Correspondence pubs.acs.org/IECR
Comment on “Comparison of the a Priori COSMO-RS Models and Group Contribution Methods: Original UNIFAC, Modified UNIFAC(Do), and Modified UNIFAC(Do) Consortium” Andreas Klamt*,†,‡ †
COSMOlogic GmbH and Co. KG, Burscheider Str. 515, 51381 Leverkusen, Germany Institute of Physical and Theoretical Chemistry, University of Regensburg, 93053 Regensburg, Germany
‡
he article, “Comparison of the a Priori COSMO-RS Models and Group Contribution Methods: Original UNIFAC, Modified UNIFAC(Do), and Modified UNIFAC(Do) Consortium”, by Xue, Mu, and Gmehling,1 describes a comparison of COSMO-RS-type models and UNIFAC-type models for the prediction of activity coefficients and derived vapor−liquid equilibria (VLE) properties of mixtures of organic compounds. Unfortunately, this paper is biased toward the UNIFAC type of models. The bias can be seen in the following points: (1) For UNIFAC, three variants are considered: the standard UNIFAC; the mod-UNIFAC, which was developed ∼20 years ago; and the latest version of the commercially available mod-UNIFAC(Do) consortium implementation, which most likely is the best version of UNIFAC currently available. On the other hand, for COSMO-RS, two rather similar reimplementations of COSMO-RS have been chosen, which definitely do not mark the stateof-the-art of COSMO-RS. No attempt has been made to get hold of the most advanced implementation of COSMO-RS (i.e., COSMOtherm),2 which is available to academia at a rather low price, and even for free in its COSMOtherm-Demo version. Since, for UNIFAC, even an up-to-date commercial version has been used, it is completely unbalanced, when, for COSMO-RS, only older, publically available remakes of the method are taken into account. (2) The entire comparison is made on data taken from the commercial DDBST databank. It must be emphasized that exactly this database is the basis for the modUNIFAC parametrizations. This means that the comparison is made on the training set of one of the two methods, which is completely unacceptable for a scientific comparison. All of the ∼5000 adjusted parameters of mod-UNIFAC have been adjusted to this database. Given the flexibility of such a multiparameter fit with ∼5000 adjustable parameters, it would be absolutely required to make the comparison on a dataset that was unseen to the parametrization. Furthermore, the DDBST is strongly biased toward data for mixtures of simple compounds, e.g. alkane−alcohol, alkane−ketones, etc., while more-complicated compounds, e.g., multiply substituted compounds with intramolecular interacting functional groups, or heterocycles are strongly underrepresented. Since UNIFAC is strong on the simple compounds but gets considerably worse on the latter, because it cannot take into account the intramolecular
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interactions, this is again a bias favoring UNIFAC. This biased selection is further intensified by the fact that the authors only took into account compounds that could be treated by all five of the methods. Since COSMO-RS can treat essentially every molecule, this restriction has reduced the dataset to those compounds that can be treated even by Original UNIFAC, i.e. to very simple standard compounds, while the strength of COSMO-RS is its ability to treat the complicated compounds with almost the same accuracy as the simple compounds. (3) In addition, the authors seem to have excluded systems with very high infinite dilution activity coefficients. They argue that this is due to data uncertainty. However, it should be noted that UNIFAC is known to perform much worse on systems with high infinite dilution activity coefficients than COSMO-RS does. It usually strongly underestimates such data.3 The reason for this is partially due the inherent problem of UNIFAC that it does not solve the quasi-chemical equations exactly, but instead uses the UNIQUAC approximation, which automatically leads to an underestimation of infinite dilution activity coefficients for highly nonideal systems.4 The insufficient performance of the UNIFAC approach in this limit can also be seen from the fact that special infinite-dilution-UNIFAC versions have been fitted, e.g., a special KOW-UNIFAC.5 In summary, leaving out this end of the data from DDBST means leaving out a part of the data for which there was the danger that UNIFAC might perform less well. (4) The examples selected for the graphs mentioned below are absolutely not representative of the overall performance of the COSMO-RS method. Considering alkane− perfluoroalkane mixtures with these simple remakes of COSMO-RS means to apply these clearly without their range of applicability, because the excess dispersion interaction of fluoro−fluoro contacts over alkane−fluoro contacts cannot be reflected in simple COSMO-RS versions. The COSMOtherm implementation of COSMO-RS does not have problems with alkane− perfluoroalkane mixtures, because of the extension described in chapter 7.1.2 of the COSMO-RS book, COSMO-RS: From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design.6 This most-detailed description of COSMO-RS is missing in the list of references of the commented paper. For all four example Published: October 8, 2012 13538
dx.doi.org/10.1021/ie302247q | Ind. Eng. Chem. Res. 2012, 51, 13538−13540
Industrial & Engineering Chemistry Research
Correspondence
VLEs and the corresponding five graphs given in the commented paper, I have provided (in Figures 1−6) the
Figure 3. : Experimental and predicted γ∞ values for ethanol in hexane, as a function of temperature. (Corresponding to Figure 4b of ref 1.) The COSMOtherm curve (solid blue) matches the experimental data very well, not as good as mod-UNIFAC, but better than UNIFAC and the COSMO-RS remakes.
Figure 1. Experimental and predicted γ∞ values for ethanol in water, as a function of temperature. (Corresponding to Figure 3 of ref 1.) The COSMOtherm line (solid blue) underestimates log γ by ∼0.2, but consistently describes the temperature dependence (in contrast to UNIFAC and the COSMO-RS remakes).
Figure 4. Vapor−liquid equilibrium (VLE) of the hexane (1) + 1propanol (2) system at 323.15 K. (Corresponding to Figure 5 of ref 1.) The COSMOtherm curve (solid blue) is in very good agreement with the experiments. Figure 2. Experimental and predicted γ∞-values for hexane in ethanol as a function of temperature. (Corresponding to Figure 4a of ref 1.) The COSMOtherm line (solid blue) is in as good agreement as the mod-UNIFAC curve, where the latter shows a curvature that does not seem to be justified by the experimental data.
results of the latest commercial COSMOtherm version (BP-TZVPP-FINE parametrization, released in Dec. 2011). In all cases, COSMOtherm provides much better agreement with the experiment than the considered remakes of COSMO-RS. Hence, almost all of the points made by the authors do not apply to advanced versions of COSMO-RS. Summarizing, in the commented paper, the comparison presented is strongly biased toward UNIFAC. The latest and most advanced (even commercial) versions of UNIFAC are compared, based on the parametrization dataset of UNIFAC, with older, non-state-of-the-art remakes of COSMO-RS, while the most advanced (commercial) version of the original developer of COSMO-RS is not considered. It is shown that most of the results derived in this comparison do only apply to the simple COSMO-RS remakes considered here, but not to
Figure 5. VLE data of the perfluorohexane (1) + hexane (2) system at 318.15 K. (Corresponding to Figure 6 of ref 1.) The COSMOtherm curve (solid blue) is not in as good agreement with experiment as the UNIFAC curves, but it is much closer than the results of the simple COSMO-RS remakes.
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dx.doi.org/10.1021/ie302247q | Ind. Eng. Chem. Res. 2012, 51, 13538−13540
Industrial & Engineering Chemistry Research
Correspondence
(5) Wienke, G.; Gmehling, J. Toxicol. Environ. Chem. 1998, 65, 57− 86. (6) Klamt, A. COSMO-RS: From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design; Elsevier: Amsterdam, 2005. (7) MedChem Database, available from Daylight, Inc., Claremont, CA, www.daylight.com/products/medchem.html. (8) PHYSPROP Database, 2010; Syracuse Research Corporation (SRC): Syracuse, NY, 2010.
Figure 6. y−x diagram of the perfluorohexane (1) + hexane (2) system at 318.15 K. (Corresponding to Figure 7 of ref 1.) COSMOtherm (solid blue line) gives strong improvement over the simple COSMO-RS versions.
COSMOtherm. Hence, the generalization/conclusion that “The results show that UNIFAC models based on experimental data are superior to the a priori COSMO-RS models.” is warranted by the content of the paper. It would be easy to select a dataset, e.g., octanol−water partition coefficients taken from the MedChem7 or PhysProp database,8 on which UNIFAC surely would have much larger errors than COSMOthermeven more so if we would neglect the latest mod-UNIFAC version. Without any doubt, UNIFAC is a useful tool for data interpolation within simple, structurally related compounds, and it will always be more accurate on these than a priori predictive models; however, on the other hand, science and industrial practice often confront the researcher with morecomplicated and new molecules, for which UNIFAC cannot provide good answers. On those compounds, the a priori prediction method COSMO-RS, especially in its most advanced implementation, COSMOtherm, provides robust predictions. Hence, a comparison such as the one presented in the commented paper1 appears to be of questionable value.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares the following competing financial interest(s): Andreas Klamt is the inventor of the COSMO-RS method and he develops and distributes the COSMOtherm implementation of COSMO-RS out of his company COSMOlogic GmbH & Co. KG.
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REFERENCES
(1) Xue, Z.; Mu, T.; Gmehling, J.; Comparison of the a priori COSMO-RS models and Group Contribution Methods: Original UNIFAC, Modified UNIFAC(Do) as well as Modified UNIFAC(Do) Consortium. Ind. Eng. Chem. Res. 2012, 51, 11809−11817 (DOI: 10.1021/ie301611w). (2) Eckert, F.; Klamt, A. 2011. COSMOtherm, Version C2.1, Release 01.12; COSMOlogic GmbH & Co. KG, Leverkusen, Germany. (3) Voutsas, E. D.; Tassios, D. P. Prediction of Infinite-Dilution Activity Coefficients in Binary Mixtures with UNIFAC. A Critical Evaluation. Ind. Eng. Chem. Res. 1996, 35, 1438−1445. (4) Klamt, A.; Krooshof, G. J. P.; Taylor, R. COSMOSPACE: Alternative to conventional activity-coefficient models. AIChE J. 2002, 48, 2332−2349. 13540
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