(321ax) Equation of State Modeling of Polarizable Dipolar and Quadrupolar Mixtures

An appropriate description of polar interactions with an equation of state (EOS) not only improves the predictive performance of the model, but in many cases significantly improves the representation of pure-component and mixture phase equilibria. One prominent route toward a description of polar interactions is given through perturbation theories [1-4], where a known nonpolar reference fluid is defined and the polar contributions to the intermolecular interactions are considered as a perturbation. Gubbins and Twu and Luckas et al. [3,4] elaborated multipolar and nonspherical components, and their mixtures, and derived simple expressions for fluids with a Lennard–Jones (LJ) reference potential.

In previous studies [5-7] EOS contributions were developed for dipole-dipole and quadrupole-quadrupole interactions. The extension to dipole-quadrupole interactions is here presented. The expressions are based on a third order perturbation theory and are suitable for spherical and non-spherical (chain-like) molecular shape. Model constants of the polar terms were adjusted to molecular simulation data for vapor-liquid equilibria of the two-center Lennard-Jones plus multipole (2CLJM) fluid. The EOS is suited for both, 2CLJM fluids and the tangent-sphere Lennard-Jones framework. The Renormalized Perturbation Theory of Wertheim [8,9] is applied to account for induced dipoles due to molecular polarizability. The theory if found in excellent agreement when compared to molecular simulations of pure components and mixtures. The polar terms are applied to real compounds and mixtures in combination with the PC-SAFT equation of state. Polarizabilities, dipole and quadrupolar moments from independent sources can be adopted so that no additional parameter is introduced. The model is found to systematically improve pure component properties and the description of mixture phase equilibria. Dielectric constants of weakly polar and polarizable compounds are also well represented.

References

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[5] Gross J. An equation of state contribution for polar components: Quadrupolar molecules. AIChE J. 2005;51:2556-2568.

[6] Gross J, Vrabec J. An equation of state contribution for polar components: Dipolar molecules. AIChE J. 2006;52:1194-1204.

[7] Kleiner M, Gross J. An equation of state contribution for polar components: Polarizable dipoles. AIChE J. 2006;52:1951-1961.

[8] Wertheim M. Theory of molecular fluids III. Mol Phys. 1977;34:1109-1124.

[9] Wertheim M. Theory of polar fluids V. Thermodynamics and thermodynamic perturbation theory. Mol Phys. 1979;37:83-94.