Medium-effect activity coefficients

See also (b) O. Popovich, Transfer activity coefficients (medium effects) in Treatise on Analytical Chemistry, Part 1, 2nd ed., (Eds. ... 

This coefficient has various names (medium effect, solvation activity coefficient, etc.) the name recommended by the responsible IUPAC commission is the transfer activity coefficient. In this book the effect of solvation in various solvents will be expressed exclusively in terms of standard Gibbs transfer energies. 

In this work the concepts of ionic medium, effective ionic strength and free versus total activity coefficients are examined. Then they are applied to the study of permissible and incorrect translations of equilibrium constants from one medium to another. 

Note that in all ion interaction approaches, the equation for mean activity coefficients can be split up to give equations for conventional single ion activity coefficients in mixtures, e.g., Eq. (6.1). The latter are strictly valid only when used in combinations that yield electroneutrality. Thus, while estimating medium effects on standard potentials, a combination of redox equilibria with H " + e 5112(g) is necessary (see Example 3). 

Ultraviolet spectra of benzoic acid in sulphuric acid solutions, published by Hosoya and Nagakura (1961), show a considerable medium effect on the spectrum of the unprotonated acid, but a much smaller one in concentrated acid. The former is probably connected with a hydrogen-bonding interaction of benzoic acid with sulphuric acid which is believed to be responsible for a peculiarity in the activity coefficient behaviour of unprotonated benzoic acid in these solutions (see Liler, 1971, pp. 62 and 129). The absence of a pronounced medium effect on the spectra in >85% acid is consistent with dominant carbonyl oxygen protonation. In accordance with this, Raman spectra show the disappearance in concentrated sulphuric acid of the carbonyl stretching vibration at 1650 cm (Hosoya and Nagakura, 1961). Molecular orbital calculations on the structure of the carbonyl protonated benzoic acid have also been carried out (Hosoya and Nagakura, 1964). 

A change of solvent may have an effect on the substrate, the enzyme, or the enzyme-substrate couple. In isotropic solvents the thermodynamic activity coefficients of the enantiomers will be equal. To a first approximation, one may assume that the activity coefficients of all the enzyme species relevant for enantioselection will be equal as well. This assumption is certainly less audacious than it may seem. For many enzymes, the substrate, once bound, is almost completely protected from the surrounding medium, making the solute-solvent interactions virtually... 

When this reasoning is applied to enantioselective enzymatic reactions, it follows that the ratio of specificity constants should not be affected by a change of medium that leads to different (but of course identical for the two enantiomers) values for the substrate activity coefficients. Indeed, solvent effects were not observed for,... 

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]. 

Equation 62 for Kf applies to all solvent mixtures and/p /Zw is identified as synonymous (from Equation 63) with the ratio of medium-effect activity coefficients ft /ft-So,... 

Strictly speaking, the term AG,° should be corrected for the secondary medium effect to take into account the fact that the activity coefficient of 0.1 M HC1 in 100 % methanol will not be the same, necessarily, as that 0.1 M HC1 in 98.5 % methanol-1.5 % water and in 97.5 % methanol-2.5 % water. 

Since an equilibrium is assumed between the transition state and the reactant(s), and because the corresponding equilibrium constant can be expressed in terms of activity coefficients and concentrations to account for the non-ideality of the medium, it follows that there should be an activity coefficient effect upon reaction rates. This is observed as a dependence of the rate constant upon ionic strength - the kinetic electrolyte effect [2]. Thus, for a bimolecular reaction,... 

Equation (96) now expresses the medium-dependence of the fractionation factor cj>LX (or K95). However, we note that the quotient of activity coefficients contains ratios of the form J/ha/2(da which really represent isotope effects on transfer activity coefficients. For this reason, the activity coefficient quotient in equation (96) is expected to vary less rapidly with the isotopic composition of the solvent than the factor Y. Furthermore as a practical step, the inclusion of the variation of fractionation factors due to the transfer effect is an unrealistic refinement at the present time. 

An equivalent approach uses activity coefficients of transfer and has been effectively applied to bimolecular reactions by Parker and co-workers (Parker, 1969).1 The medium effect for a bimolecular... 

Also called the medium effect, solvent activity coefficient, or transfer activity coefficient, and also written as y (MX, O —> S). It is a constant characteristic of the solute MX (or the solute ions M and X ) and the two solvents O and S. 

Once the standard potential of Cell I has been determined precisely, calculations of the mean activity coefficient, y , of HC1 and the primary and secondary medium effects using well-known relations are relatively simple tasks. Using empirical equations of the type E = a - - bT + cT2 and E° = a0 + b0T + c0T2, it is possible to calculate the molal enthalpies and heat capacities. These types of calculations are demonstrated in many... 

Because of difficulties in precisely calculating the total ion activity coefficient (y) of calcium and carbonate ions in seawater, and the effects of temperature and pressure on the activity coefficients, a semi-empirical approach has been generally adopted by chemical oceanographers for calculating saturation states. This approach utilizes the apparent (stoichiometric) solubility constant (K ), which is the equilibrium ion molal (m) product. Values of K are directly determined in seawater (as ionic medium) at various temperatures, pressures and salinities. In this approach ... 

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