Vapour-Liquid binary phase diagrams
The following examples demonstrate some of COSMOtherm's capacities:
Figures 1 - 3 demonstrate COSMOtherm capability to predict all kinds of vapour-liquid equilibrium (VLE) thermodynamical properties at the example of the binary mixture of the system n-heptane (1) - 1-butanol (2). Experimental data for Figures 1-3: A. Gusovius, Diplomarbeit, TU Darmstadt, Germany, 1997; C. W. Smith and E. W. Engel, J. Amer. Chem. Soc. 51 2660 (1929).
Figures 4 and Figure 6 demonstrate the application of COSMOtherm to the problem of compounds showing different isomeric structures with differing chemical characteristics at the example of the VLE properties of the three n-hexyne isomers mixed with n-octane and the mixture of two nitrile ester compounds. Whereas the treatment of isomers in COSMOtherm is as simple as the treatment of any other molecule, such systems cannot easily be treated by group contribution methods like UNIFAC !
Figures 5 demonstrate the qualitatively as well as quantitatively correct prediction of thermodynamical properties at different temperatures at the system 3-hexyne - n-octane. Experimental data for Figures 4: G. Boukais-Belaribi, B. F. Belaribi, A. Ait-Kaci and J. Jose, Fluid Phase Equilibria 167 (2000) 83-97.
All COSMOtherm calculations were done using Turbomole BP-TZVP-COSMO with the standard parametrization for this method/basis set combination.
Figure 1 shows the binary vapour-liquid phase diagram of the n-heptane (1) - 1-butanol (2) system at T=50 °C;
x = mole fraction of 1-butanol in the liquid phase.
y = mole fraction of 1-butanol in the gas phase.
Figure 2 shows the activity coefficients of the n-heptane (1) - 1-butanol (2) system at T=50°C;
Figure 3 shows the excess enthalpies (H-Excess), free energies (G-Excess) and entropies (S-Excess, times temperature T) of the n-heptane (1) - 1-butanol (2) system at T=50°C. The experimental data was fitted to a polynomial representation in order to allow for a comparison on the experimental and calculated TS-Excess=G Excess-H-Excess values. As is visible from Figure 1, COMSOtherm is able to predict the quite unusual course of TS-Excess qualitatively correct. Taking into account the relatively small absolute values of the excess properties in this system, the quantitative correlation of experimental and predicted COSMOtherm values is excellent.
Figures 4 show the excess enthalpies (HE) and binary vapour-liquid phase diagram of the three isomeric n-hexynes (1) in n-octane (2) at T=30°C. The good correspondence between experiment and the COSMOtherm prediction is obvious. In addition, COSMOtherm is able to reproduce the qualitative differences between the hexyne isomers, which results form the chemically different environment of the triple-bond in 1-hexyne (a hydrogen atom terminating the triple bond, which can be expected to be slightly acidic) and 2- and 3-hexyne (no terminal hydrogen, thus very similar chemical behaviour). Note, that such isomeric effects can not be reproduced by group contribution methods like UNIFAC (UNIFAC predictions are also given in the Figures). Apart from COSMOtherm only thermodynamical models such as DISQUAC are applicable to this system, however to the price of having to fit experimental data of the binary system to the DISQUAC model and thus losing the generality and independence from experimental data which is a primary quality of COSMOtherm. The COSMOtherm predictions of GE and HE are in good agreement with experiment as well as with a DISQUAC study done for the hexyne - n-octane system (compare G. Boukais-Belaribi et al. Fluid Phase Equilibria 167, 83 (2000)).
Figures 5 show the excess free energies (GE), the binary vapour-liquid composition diagram and the activity coefficients of the system 3-hexyne (1) - n-octane (2) at three different temperatures between T=-10°C and T=+60°C. For all goal properties correspondence between experiment and COSMOtherm calculations is very good, qualitatively as well as quantitatively.
Figure 6 show the binary vapour-liquid composition diagram and the COSMO surfaces of two isomeric nitrile-ester compounds (Exp. data providedby C. Rose, Lonza Group, CH). COSMOtherm is able to resolve the very small differences in electronic effects the isomers!
