VT 2005 Sigma Profile Database

VT 2005 Sigma Profile Database for Using COSMO-Based Thermodynamic Methods

Eric Mullins¹, Richard Oldland¹, Y. A. Liu¹*, Shu Wang², Stanley I. Sandler²,
Chau-Chyun Chen³, Michael Zwolak¹, and Kevin C. Seavey¹

¹Honeywell Center of Excellence in Computer-Aided Design and
SINOPEC/FPCC/AspenTech Center of Excellence in Process Systems Engineering
Department of Chemical Engineering
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061

²Center for Molecular and Engineering Thermodynamics
Department of Chemical Engineering
University of Delaware
Newark, Delaware 19716

³Aspen Technology, Inc.
Cambridge, Massachusetts 02141

Email: pmullins@vt.edu (Mullins), roldland@vt.edu (Oldland), design@vt.edu (Liu),
wangs@che.udel.edu (Wang), sandler@udel.edu (Sandler), chauchyun.chen@aspentech.com (Chen),
zwolak@caltech.edu (Zwolak), kseavey@vt.edu (Seavey)

*Phone: (540) 231-7800; Fax: (540) 231-5022; E-mail: design@vt.edu
To whom correspondence should be addressed.

Abstract

Thermodynamic methods based on COSMO (conductor-like screening models) originated from using solvation thermodynamics and computational quantum mechanics. These methods rely on sigma profiles specific to each molecule. A sigma profile is the probability distribution of a molecular surface segment having a specific charge density. Two COSMO-based thermodynamic models are COSMO-RS (real solvents) developed by Klamt and his colleagues, and COSMO-SAC (segment activity coefficient) published by Lin and Sandler. Generating the sigma profiles represents the most time-consuming and computationally expensive aspect of using COSMO-based methods. A growing number of scientists and engineers are interested in the COSMO-based thermodynamic models, but are intimidated by the complexity of generating the sigma profiles. This paper presents the first free, web-based sigma profile database of over 1,200 compounds. We describe the procedure for sigma profile generation. We have validated our database by comparing COSMO-based predictions of activity coefficients, normal boiling point and heat of vaporization with experimental data and thermodynamic database. We discuss improvements which include using supplemental geometry optimization software packages to provide good initial guesses for molecular confirmations as a precursor to the COSMO calculation. Finally, this paper provides a FORTRAN program and a procedure to generate additional sigma profiles, as well as a FORTRAN program to generate binary phase-equilibrium predictions using the COSMO-SAC model. Our sigma profile database will facilitate predictions of thermodynamic properties and phase behaviors from COSMO-based thermodynamic models.

Sigma Profile Summary

COSMO models require sigma profiles in a manner similar to the way UNIFAC requires parameter databases, with one exception: sigma profiles are molecule-specific whereas UNIFAC parameters are functional group-specific. Furthermore, UNIFAC requires binary interaction parameters for the functional group pairs (binary interactions between groups). As a result, a sigma-profile database is much smaller and easier to use than the UNIFAC parameter database. Sigma profiles require only the molecular structure and a density-functional theory calculation to incorporate new compounds into the database.

The COSMO model creates a cavity around a solute molecule and then ideally dissolves it into an infinite conducting medium. We summarize this process with this figure:

We calculate the surface charge distribution or the SIGMA PROFILE. The sigma profile shows the probability of a surface segment having a specific charge density that determines the interaction energy with other surface segments in the liquid phase.