Synthesis of Materials Within Reverse Micelles

Abstract

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 and various other synthesis procedures with the aim of reaching self-organizing nanoparticle systems, will be outlined.

Phase diagrams of microemulsions containing reducing agents and metal salts as bases for the synthesis of metallic nanoparticles

“…The difficulties mainly arise from the need to "co-reduce" two metals with largely different redox potentials and from the low water stability of most reducing agents. As already mentioned Pt/Bi and Pt/Pb intermetallic compounds exhibit unique catalytic properties in fuel cell applications [20,[24][25][26][27] SANS, NMR self-diffusion, static and dynamic light scattering measurements performed along the oil emulsification failure boundary can be self-consistently interpreted in terms of spherical oilswollen micelles, interacting through a hard sphere potential over a large concentration range. The presence of spherical micelles can be understood in terms of the flexible surface model where structure and stability are governed by the curvature and the elastic properties of the surfactant monolayer [30,35,36-37-39].…”

“…at room temperature) is the use of water-in-oil (w/o) microemulsions. Using microemulsions instead of the above mentioned solvochemical reduction techniques allows us to synthesize nanoparticles of low polydispersity and tuneable size [26][27][28]. Microemulsions are thermodynamically stable mixtures of at least three components, namely, water, oil, and a surfactant with domain sizes of 5 -50 nm [29].…”

Embedded Phases: A Way to Active and Stable Catalysts

“…[11] Nanosized metal oxide nanoparticles can be prepared using reverse micelles which are water-in-oil systems stabilized by a surfactant. [12] In a nonpolar solvent, a surfactant aggregates into nanosized spherical structures made up by a core of polar headgroups and a shell of hydrophobic tails. When a small amount of polar solvent is added in the mixture, hydrophobic interactions favor its segregation in the interior of the micelles.…”

Templated Synthesis for Nanoarchitectured Porous Materials

“…239,240 For example, it was established that combining AOT and lecithin in an optimized content would lead to a stable system of hexagonally arranged water columns (H II ) in isooctane. 241 Following this discovery, CdS nanorods 242 and channeled porous TiO 2 could be synthesized.…”