Micellar and Microemulsion Media

Surfactants are the most common surface-active agents, and their use is varied and vast in industrial as well as consumer markets. Surfactants are amphiphilic molecules with a hydrophobic tail attached to hydrophilic head group. In aqueous solutions, above a critical concentration (called the "critical micellar concentration," cmc), a surfactant dynamically associates to form large molecular aggregates of colloidal dimensions termed micelles. Above the cmc, there exists a dynamic equilibrium between the monomers and the micelles. Each micelle is composed of a certain number of surfactants molecules (the aggregation number) that dictates the general size and geometry of the particular micellar system. The structure of a normal micellar system is such that the hydrophobic tails are all directed away from the aqueous phase and toward the center of the micelle, forming the core. The hydophilic head groups are directed toward and in contact with the aqueous solution phase, thus forming the polar surface. The nature of the hydrophilic moiety determines whether the micellar system can be classified as anionic, cationic, zwitterionic, or nonionic. Extensive compilations of the preparation, properties, and critical micellar parameters of these classes of surfactants have been published, and one should consult them for further details [17].

The charge distribution at the surface of the micelle plays a dominant role in the behavior of the micellar system. A two-dimensional schematic representation of a spherical, ionic micellar system is shown in Figure 5 [47]. The structure is one in which the hydrophilic head groups are directed toward and in contact with the aqueous phase and the hydrophobic tails are directed away from the water phase, forming a central nonpolar core. In the Stern layer, the drop in the electric potential is very sharp, while in the Guoy-Chapman layer it is rather gradual.

Solubilizing power is the most useful and practically important property of micellar systems. Solubilization is a dynamic equilibrium process and depends on the temperature, surfactant concentration, the nature of the solute, and the type of micellar system. There are several possible sites for solubilization in a micellar system, and the site occupied by the solubilizate depends on the nature of both the solute and the micelle. In a normal micelle, a nonpolar solute may be located near the center of the hydrophobic core. An amphipathic solute may be oriented in the micelle so that the hydrophilic moiety is either near to or far from the Stern layer. Ionic solutes may be adsorbed on the polar micellar surface.

Another important feature of micellar systems is their ability to serve as a novel reaction medium in which the rates, equilibrium position, products, and even stereochemistry may be affected. They can either inhibit or accelerate the rates of chemical reactions as well as shift the

Microemulsion Micelle Images

o few A

/\/V\ Hydrophobic Tall, O Head Group, Counter Ion o few A

/\/V\ Hydrophobic Tall, O Head Group, Counter Ion

Figure 5 A two-dimensional representation of an ionic spherical micelle.

position of the reaction equilibrium depending on the nature of the reaction and the type of micellar system. These effects are the consequence of solubilization and are attributed to the balance of the hydrophobic and electrostatic interactions occurring between the micellar system and the reactants.

Microemulsions are similar to micellar systems. A simplistic description of them is that microemulsions are swollen micelles. A common microemulsion is obtained by proper processing of water, surfactant, another hydrocarbon, and a cosurfactant. One significant difference between the two is the average size. While ordinary micelles are about 2-3 nm in size, oil-in-water microemulsions have an average size in the range of 20-40 nm. As a consequence, the solubilizing capacity of microemulsions is far greater than that of micelles. Their structural characteristics are very similar to those of micelles.

In summary, micelles and microemulsions affect the redox equilibrium concentrations and solubilize very many sparingly soluble organic pollutants. This makes them attractive photo-catalysts in the photochemical degradation of toxic species that are not easily brought into solution in aqueous media. Like the semiconductor sols, micellar and microemulsion systems are generally optically transparent.

0 0

Post a comment