Solvent activity coefficients Medium effects

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. 

Other names used for the medium effect are solvent activity coefficient (Chapter 6), degenerate activity coefficient and distribution coefficient. 

The methods used to establish values of have been considered elsewhere in this book (sect. 2.11.3). It is appropriate here only to mention the most widely applicable methods, where the solute is equilibrated separately with the solvents under consideration. The solvent activity coefficient or medium effect is then obtained from eqn. 2.11.13b (neglecting solute activity coefficients) ... 

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. 

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,... 

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,... 

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. 

Medium activity coefficients, cf. Section 5.5, can be used to discuss these effects. Fig. 15 provides a summary of the changes in solvation energy of ions and neutral molecules of various types in solvents which are representative for the solvent classes of Table I. The energy scale, RTln j y, with methanol as the reference solvent is taken from Ref. The non-measurable medium activity coefficient of the activated complex can be estimated from similar stable mol niles or ions. 

Yang, J.Z. (2005). Medium effect of an organic solvent on the activity coefficients of HCl consistent with Pitzer s electrolyte solution theory, /. Solution Chem, Vol. 1, No. 1, pp. 71-76, ISSN 0095-9782... 

The relevant equation for the dissection of solvent effects in terms of the Gibbs energy may be deduced from the Pronsted-Bjerrum equation for the reaction of Q and R through a transition state Tr(3-5,32,33). Here, k is the rate constant in a standard solvent and k is the rate constant in any other solvent. The terms and are primary medium activity coefficients. Equation can be recast as equation and the latter simplified by defining the... 

Extension to a nonideal system Minimization of free energy Thermodynamics of reactions in solution Partial molar properties Medium and substituent effects on standard free energy change, equilibrium constant, and activity coefficient General considerations Solvent and solute operators Comments... 

Medium effect (/" ) For solvents other than water the medium effect is the activity coefficient related to the standard state in water at zero concentration. It reflects differences in the electrostatic and chemical interactions of the ions with the molecules of various solvents. Solvation is the most significant interaction. 

Equation (2.43) describes the effect of long-range forces and howtheycan be modified by short-range interactions between ions. In a solution, however, short-range interactions between ions and solvent molecules need to be considered, and it has been found that such reactions have an approximate variation which is proportional to the concentration of the ionic medium. Therefore, the expression for the activity coefficient can be extended to... 

In contrast to the amides, (see V-C-2) the carboxylic acids act like well-behaved Hammett bases (325) ip indicator studies. We have already seen (II-D) that this is the result of a misleading fortuity and although the relative values for the pKa s of the benzoic acids are meaningful, this is one of several groups of bases for which the real standard state is unknown. In this connection, a recent study of 2,2-diphenic acid shows a log Q vs. Hn plot of slope 0.57 (246). Unfortunately, no attempt was made to correct for the usual medium effects, but one expects that this unusually bad slope is due to activity coefficient problems over and above solvent shifts. 

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