Virial equation of state

The virial equation of state is used to describe the behavior of diluted gases. It is usually written as an expansion of the compressibility factor, $Z$, in terms of either the density or the pressure. Such an expansion was first introduced in 1885 by Thiesen and extensively studied by Heike Kamerlingh Onnes , and mathematically by Ursell . One has

${\frac {pV}{Nk_{B}T}}=Z=1+\sum _{k=2}^{\infty }B_{k}(T)\rho ^{k-1}$.

where

• $p$ is the pressure
• $V$ is the volume
• $N$ is the number of molecules
• $T$ is the temperature
• $k_{B}$ is the Boltzmann constant
• $\rho \equiv {\frac {N}{V}}$ is the (number) density
• $B_{k}\left(T\right)$ is called the k-th virial coefficient

Virial coefficients

The second virial coefficient represents the initial departure from ideal-gas behaviour

$B_{2}(T)={\frac {N_{A}}{2V}}\int ....\int (1-e^{-\Phi /k_{B}T})~d\tau _{1}d\tau _{2}$

where $N_{A}$ is Avogadros number and $d\tau _{1}$ and $d\tau _{2}$ are volume elements of two different molecules in configuration space.

One can write the third virial coefficient as

$B_{3}(T)=-{\frac {1}{3V}}\int \int \int f_{12}f_{13}f_{23}dr_{1}dr_{2}dr_{3}$

where f is the Mayer f-function (see also: Cluster integrals). See also

Convergence

For a commentary on the convergence of the virial equation of state see and section 3 of .

Quantum virial coefficients

Using the path integral formulation one can also calculate the virial coefficients of quantum systems .

References

Related reading

• James A Beattie and Walter H Stockmayer "Equations of state", Reports on Progress in Physics 7 pp. 195-229 (1940)
• Edward Allen Mason and Thomas Harley Spurling "The virial equation of state", Pergamon Press (1969) ISBN 0080132928
• Nathaniel S. Barlow, Andrew J. Schultz, Steven J. Weinstein and David A. Kofke "Analytic continuation of the virial series through the critical point using parametric approximants", Journal of Chemical Physics 143 071103 (2015)