A density functional theory for vapor-liquid interfaces using the PCP-SAFT equation of state

Journal article (2009)

Authors

Department
Process and Energy (Mechanical, Maritime and Materials Engineering) (TU Delft)
Copyright: © 2009 Gross, J. ; American Institute of Physics
DOI: https://doi.org/doi:10.1063/1.3263124
Statistical analysis Density functional theory Surface tension Perturbation theory Equations of state Functional analysis Organic compounds Carbon compounds Helmholtz equations Capillary waves
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Published Date

25-11-2009

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Source:

Journal of Chemical Physics, 131 (20), 2009

ISSN:

0021-9606

Source:

http://link.aip.org/link/JCPSA6/v131/i20/p204705/s1

Faculty

Mechanical, Maritime and Materials Engineering

Department

Process and Energy

Abstract

A Helmholtz energy functional for inhomogeneous fluid phases based on the perturbed-chain polar statistical associating fluid theory (PCP-SAFT) equation of state is proposed. The model is supplemented with a capillary wave contribution to the surface tension to account for long-wavelength fluctuations of a vapor-liquid interface. The functional for the dispersive attraction is based on a nonlocal perturbation theory for chain fluids and the difference of the perturbation theory to the dispersion term of the PCP-SAFT equation of state is treated with a local density approximation. This approach suggested by Gloor et al. [Fluid Phase Equilib. 194, 521 (2002)] leads to full compatibility with the PCP-SAFT equation of state. Several levels of approximation are compared for the nonlocal functional of the dispersive attractions. A first-order non-mean-field description is found to be superior to a mean-field treatment, whereas the inclusion of a second-order perturbation term does not contribute significantly to the results. The proposed functional gives excellent results for the surface tension of nonpolar or only moderately polar fluids, such as alkanes, aromatic substances, ethers, and ethanoates. A local density approximation for the polar interactions is sufficient for carbon dioxide as a strongly quadrupolar compound. The surface tension of acetone, as an archetype dipolar fluid, is overestimated, suggesting that a nonisotropic orientational distribution function across an interface should for strong dipolar substances be accounted for.

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