Multiphase aqueous systems, phase

In comparison with classical processes involving thermal separation, biphasic techniques offer simplified process schemes and no thermal stress for the organometal-lic catalyst. The concept requires that the catalyst and the product phases separate rapidly, to achieve a practical approach to the recovery and recycling of the catalyst. Thanks to their tunable solubility characteristics, ionic liquids have proven to be good candidates for multiphasic techniques. They extend the applications of aqueous biphasic systems to a broader range of organic hydrophobic substrates and water-sensitive catalysts [48-50]. 

The formation of nitrosamines in aprotic solvents has applicability to many practical lipophilic systems including foods (particularly bacon), cigarette smoke, cosmetics, and some drugs. The very rapid kinetics of nitrosation reactions in lipid solution indicates that the lipid phase of emulsions or analogous multiphase systems can act as "catalyst" to facilitate nitrosation reactions that may be far slower in purely aqueous media (41, 53, 54). This is apparently true in some cosmetic emulsion systems and may have important applicability to nitrosation reactions in vivo, particularly in the GI tract. In these multiphase systems, the pH of the aqueous phase may be poor for nitrosation in aqueous media (e.g., neutral or alkaline pH) because of the very small concentration of HONO or that can exist at these pH ranges. 

With the MBR, individual components of multiphase systems can be heated at different rates according to differences in their dielectric properties. We have termed this, differential heating. For example, the aqueous phase of a water/chloroform system (1 1 by volume), heated more rapidly than did the organic layer and a tempera-... 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

Surfactants Salting in is when the addition of electrolyte to a solution of non-ionic surfactant causes the critical micelle concentration to increase. Also, addition of electrolyte to an ionic surfactant solution in a multiphase system can drive surfactant from the oil phase into the aqueous phase. Salting out is when the addition of electrolyte causes the critical micelle concentration to decrease. Also, addition of electrolyte to an ionic surfactant solution in a multiphase system can drive surfactant from the aqueous phase into the oil phase. 

Three systems are generally used for such evaluations emulsions (water-in-oil, oil-in-water), liposomes (uni- or multUamellar vesicles formed with aqueous phase and phospholipids), and micelles (emulsions formed with free fatty acids and aqueous phase). A lengthy discussion about how to use these multiphase systems in evaluating antioxidant activity is provided by Frankel (19). 

Figure 2. Different approaches of the variation of the application phase of oxo catalysts. FBS = fluorous biphase [multiphase] system PEG = polyethylene glycol NAIL = non-aqueous ionic liquid.

DYNAMICS OF DISTRIBUTION The natural aqueous system is a complex multiphase system which contains dissolved chemicals as well as suspended solids. The metals present in such a system are likely to distribute themselves between the various components of the solid phase and the liquid phase. Such a distribution may attain (a) a true equilibrium or (b) follow a steady state condition. If an element in a system has attained a true equilibrium, the ratio of element concentrations in two phases (solid/liquid), in principle, must remain unchanged at any given temperature. The mathematical relation of metal concentrations in these two phases is governed by the Nernst distribution law (41) commonly called the partition coefficient (1 ) and is defined as = s) /a(l) where a(s) is the activity of metal ions associated with the solid phase and a( ) is the activity of metal ions associated with the liquid phase (dissolved). This behavior of element is a direct consequence of the dynamics of ionic distribution in a multiphase system. For dilute solution, which generally obeys Raoult s law (41) activity (a) of a metal ion can be substituted by its concentration, (c) moles L l or moles Kg i. This ratio (Kd) serves as a comparison for relative affinity of metal ions for various components-exchangeable, carbonate, oxide, organic-of the solid phase. Chemical potential which is a function of several variables controls the numerical values of Kd (41). 

It is noteworthy to mention that tliis reaction is not a homogeneous single phase system as neither reactant is soluble in aqueous alkaline reaction medium. We believe that the selective absorption of microwaves by polar molecules and intermediates in a multiphase system could substitute as a phase transfer catalyst witliout using any phase transfer reagent, thereby providing the observed acceleration as has been observed for ultrasonic irradiation. ... 

As exemplified in the previous sections, organometallic catalysis in water and bi-phasic water-containing systems has not only been intensively examined during recent years but its scope has also considerably broadened. Between 1945 and 1997, the publishing date of the first edition of this book, about 1650 articles dealing with catalysis in aqueous media appeared in scientific journals. Approximately the same number of articles has been published between 1998 and the end of 2002. In the first half of 2003 nearly 200 new articles appeared [1], Most of the research dedicated to reactions in aqueous-phase and aqueous biphasic or multiphase systems refer to the reactions described in the previous sections. In this chapter some other new results, not described above in more detail, on reactions in aqueous media will be presented. 

Application of whole cells in two-liquid phase or multiphase systems or in micelles biocatalysis at the interface between the aqueous and the organic phase (for poorly soluble substrates). 

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