Isotope Effects on Solubility

At temperatures well below UCST, solubilities of hydrocarbons in water or water in hydrocarbons drop to very low values. The solutions are very nearly ideal in the Henry s law sense, and the isotope effects on solubility can be directly interpreted as the isotope effect on the standard state partial molar free energy of transfer from the Raoult s law standard state to the Henry s law standard state. Good examples include the aqueous solutions of benzene, cyclohexane, toluene,... 

Table 5.13 Solvent isotope effects on solubility of selected electrolytes in H20 Salt T/K Lh AL, %...

Van Hook, W. A. and Rebelo L. P. N. Isotope effects on solubility, in T. Letcher, T., ed., Developments and Applications in Solubility, Royal Society of Chemistry, Cambridge (2007)... 

Beck, T. L., Quantum path integral extension of Widom s test particle method for chemical potentials with application to isotope effects on hydrogen solubilities in model solids, J. Chem. Phys. 1992, 96, 7175-7177... 

Abstract Isotope effects on the PVT properties of non-ideal gases and isotope effects on condensed phase physical properties such as vapor pressure, molar volume, heats of vaporization or solution, solubility, etc., are treated in some thermodynamic detail. Both pure component and mixture properties are considered. Numerous examples of condensed phase isotope effects are employed to illustrate theoretical and practical points of interest. 

Table 5.3 Solute and solvent solubility isotope effects for (benzene-water) solutions at 306.2 K obtained from IE s on Henry s Law coefficients, Ki and Kn- [Isotope effects on free energies of transfer, ideal gas to solution in the limit of infinite dilution] (Dutta-Choudhury, M., Miljevic, N.

Table 5.11 H2O/D2O solvent isotope effects on gas solubility. Thermodynamics of transfer at 298.15 K, AY = Y (H2O) — Y (D20) (Scharlin, P. and Battino, R.. /. Solut. Chem. 21, 67 (1992). Fluid Phase Equlib. 94, 137 (1994))...

The statistical mechanical theory of the vapor pressure isotope effect can be applied to the interpretation of isotope effects on other phase equilibria (e.g., solid-liquid, liquid-liquid miscibility, solubility of gases in liquids, etc.). 

Whereas the differences in vapor pressure between isotopic substances have received much attention, relatively little is known about the isotope effects on the solubility of gases in liquids the available experimental information has been recently reviewed by Jancso (2002). The isotope effect is manifested in the isotope fractionation of natural gases, for example, in studies of the solubility of nitrogen and oxygen in water it was found that the heavier molecular species, and are more soluble in water than the respective lighter ones, N2 and... 

Because solvent viscosity experiments indicated that the rate-determining step in the PLCBc reaction was likely to be a chemical one, deuterium isotope effects were measured to probe whether proton transfer might be occurring in this step. Toward this end, the kinetic parameters for the PLCBc catalyzed hydrolysis of the soluble substrate C6PC were determined in D20, and a normal primary deuterium isotope effect of 1.9 on kcat/Km was observed for the reaction [34]. A primary isotope effect of magnitude of 1.9 is commonly seen in enzymatic reactions in which proton transfer is rate-limiting, although effects of up to 4.0 have been recorded [107-110]. 

There is a good deal of data which compares the solubilities of H2 and D2 in various solvents, but by far the most extensive information on gas solubility IE s compares gas solubilities in H20 and D20 (Table 5.11). The solubility of D2 is considerably higher than H2 in all solvents investigated (Table 5.12). The isotope effect increases sharply as temperature falls (although not shown in Table 5.12). The isotope effect increases sharply as temperature falls (although not shown in Ta-... 

Concentrations of aqueous electrolyte solutions are conventionally expressed using the aquamolality scale (L = moles salt per 55.508 mol solvent (l,000g for H20)). Some typical solubilities (298.15K) are listed in Table 5.13. Almost all salts are less soluble in D20 than in H20. For those salts whose solubility increases with temperature, which is the ordinary behaviour, the isotope effects decrease with temperature. Writing the standard state partial molar free energy of pure solid salt as Pxsalt) and its standard state in solution as p, (HorD) we have on comparing the saturated solutions in H20 and D20,... 

Methane is slightly soluble in HF-SbF5 even at atmospheric pressure (0.005 M), which facilitates direct kinetic studies by NMR. Thus the transition states for methane activation in this medium have been studied experimentally by Ahlberg et al.49 The first-order rate constants [Eqs. (5.9) and (5.10)], determined experimentally on the basis of2H -decoupled 600-MHz 1H NMR time-dependent spectra (Figure 5.2), are on the order of 3.2 x 10 1 s 1 at —20°C and show a secondary kinetic isotope effect (SKIE) of 1 0 02. 

In supercritical HjO it was found that the toluene-soluble products (TS) profile leveled off at about 50%. However in supercritical D2O, liquefaction was consistently superior to that in the protio medium, and the ultimate convertibility was about 60%. This inverse isotope effect can be explained by a model in which the limits to conversion are solely based on the competition between the kinetics of the parallel routes to TS and char, respectively. Conversion is thus limited by the kinetics of the conversion system, this limitation being even more severe in conventional donor systems which are inherently less effective for liquefaction than is water/CO. In principle the conversion of coal virtually completely to TS in a single step is feasible in a system with sufficient reducing potential. 

The hydrolysis of steroid hydrogen sulphates, important to biochemists in view of the excretion of many steroid hormone metabolites as water soluble salts of hydrogen sulphates, has been studied in some detail [20,21]. It had been known for some years that ether extraction of the steroid hydrogen sulphate from acidified solutions is accompanied by hydrolysis of the sulphate in the ether phase, with liberation of the steroid alcohol. This hydrolysis was shown to follow first-order kinetics, but to be dependent in a remarkable way on the solvent. Ethers are the most effective solvents, followed by others of low polarity including benzene, esters and ketones, while water and alcohols have a strong retarding effect on hydrolysis. Neither acids nor bases produce significant catalysis, and the presence of did not result in incorporation of isotopic... 

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