Reactions in Micellar Solutions and Microemulsions

Surfactant aggregates are a somce of microstructure in solution that can provide a favorable environment for some chemical reactions. While, with the exception of emulsion polymerization, large-scale commercial use of micellar solutions or microemulsions as reaction media is not currendy widespread, their potential is great and an area of active research (Gratzel and Kalyanasundararam, 1991 Holmberg, 1994 Pileni, 1993 Rathman, 1996). 

Effect of surfactants on conversion of trichlorotoluene to benzoic acid in 20% NaOH at 80 C (Monger et al., 1975) 

Surfactant aggregates can also improve selectivity by having different effects on rates of different reactions. In aqueous nitric acid (HNO3) solution, for example, nitration of phenol produces 35% o-nitrophenol and 65% p-nitrophenol. In an oil-in-water microemulsion, however, the corresponding results were 80% and 10% (Chhatre et al., 1993). Solubilized phenol molecules were oriented at the microemulsion droplet surfaces with the hydroxy (OH) group extending toward the aqueous solution. The ortho position was thus most accessible to nitronium ions (NO2+) in the aqueous solution. 

Surfactants themselves can be synthesized in some cases by autocatalytic reactions using micelles. Kust and Rathman (1995), for instance, synthesized N,N-dimethyldodecylamine N-oxide by reaction of the amine with hydrogen peroxide (HjOj). The reaction rate increased greatly when enough surfactant had been produced to form micelles which solubilized the amine. 

Considerable research has also been directed toward using water-in-oil microemulsions to produce nanoscale solid particles (PUeni, 1993 HUai et al., 1995). In one preparation method, two microemulsions are prepared, one containing the cation of the desired solid in its aqueous droplets, the other the anion. When they are mixed, nanoparticles are formed by precipitation. For example, titanium hydroxide was precipitated by mixing microemulsions containing titanium tetrachloride (TiCl4) and ammonium hydroxide (NH4OH). Subsequent calcination yielded titanium dioxide (TiOj) particles (Pillai et al., 1995). 

---

Surfactant-polymer interactions, micelle formation, and chemical reactions in micellar solutions and microemulsions... 

Chapter IV on surfactants. New sections on surfactant/polymer interactions and on chemical reactions in micellar solutions and microemulsions plus expansion and inclusion of new material in almost all other sections. 

---