Spherically averaged versus angle-dependent interactions in quadrupolar fluids

TL;DR

This study compares spherically averaged and angle-dependent models of quadrupolar fluids, showing that simplified isotropic potentials can accurately reproduce key phase properties, especially when parameters are rescaled to match experimental critical points.

Contribution

It demonstrates that isotropic quadrupolar potentials, derived from perturbative expansion, can effectively model quadrupolar fluids, simplifying simulations without significant loss of accuracy.

Findings

Discrepancies near critical point are about 4% for properties like vapor pressure.

Rescaling Lennard-Jones parameters yields almost perfect agreement with experimental data.

Integral equation/density functional approach accurately describes the system except near criticality.

Abstract

Employing simplified models in computer simulation is on the one hand often enforced by computer time limitations but on the other hand it offers insights into the molecular properties determining a given physical phenomenon. We employ this strategy to the determination of the phase behaviour of quadrupolar fluids, where we study the influence of omitting angular degrees of freedom of molecules via an effective spherically symmetric potential obtained from a perturbative expansion. Comparing the liquid-vapor coexistence curve, vapor pressure at coexistence, interfacial tension between the coexisting phases, etc., as obtained from both the models with the full quadrupolar interactions and the (approximate) isotropic interactions, we find discrepancies in the critical region to be typically (such as in the case of carbon dioxide) of the order of 4%. However, when the Lennard-Jones parameters are rescaled such that critical temperatures and critical densities of both models coincide with the experimental results, almost perfect agreement between the above-mentioned properties of both models is obtained. This result justifies the use of isotropic quadrupolar potentials. We present also a detailed comparison of our simulations with a combined integral equation/density functional approach and show that the latter provides an accurate description except for the vicinity of the critical point.