(334a) Evaluation of Thermodynamic Properties of Gas Mixtures Via the Virial Equation of State With Accurate Molecular Models

Experimental measurements of vapor pressures of heavy organic molecules can be performed via a gas saturation technique in a light carrier gas. The apparent vapor pressure based on an ideal-gas treatment of vapor mixture is found to depend on the carrier gas, indicating that treatment of non-ideality is needed to properly interpret the experimental data. Semiempirical methods can be applied, but a successful outcome is more likely if based on treatment that considers the specific molecular features of the component molecules.  The virial equation of state (VEOS) for mixtures presents a convenient approach to develop such an approach.

The VEOS for mixtures is given as a power series in density and mole fraction of each component, with coefficients relating to the interactions of corresponding number of molecules of each species. The pure virial coefficient of each component can be calculated via evaluation of cluster integral(s) dependent on the intermolecular interactions among molecules of the same species, whereas the cross virial coefficients are given as cluster integral(s) involving the interactions among unlike molecules species.

In previous work [1] we explored the Lennard-Jones binary mixtures using Mayer sampling Monte Carlo method [2], computing virial coefficients and examining vapor-liquid critical behavior. In the present work, we focus on more realistic molecular models, which requires us to re-develop the virial series for mixtures of non-rigid molecules. For such systems, it is necessary to consider cluster integrals that are not present in conventional formulations of the virial series (applicable to rigid molecules), because molecular flexibility prevents cancellation of cluster integrals that is usually assumed in the conventional  development [3,4].

We examine coefficients for nitrogen(N₂) and carbon dioxide(CO₂), which are sometimes used as carrier gases in experimental measurements. For the heavy organic, we examine n-alkanes as prototype molecules. Both the pure virial coefficients of every species and the cross virial coefficients of N₂/n-alkanes and CO₂/n-alkanes binaries are calculated up to the 5^thorder. We examine the convergence behavior of the VEOS for these binaries, and compare the thermodynamic properties of with available simulation and experimental data.

[1] A. J. Schultz and D. A. Kofke, J. Chem. Phys. 130, 224104 (2009).

[2] J. K. Singh and D. A. Kofke, Phys. Rev. Lett. 92, 220601 (2004).

[3] S. Caracciolo, B. M. Mognetti, and A. Pelissetto, J. Chem. Phys. 125, 094903 (2006).

[4] K. R. S. Shaul, A. J. Schultz, and D. A. Kofke, J. Chem. Phys. 135, 124101 (2011)