Group contribution methods effects

In general, properties that are additive and could be estimated by group contribution methods, such as density and heat of fusion, tend to follow the order of PET, PTT and PBT properties dependent on the conformational arrangement of the methylene units, such as modulus, show an odd-even effect, at least among these three polyesters. 

This is a key feature of the system for anyone who wants to understand and rationalize the effects of the microenvironment of a biocatalyst on its activity, its stability, or its specificity. Since for many years the use of thermodynamic activity was recommended for quantifying substrate availability in non-conventional media [17, 18], the replacement of concentrations of species by their thermodynamic activities in liquid non-conventional media requires a knowledge of their activity coefficients (y values). And this point is still far from being straightforward, as (a) values depend on molar ratios of other species present in the medium, and (b) methods used to estimate these values, such as UNI FAC group contribution method [19], are often called into question, and claimed to be sources of inaccuracy [20, 21]. 

In addition to matching bulk physical properties as already mentioned, it is also necessary to consider the activity coefficients to insure that the molecular interactions between the solutes and the solvent in the original and the substitute are generally similar. This insures that proposed substitute solvents will likely dissolve the same solutes and have similar effects to those of the original solvent. However, it is important to match only the activity coefBcients of the solutes in the solvents at in te dilution (zero solute concentration), so as not to include solute-solute interactions. The authors matched the activity coefficients at infinite dilution of a representative from six chemical families alcohols, ethers, ketones, water, normal alkanes, and aromatics, i.e., they have matched these activity coefficients in the solvent to be replaced to those in the replacement solvent. The particular components used are ethanol, diethyl ether, acetone, water, normal octane, and benzene. However, one could conceivably use different compounds successfully. Activity coefficients can be estimated from group contribution methods (77). 

Equation (2.1.6) fails completely for liquids such as water, methanol and formamide. This is not surprising, because in these liquids hydrogen bonds play a substantial role in providing cohesive forces whose effect is not included in the purely dispersive Hamaker constant. One approach to this problem, initiated by Fowkes (1962), has been to assume that each type of intermolecular force makes an additive contribution to the surface tension. So-called group contribution methods have been developed on the basis of this idea, which are of considerable practical use in classifying and predicting surface tension behaviour in a semi-empirical way. 

A column comprises individual separation stages in which the purification of the product is carried out by means of the effect that vapor and liquid have different compositions at equilibrium. Accordingly, the column design calls for knowledge of the phase equilibria of the systems [5]. Normally, phase equilibrium calculations are based on binary parameters describing the interactions of two different molecules. If multicomponent mixtures are considered, some of these interactions might be unknown. To obtain better simulation results, they should at least be estimated. This was the main reason for the development of the UNIFAC group contribution method twenty years ago. 

When looking at Figure 8, it is obvious that the nature of the solvent has an infiuence on the activity of the enzyme, even when the hydration of the enzyme is maintained constant by working at a fixed water activity. The parameter most frequently used to relate the nature of the solvent to its effect on enzyme-catalyzed reactions is log P, where P is the partition coefficient of the solvent between 1-octanol and water. This parameter, quantifying the hydrophobicity of organic molecules, can, of course, be measured experimentally, but it can also be calculated from group contribution methods, such as the hydrophobic fragmental content (44). 

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