Synthesis of Materials within Reverse Micelles

Reverse micelles as nanosized aqueous droplets existing at certain compositions of water-in-oil microemulsions are widely used today in the synthesis of many types of nanoparticles. However, without a rich conceptual network that would correlate the properties and compositions of reverse micellar microemulsions to the properties of to-be-obtained particles, the design procedures in these cases usually rely on a trial-and-error approach. As like every other science, what is presently known is merely the tip of the iceberg compared to the uninvestigated vastness still lying below. The aim of this article is to present readers with most of the major achievements from the field of materials synthesis within reverse micelles since the first such synthesis was performed in 1982 until today, to possibly open up new perspectives of viewing the typical problems that nowadays dominate the field, and to hopefully initiate the observation and generation of their actual solutions. We intend to show that by refining the oversimplified representations of the roles that reverse micelles play in the processes of nanoparticles synthesis, steps toward a more complex and realistic view of the concerned relationships can be made. The first two sections of the review are of introductory character, presenting the reader with the basic concepts and ideas that serve as the foundations of the field of reverse micellar synthesis of materials. Applications of reverse micelles, other than as media for materials synthesis, as well as their basic structures and origins, together with experimental methods for evaluating their structural and dynamic properties, basic chemicals used for their preparation and simplified explanations of the preparation of materials within, will be reviewed in these two introductory sections. In Secs. 3 and 4, we shall proceed with reviewing the structural and dynamic properties of reverse micelles, respectively, assuming that knowledge of both static and dynamic parameters of microemulsions and changes induced thereof, are a necessary step prior to putting forth any correlations between the parameters that define the properties of microemulsions and the parameters that define the properties of materials synthesized within. Typical pathways of synthesis will be presented in Sec. 5, whereas basic parameters used to describe correlations between the properties of microemulsion reaction media and materials prepared within, including reagent concentrations, ionic strength, temperature, aging time and some of the normally overlooked influences, will be mentioned in Sec. 6. The whole of Sec. 7 is devoted to reviewing water-to-surfactant molar ratio as the most often used parameter in materials design by performing reverse micellar synthesis routes. The mechanisms of particle formation within precipitation synthesis in reverse micelles is discussed in Sec. 8. Synthesis of composites, with special emphasis on silica composites, is described in Sec. 9. All types of materials, classified according to their chemical compositions, that were, to our knowledge, synthesized by using reverse micelles up-to-date, will be briefly mentioned and pointed to the corresponding references in Sec. 10. In Sec. 11, some of the possible future directions for the synthesis of nanostructured materials within reverse micelles, found in combining reverse micellar syntheses