Biphasic environments

Another possibility, taking advantage of the biphasic environment, is to use fluorous organic solvents as the catalyst phase instead of water [155]. Crooks and coworkers prepared dendrimer-stabilized colloid catalysts soluble in the fluorous phase and used the catalysts in hydrogenation [156] and in a Heck reaction [157]. In both cases the colloidal catalyst in the fluorous phase was recyclable and showed some interesting selectivities and products unique to the nanoenvironment in the dendrimer interior. 

Industrial environments expose individuals to a plethora of airborne chemical compounds in the form of vapors, aerosols, or biphasic mixtures of both. These atmospheric contaminants primarily interface with two body surfaces the respiratory tract and the skin. Between these two routes of systemic exposure to airborne chemicals (inhalation and transdermal absorption) the respiratory tract has the larger surface area and a much greater percentage of this surface exposed to the ambient environment. Or dinary work clothing generally restricts skin exposures to the arms, neck, and head, and special protective clothing ensembles further limit or totally eliminate skin exposures, but breathing exposes much of the airway to contaminants. 

Hori H, A Yamamoto, S Kutsuna (2005b) Efficient photochemical decomposition of long-chain perfluorocarboxylic acids by means of an aqueous/liquid COj biphasic system. Environ Sci Technol 39 7692-7697. 

Lipophilicity represents the affinity of a molecule or a moiety for a lipophilic environment. It is commonly measured by its distribution behavior in a biphasic system, either liquid-liquid (e.g. partition coefficient in 1-octanol-water) or solid-liquid (retention on reversed-phase high-performance liquid chromatography or thin-layer chromatography system). 

Several kinds of states in which enzymes may be used for various reactions in aqueous-organic biphasic systems have been developed in previous work (Table 2). In biphasic media, the biocatalyst is easily recovered after the reaction then it is not always necessary to be immobilized. Nevertheless, the immobilization can confer important properties, such as improved stability of biocatalyst. Furthermore, protection of the biocatalyst against a damaging turbulent environment can also play a role. 

The effect of biphasic mixtures on the productivity includes the contributions of solvent partitioning on enzyme activity and stability. An important activity does not necessarily lead to increased productivity. We must then distinguish between the effect of the environment on activity and productivity. 

Yang and Russell [7] made comparison of lipase-catalyzed hydrolysis in three different systems organic, biphasic, and reversed micelles. They affirmed that water content is an important factor that distinctly affects every system. Their results demonstrated that activity of lipase in organic-aqueous biphasic media was lower than that obtained in reversed micelles. However, better productivities were obtained in biphasic media, which were the most suitable environment. 

Lipophilicity is a molecular property expressing the relative affinity of solutes for an aqueous phase and an organic, water-immiscible solvent. As such, lipophilicity encodes most of the intermolecular forces that can take place between a solute and a solvent, and represents the affinity of a molecule for a lipophilic environment. This parameter is commonly measured by its distribution behavior in a biphasic system, described by the partition coefficient of the species X, P. Thermodynamically, is defined as a constant relating the activity of a solute in two immiscible phases at equilibrium [111,112]. By convention, P is given with the organic phase as numerator, so that a positive value for log P reflects a preference for the lipid phase ... 

As with classical multiphase catalysis, the organometallic catalyst is retained here in a liquid phase that is immiscible with the second phase containing substrates and/or products. For hydrogenation, the liquid/SCF system is always biphasic, whereas conventional systems are usually triphasic (liquid-1 /liquid-2/ H2). The liquid phase must provide a stable environment for the organometallic catalyst and should be insoluble in the SCF phase. Water, ILs and PEG have been used successfully for this purpose, together with scC02 as the mobile phase. Again, the products must not be too polar in order to be effectively extracted if C02 is used as the SCF. 

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

In case of ions, it is important to know the coordination environment to extract metals. UV-Vis spectroscopic measurements provide the respective information. There are systems in which both the biphasic extraction equilibrium and the metal coordination environment in an IL and a molecular organic solvent are the same [17]. 

This chapter shows how a biphasic medium can help in reducing loss of volatile compounds in a gaseous phase exiting from a bioreactor, in comparison with pure aqueous systems. It also emphasises the usefulness of solvents having low vapour pressure (heavy organic solvents or ionic liquids) in the reduction of the release of compounds into the environment. There are, from this point of view, common interests between engineering needs and environmental concerns in the flavouring industry. 

Nelson and Hummel 351) made a careful investigation of the urea transition near pH 7. Their absorbance difference spectra clearly show changes in the environment of phenylalanine as well as tyrosine residues. The optical rotation changes occur in the same range of urea concentrations. The loss of enzymic activity (U>p) is biphasic. At low urea con-... 

Neutral cyclodextrins have been used as chiral phase-transfer catalysts for an interesting inverse phase-transfer catalysis reaction [50]. The Markovnikovhydration of the double bond by an oxymercuration-demercuration reaction has been demonstrated in the presence of cyclodextrins as chiral phase-transfer catalysts to obtain products in low to moderate enantioselectivity (Scheme 7.16). The mercuric salts are water-soluble, and remain in the aqueous phase, whereas the neutral alkenes prefer an organic phase. A neutral cyclodextrin helps to bring the alkenes into the aqueous phase in a biphasic reaction, and also provides the necessary asymmetric environment. 

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