Comparison of Different Organic Solvent-Water Systems

7 3 Comparison of Different Organic Solvent-Water Systems 

Compound i (Solute) ( ,/A)h Structure log, hw log KilVi log /Cdw log Kjcw log Klow 

When considering bipolar solutes (e.g., aniline, 1-hexanol, phenol, hexanoic acid), we can see that depending on the relative magnitudes of the solvent s at and /3, values, solute solvent interactions may become quite attractive. For example, for aniline, for which j6 trichloromethane is still the most favorable solvent, whereas for phenol (a, j6,), diethylether wins over the others. Finally, due to the lack of polar interactions in hexane, bipolar solutes partition rather poorly from water into such apolar solvents (Table 7.1). 

LFERs Relating Partition Constants in Different Solvent-Water Systems 

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The most common adsorption systems consist of silica gel or alumina adsorbents in association with an organic solvent system. The adsorbent can exert a considerable influence on the separation of compounds. Alumina and silica gel, for example, have significantly different properties and can result in quite different separations. Activation of the adsorbent also influences sample retention. The presence of water on the adsorbent decreases the adsorbent activity due to blockage of active sites. If large quantities of water are present, a partition system may be set up which may extensively change the retention times due to the different chromatographic principle involved. Table 2.1 compares results obtained for the separation of the insecticide carbaryl (Sevin) and its hydrolysis product 1-naphthol on alumina and silica gel. Comparisons between activation and deactivation are made. The results show that separation of the two components is reversed with the two adsorbents examined. In most cases, activation of the plates caused the/ f values to increase relative... 

The complex formation in PAA-PVP-methanol and PMAA-PVP-DMF systems has also been investigated44. The composition of the complexes in these solvents has been determined by conductometric and potentiometric methods. The titration curves in methanol and DMF (Fig. 15) exhibit a typical inflection point termining the molar ratio of the components in the complex. The complex compositions in organic solvents and water are different (Table 2). For comparison the titration curves in Fig. 15 of PAA with PVP in DMSO are illustrated. The solvent (DMSO) strongly competes for hydrogen bonds and it has been already shown48 that the complex is broken down. No inflection point on the titration curves is observed in this case. 

It is generally stated that biocatalysis in organic solvents refers to those systems in which the enzymes are suspended (or, sometimes, dissolved) in neat organic solvents in the presence of enough aqueous buffer (less than 5%) to ensure enzymatic activity. However, in the case of hydrolases water is also a substrate and it might be critical to find the water activity (a ) value to which the synthetic reaction (e.g. ester formation) can be optimized. Vahvety et al. [5] found that, in some cases, the activity of Candida rugosa lipase immobihzed on different supports showed the same activity profile versus o but a different absolute rate. With hpase from Burkholderia cepacia (lipase BC), previously known as lipase from Pseudomonas cepacia, and Candida antarctica lipase B (CALB) it was found that the enzyme activity profile versus o and even more the specific activity were dependent on the way the enzyme was freeze dried or immobihzed [6, 7]. A comparison of the transesterification activity of different forms of hpase BC or CALB can be observed in Tables 5.1 and 5.2, respectively. 

The comparison becomes more solid when the data for Fc /Fc and relative redox pairs in different solvents are considered in parallel [e.g., for bis biphenyl)chro-mium (1/0) (BCr /BCr) or for cobaltoceniumicobaltocene (CcVCc)]. A relative redox probe containing the anion (carborane compound, bis-o-dicarbollyl-nickel) was studied for comparison [12], A consideration of the potential differences in various solvents, rather than relative redox potential values, ehminates the UP. The tabulated values for differences between the formal potentials of BCr /BCr and Fc /Fc couples in 22 solvents with significantly different static permittivity values are equal within the accuracy of few mV [7]. Similarly, the differences of the formal potentials of Fc /Fc and CcVCc in five aprotic solvents and three aqueous-organic mixtures remain practically constant within the limits of experimental error [12], when the difference in water is somewhat larger, ca 40 mV. The comparison with aqueous systems is slightly risky, as the reactants tmder study are not so stable in water. 

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