Gibbs-Duhem integration

CURRENTLY THIS ARTICLE IS UNDER CONSTRUCTION

History

The so-called Gibbs-Duhem Integration referes to a number of methods that couple molecular simulation techniques with thermodynamic equations in order to draw phase coexistence lines.

The method was proposed by Kofke (Ref 1-2).

Basic Features

Consider two thermodynamic phases: $a$ and $b$, at thermodynamic equilibrium at certain conditions. The thermodynamic equilibrium implies:

• Equal temperature in both phases: $T=T_{a}=T_{b}$, i.e. thermal equilbirum.
• Equal pressure in both phases $p=p_{a}=p_{b}$, i.e. mechanical equilbrium.
• Equal chemical potentials for the components $\mu _{i}=\mu _{ia}=\mu _{ib}$, i.e. material equilibrium.

In addition if we are dealing with a statistical mechanics model, with certain parameters that we can represent as $\lambda$ , the model should be the same in both phases.

Example: phase equilibria of one-compoment system

Notice: The derivation that follows is just a particular route to perform the integration

• Consider that at given conditions of $T,p,\lambda$ two phases of the systems are at equilibrium, this implies:

$\mu _{a}\left(T,p,\lambda \right)=\mu _{b}\left(T,p,\lambda \right)$

Given the thermal equilibrium we can also write:

$\beta \mu _{a}\left(\beta ,\beta p,\lambda \right)=\beta \mu _{b}\left(\beta ,\beta p,\lambda \right)$

where

• $\beta =1/k_{B}T$, where $k_{B}$ is the Boltzmann constant

When a differential change of the conditions is performed we wil have for any phase:

$d\left(\beta \mu \right)=\left[{\frac {\partial (\beta \mu )}{\partial \beta }}\right]_{\beta p,\lambda }d\beta +\left[{\frac {\partial (\beta \mu )}{\partial (\beta p)}}\right]_{\beta ,\lambda }d(\beta p)+\left[{\frac {\partial (\beta \mu )}{\partial \lambda }}\right]_{\beta ,\beta p}d\lambda .$

Taking into account that $\mu$ is the Gibbs free energy per particle:

$d\left(\beta \mu \right)={\frac {E}{N}}d\beta +{\frac {V}{N}}d(\beta p)+\left[{\frac {\partial (\beta \mu )}{\partial \lambda }}\right]_{\beta ,\beta p}d\lambda .$

TO BE CONTINUED .. soon

References

1. David A. Kofke, Gibbs-Duhem integration: a new method for direct evaluation of phase coexistence by molecular simulation, Mol. Phys. 78 , pp 1331 - 1336 (1993)
2. David A. Kofke, Direct evaluation of phase coexistence by molecular simulation via integration along the saturation line, J. Chem. Phys. 98 ,pp. 4149-4162 (1993)