(381e) A Novel Approach of Modeling Orientational Hydrogen Bonding Interactions in Associating Fluids with the COSMO-SAC Activity Coefficient Model

Based on solid theoretical
background in statistical thermodynamics and quantum mechanics, the COSMO-SAC
model is a powerful predictive thermodynamic model which can provide reliable thermodynamic
properties and phase behavior of mixture fluids. The model has been applied in
many problems such as vapor-liquid equilibrium, liquid-liquid equilibrium, drug
solubility, partition coefficient ionic liquid screening, etc
over the last decades. The core knowledge in COSMO-SAC model is how to evaluate
the interaction between molecules on the basis of surface segments whose
properties such as position and charge density can be determined from quantum
mechanical calculation. In recent years, some refinements such as including the temperature dependence on
electronic interactions and differentiating the different types of hydrogen
bonding interactions are introduced to improve the accuracy of COSMO-SAC model
(Mu, Rarey et al. 2007). In this work, we propose a
new approach which allows for consideration of spatial orientation of hydrogen
bonds in the COSMO-SAC model. Instead of using a cutoff charge or a predefined
hydrogen bonding atoms, we adopt the concept of molecular orbital, Valence
Shell Electron Pair Repulsion theory (PETRUCCI, R.H 2002), to identify the
hydrogen bonding surfaces (Fig. 1). The
inclusion of molecular orbital information allows for specific the hydrogen
bond interactions within certain spatial orientations and results in a more
accurate description regarding the vapor-liquid equilibrium (VLE) of mixtures
containing different hydrogen bonding specie. We examined 35 associating fluids
(whose include water, alcohols, amines and amides) in total 598 mixtures (7583
data points, temperature range from 263.15K to 548.15 K). The
prediction accuracy is found to be 6.75% (AARD-P%) and 2.67% (AAD-y%) in
pressure and vapor composition, respectively. The results are more accurate
than the original model (7.16% and 2.83%) (Hsieh, Sandler et al. 2010),
while the number of universal parameters used are reduced from 10 to 9.

(a)	(b)	(c)

Figure
1. (a) The
structure of water molecule and its lone pair describing by VSEPR theory. (b)
The screening charge distribution on water surface. (c) The spatial allowance in hydrogen bond restricted by the
lone pair within a cut-off radius.

References
1.     Hsieh,
C.-M., S. I. Sandler and S.-T. Lin (2010). "Improvements of COSMO-SAC for
vapor¡Vliquid and liquid¡Vliquid equilibrium predictions." Fluid Phase
Equilibria 297(1): 90-97.

2.     Mu,
T. C., J. Rarey and J. Gmehling
(2007). "Performance of COSMO-RS with sigma profiles from different model
chemistries." Industrial & Engineering Chemistry Research 46(20):
6612-6629.

3.     PETRUCCI,
R.H., HARWOOD, W.S. and HERRING, F.G. (2002). General ChemistryPrinciples
and Modern Applications, 8th edn. Prentice Hall, Upeer Saddle River, NJ.