(490i) Co-Oriented Fluid Functional Equation for Electrostatic Interactions

The description and prediction of thermo-physical properties has undergone substantial progress in the past decades especially using equations of state (EOS). Still, there are many substances, which cannot be described with this kind of theory. This is especially true, if not only pure compounds, but also mixtures are considered. Substances that have a polar character and tend to form local structures have turned out to be especially challenging to model. Probably the best-known substance from this class is water.

In this contribution, focus lies on much simpler molecules, namely two model fluids:

a) The Stockmayer fluid with a Lennard-Jones (LJ) sphere and a central dipole.

1. b) A model with a LJ sphere and a dipole shifted from the center along its axis.

Despite their simplicity, no EOS is currently available that can model all these fluids.

A new perturbation approach is presented that allows incorporating the unknown structure of the dipolar models in the form of their orientation distribution function (ODF) without the need for iterations. The resulting EOS – Co-Oriented Fluid Functional Equation for Electrostatic interactions (COFFEE) [1] – has a functional part describing the impact of ODF on the free energy and a non-functional part consisting of the usual ideal part, the reference part and a far field contribution where it is assumed that outside the first coordination shell no ordering occurs. The missing parameters of the functional part are fit to molecular simulation data of model a)’s ODF. For verification purposes, the ODFs of fluid b) are predicted. The remaining parameters of the far field are fitted to the vapor liquid equilibrium (VLE) data for fluid a) and again the VLE data for fluid b). Results are very good for both ODF and VLE for low to intermediate dipole moments.

In addition to these properties, the dielectric constant is predicted from the orientation data for the Stockmayer fluid with μ^* = 1.0 and compared to molecular simulation data of Kohns [2]. The results are in good mutual agreement. Furthermore, the theory is applied to hydrogen chloride for which it shows better performance than literature models using PC-SAFT or PCP-SAFT.

References

[1] K. Langenbach, Chem. Eng. Sci., 174, 40-55 (2017).

[2] M. Kohns, K. Langenbach, in preparation.