded by surfactant and organic solvent, and the aggregates behave as aqueous dispersions stabilized by surfactant and cosurfactant. 

In these systems, as in normal micelles, it is useful to distinguish between an aqueous and a non-aqueous pseudophase, except that the alcohol or amine in a microemulsion droplet may act as a nucleophile. Thus, as in micellar solutions, the aggregates can bring reactants together or keep them apart. For example, the complexing of metal ions to amino acid derivatives can be controlled by microemulsions [152], and the interior of a w/o microemulsion in these systems has been compared with the metal binding site of a protein. 

Mackay and coworkers have used microemulsions extensively as reaction media, and have shown that in alkaline solution a hydroxyl function of an alcohol or a non-ionic surfactant can act as a nucleophile. For example in a microemulsion of 

---

Hager M, Currie F, Holmberg K (2003) Organic Reactions in Microemulsions. 227 53-74 Hausler H, Stiitz AE (2001) d-Xylose (d-Glucose) Isomerase and Related Enzymes in Carbohydrate Synthesis. 215 77-114... 

Reactions in vesicles 268 Reactions in microemulsions 271 Submicellar self-assembly aggregates 273... 

Lopez-Quintela MA (2003) Synthesis of nanomaterials in microemulsions formation mechanisms and growth control. Curr Opin Colloid Interface Sci 8 137-144 Lopez-Quintela MA, Tojo C, Blanco MC, Rio LG, Leis JR (2004) Microemulsion dynamics and reactions in microemulsions. Curr Opin Colloid Interface Sci 9 264-278 Maitra A (1984) Determination of Size Parameters of Water Aerosol Ot Oil Reverse Micelles from Their Nuclear Magnetic-Resonance Data. J Phys Chem 88 5122-5125... 

Enzyme activity is highly dependent on the composition, and as a consequence on different microstructures of the microemulsion. Up to the present, no suitable theory exists to explain the correlation between the reaction media properties and their effects on enzymatic reactions in microemulsions. All experimental results on enzymatic reactions show that the activity is greatly affected by the structure of the microemulsion. 

The surfactant mass fraction in a microemulsion defines the size of the interfacial area between the water and oil. The reaction rate of organic reactions in microemulsions can be dramatically enhanced by increasing the specific interfacial area [95]. Enzyme catalysis in microemulsions is usually not influenced by the size of the interfacial area because only a small fraction of the reverse micelles are hosting a bio-molecule. Most investigations published so far were made with low enzyme concentrations resulting in a low population of enzymes per reverse micelle. 

When we consider the metals of nanoscopic size, fine metal particles from micrometer to nanometer size can be synthesized by both physical and chemical methods. The former method provides the fine metal particles by decreasing the size by addition of energy to the bulk metal, while in the latter methods, fine particles can be produced by increasing the size from metal atoms obtained by reduction of metal ions in solution. Since chemical reactions usually take place in homogeneous solution in any case, this chapter includes most of the cases of synthesis and growth of fine metal particles. However, the polyol process, reaction in microemulsions, and formation in the gas phase are omitted, since they are described in later chapters by specialists in those fields. 

Activity and stability are often comparable to values in aqueous media. Many substrates which cannot be made to react in water or in pure organic solvents such as hexane owing to lack of solubility can be brought to reaction in microemulsions. Whereas enzyme structure and mechanism do not seem to change upon transition from water to the microemulsion phase (Bommarius, 1995), partitioning effects often are very important. Besides an enhanced or diminished concentration of substrates in the vidnity of microemulsion droplets and thus of enzyme molecules, the effective pH values in the water pool of the droplets can be shifted in the presence of charged surfactants. Frequently, observed acceleration or deceleration effects on enzyme reactions can be explained with such partitioning effects (Jobe, 1989). 

Micellar solutions have also been used as media for organic reactions. Reactions in micellar solutions have much in common with reactions in microemulsions but the capability to solubilize hydrophobic components is much smaller in micellar solutions than in microemulsions. Micellar solutions are therefore of limited value for preparative purposes. For both systems separation of the surfactant from the product may constitute a work-up problem. 

Bunton and de Buzzaccarini have studied the kinetics of substitution reactions in microemulsions and compared the result with the reaction kinetics in... 

---