Choice of the organic solvent

Some basic requirements apply regarding the choice of organic solvent in SLM systems. 

if an aqueous system is involved,. solubility in the aqueous pha.se should be extremely low and the volatility should also be low. In addition, the organic liquid must be a solvent for both the carrier and the carrier-solute complex. 

Another important factor is the viscosity of the organic phase since the presence of a carrier or carrier-solute complex increases the viscosity of the liquid phase in many cases. The effect of the viscosity on the diffusion coefficient can be illustrated by the Stokes-Einstein equation which shows that the diffusion coefficient is inversely proportional to die viscosity, i.e. 

On increasing the carrier concentration, two effects are once again counteracting. On the one hand, the flux will increase (see eq. VI - 89), on the other hand an increasing carrier concentration will increase the viscosity, hence reducing the diffusion coefficient and leading to a decreased flux. 

Another ver severe problem with SLM is the instability of the liquid film with time which causes the process to cease because of loss of the organic phase. Although it is essential for the solubility of the organic phase in the aqueous phase to be as low as possible, even if the solubility meets this requirement or even if the aqueous phase is saturated with the solvent the process becomes unstable after a finite period of time. 

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The HCl by-product of the amidation reaction is neutralized by also dissolving an inorganic base in the aqueous layer in interfacial polymerization. The choice of the organic solvent plays a role in determining the properties of the polymer produced, probably because of differences in solvent goodness for the resulting polymer. Since this reaction is carried out at low temperatures, the complications associated with side reactions can be kept to a minimum. 

The choice of the organic solvent is very important in controlling the polymer molecular weight, since it appears that the polymerization actually occurs on the organic solvent side of the interface in most systems. The reason for this is the greater tendency of the diamine to... 

The mobile phase in RP chromatography contains water and one or more organic solvents, most frequently acetonitrile, methanol, tetrahyrofuran, or propanol. By the choice of the organic solvent, selective polar interactions (dipole-dipole, proton-donor, or proton-acceptor) with analytes can be either enhanced or suppressed, and the selectivity of separation can be adjusted. Binary mobile phases are usually well suited for the separation of a variety of samples, but ternary or, less often, quaternary mobile phases may offer improved selectivity for some difficult separations. The retention times are controlled by the concentration of the organic solvent in the aqueous-organic mobile phase. Equation 1 is widely used to describe the effect of the volume fraction of methanol or acetonitrile

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Various factors affect the enantioselectivity in such hydrolyses. These include the concentration and the choice of the organic solvent used.30 In another, pretreatment of the lipase with 2-propanol raised the enantioselectivity 25-fold to 93% ee.31 In an esterification of menthoi, coating the... 

Addition of a base to the aqueous phase removes the hydrochloric acid that forms and catalyzes the reaction. The choice of the organic solvent is important, because it appears that the reaction occurs on the organic side of the interface [28]. 

When one develops new reversed-phase (RP)-HPLC methods, one usually uses the selectivity of the mobile phase as the primary method development tool. The chromatographic separation can be influenced by the choice of the organic solvent (mainly methanol and acetonitrile), or by variation of pH or buffer type. Schemes for method development using these parameters have been described in the literature [1,2]. Most important are the selectivity changes caused by pH changes, which are well-understood and easily predictable (3). It is well known that the stationary phase influences the selectivity as well, but this effect is often not very well understood. The primary reason for this is the fact that reliable methods for the description of the stationary phase selectivity have only become available fairly recently. In the last few years, several papers have been published that deal with the subject of selectivity in a fimdamental way [4—9] or represent a data collection based on older methods [10-15]. In this chapter, we describe in detail the method used in our laboratory. We then look at our selectivity charts and discuss our results. It needs to be pointed out in advance that selectivity charts only accurately represent the properties of a stationary phase under the conditions of the measurement. If we depart from the mobile phase composition of the test, the relationships between different columns will change, since selectivity arises from a combined effect of the mobile phase and the stationary phase. 

Liquid-liquid interfaces for ITIES research are limited by the choice of the organic solvent that must, of course, be immiscible with water and able to dissolve electrolytes. As a consequence, electrochemistry at ITIES is often limited to the H2O-NB, H20-1,2-DCE, water-heptanone, and water-2-nitrophenyloctylether (NPOE) systems, the last two having been developed for their low toxicity. 

Frequently the choice of the mobile phase equates to methanol/water or acetonitrile mixtures in various proportions. The next step is to optimize the concentration of the organic solvent. Following that, low concentrations of tetrahydrofuran are explored to further improve separations (see Fig. 15.8). 

Predictably, the association between the ion-pair and the substrate is influenced by the choice of the organic phase and by the reaction temperature. Polar solvents will not only affect the interaction between the catalysts and substrate, they will also reduce the association of the ion-pair with a resultant increase in free anion over which there is no stereochemical control (Table 12.4). 

Thus, to conduct successful analyses for many organic and inorganic compounds at trace concentrations, it is necessary to extract these compounds and use a concentration step prior to analysis. Many of the techniques developed for preconcentration are described in specialized books [10]. Proper choice of the extracting solvent can often be the critical step in the procedure. 

By appropriate choice of the type (or combination) of the organic solvent(s), selective polar dipole-dipole, proton-donor, or proton-acceptor interactions can be either enhanced or suppressed and the selectivity of separation adjusted [42]. Over a limited concentration range of methanol-water and acetonitrile-water mobile phases useful for gradient elution, semiempirical retention equation (Equation 5.7), originally introduced in thin-layer chromatography by Soczewinski and Wachtmeister [43], is used most frequently as the basis for calculations of gradient-elution data [4-11,29,30] ... 

Reaction engineering helps in characterization and application of chemical and biological catalysts. Both types of catalyst can be retained in membrane reactors, resulting in a significant reduction of the product-specific catalyst consumption. The application of membrane reactors allows the use of non-immobilized biocatalysts with high volumetric productivities. Biocatalysts can also be immobilized in the aqueous phase of an aqueous-organic two-phase system. Here the choice of the enzyme-solvent combination and the process parameters are crucial for a successful application. 

The choice of an organic solvent for a given reaction can be determined by three main factors [23] ... 

From the results reviewed above, one might get the impression that the choice of an organic solvent that optimizes the enantioselectivity of the enzyme in a given resolution reaction is a matter of tedious trial and error, with little guidance from established rules or insights. In practice, however, one has to consider several mitigating circumstances. In many cases of interest, the choice will be limited to a relatively small number of solvents that are either industrially approved or readily available in the laboratory. Since most practical resolutions start from a racemic mixture obtained by chemical synthesis, batch-mode enrichment requiring relatively modest 5-values will be an attractive method. In that case, solubility and easy... 

The derivatization reaction is performed at ambient temperature by simply mixing the aqueous sample extract witli a phosphate buffer of appropriate pH and then adding the fluorescamine solution in acetonitrile under vigorous stirring. Acetonitrile is the solvent of choice for preparing fluorescamine solutions, because tlie net fluorescence decreases witli a decrease in polarity of the organic solvent in the order acetonitrile, acetone, dioxane, and tetrahydrofuran. 

The catalyst system can be reused if an ionic liquid-water or ionic liquid-water-tert-butanol solvent system is used [155-157]. Some care has to be taken with the choice of ligand under these conditions, as the presence of an alkene within the ligand itself results in dihydroxylation during the reaction and, as a result, the modified ligand is not extracted from the ionic liquid into the organic layer [158]. Supercritical C02 can be used in place of the organic solvent to extract the product [159, 160]. 

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