Continuous phase solvent

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 > 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

A universal property of surfactant solutions is the existence of a critical micelle concentration (CMC) representing the minimum amount of surfactant required to form aggregates. The CMC also represents the solubility of the surfactant unimer in the oil or continuous phase solvent. At surfactant concentrations above the CMC,... 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelleforming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/water systems is already well documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system. A The spherical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now well-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

As has been mentioned, the phase stability of these microemulsions is dependent upon the fluid density. The continuous phase solvent must have a sufiSciently high dielectric constant to be able to solvate these nanometer-sized droplets. In near-critical and supercritical solvents having low dielectric constants, we observe strong attractive interactions between the droplets giving rise to a limited size of droplet that can be dispersed. Likewise, the magnitude of the predicted van der Waals type of attractive interactions rises sharply as the dielectric constant of the continuous phase is reduced below a region bounded by supercritical and near-critical... 

This technique involves the freezing of the emulsion the relative freezing points of the continuous and dispersed phases are important. The continuous-phase solvent is usually organic and is removed by sublimation at low temperature and pressure. Finally, the dispersed phase solvent of the droplets is removed by sublimation, leaving polymer-drug particles. 

Emulsions are characterized in terms of dispersed / continuous phase, phase volume ratio, droplet size distribution, viscosity, and stability. The dispersed phase is present in the form of microscopic droplets which are surrounded by the continuous phase both water-in-oil (w/o) and oil-inwater (o/w) emulsions can be formed. The typical size range for dispersed droplets which are classified as emulsions is from 0.25 to 25 p (6). Particles larger than 25 p indicate incomplete emulsification and/or impending breakage of the emulsion. Phase volume ratio is the volume fraction of the emulsion occupied by the internal (dispersed) phase, expressed as a percent or decimal number. Emulsion viscosity is determined by the viscosity of the continuous phase (solvent and surfactants), the phase volume ratio, and the particle size (6). Stroeve and Varanasi (7) have shown that emulsion viscosity is a critical factor in LM stability. Stability of... 

Both small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) have been used for obtaining detailed structural information about macromolecular species such as micelles or polymers in supercritical solutions. A variety of different microstructures have been identified in SCFs. In addition, changes in the fluid density have been shown to not only affect the primary structure, but also the secondary structure involving the spatial distribution of micelles or polymers in the continuous-phase solvent. This can have dramatic effects on reaction rates and pathways. 

Supercritical microemulsions represent a radically different type of reaction media. Whereas the aqueous microdomains of these systems are much like their analogs in liquid systems, the interfacial region and the continuous-phase solvent have unusual and potentially advantageous properties. The specific benefits include ... 

Control of the reaction rates and pathways by changing the density of the continuous phase solvent... 

Changes in the hydrostatic pressure affect the reaction rates for molecularly dispersed species. In addition, pressure may affect the structure of the microemulsion in a way that significantly affects the rates of reaction. Changes in the fluid pressure can cause changes in the microstructure of the fluid, and the accessibility of the reactant to a catalyst will be altered. As indicated in previous sections, several different studies have shown that the density of the continuous-phase solvent can be used to induce structural changes in both the primary and secondary structures of the microemulsion. These structural changes can dramatically alter the reaction kinetics. 

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