equilibrium fractionation

Equilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. In general, equilibrium fractionations result from the reduction in vibrational energy (especially zero-point energy) when a more massive isotope is substituted for a less massive one. This leads to higher concentrations of the massive isotopes in substances where the vibrational energy is most sensitive to isotope substitution, i.e., those with the highest bond force constants.

In a reaction involving the exchange of two isotopes, ^lX and ^hX, of element “X” in molecules AX and BX,

$A^lX + B^hX \leftrightarrow A^hX + B^lX$

each reactant molecule is identical to a product except for the distribution of isotopes (i.e., they are isotopologues). The amount of isotopic fractionation in an exchange reaction can be expressed as a fractionation factor:

$\alpha = \frac{(^hX/^lX)_{AX}}{(^hX/^lX)_{BX}}$

$α = 1$ indicates that the isotopes are distributed evenly between AX and BX, with no isotopic fractionation. $α > 1$ indicates that ^hX is concentrated in substance AX, and $α < 1$ indicates ^hX is concentrated in substance BX. $α$ is closely related to the equilibrium constant (K_eq):

$\alpha = (K_{eq} \cdot \Pi \sigma_{Products}/ \Pi \sigma_{Reactants})^{1/n}$

where $Πσ P r o d u c t s$ is the product of the rotational symmetry numbers of the products (right side of the exchange reaction), $Πσ R e a c t a n t s$ is the product of the rotational symmetry numbers of the reactants (left side of the exchange reaction), and $n$ is the number of atoms exchanged.

An example of equilibrium isotope fractionation is the concentration of heavy isotopes of oxygen in liquid water, relative to water vapor,

$H_2^{16}O_{(l)} + H_2^{18}O_{(g)} \leftrightarrow H_2^{18}O_{(l)} + H_2^{16}O_{(g)}$

At 20^oC, the equilibrium fractionation factor for this reaction is

$\alpha = \frac{(^{18}O/^{16}O)_{Liquid}}{(^{18}O/^{16}O)_{Vapor}} = 1.0098$

References

Chacko T., Cole D.R., and Horita J. (2001) Equilibrium oxygen, hydrogen and carbon isotope fractionation factors applicable to geologic systems. Reviews in Mineralogy and Geochemistry, v. 43, p. 1-81.

Horita J. and Wesolowski D.J. (1994) Liquid-vapor fractionation of oxygen and hydrogen isotopes of water from the freezing to the critical temperature. Geochimica et Cosmochimica Acta, v. 58, p. 3425-2437.

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