An Improvement to COSMO-SAC for Predicting Thermodynamic Properties

Abstract

A modified COSMO-SAC model is presented to calculate thermodynamic properties of pure fluids and mixtures using statistical thermodynamics and the surface charge density of each compound obtained from a quantum mechanics (QM) calculation. The main differences from the previous models are that the new model includes a dispersion contribution in the mixture interaction, and is reparametrized using measured pure and mixture thermodynamic data simultaneously. With a single set of universal parameters, the new model provides higher accuracy than our previous models for predicting mixture thermodynamic properties while maintaining the same accuracy for pure compound thermodynamic properties. The overall rootmean-square deviation (RMSD) in the logarithms of partition coefficients for 992 octanol−water partitioning systems and 829 other solvent−water partitioning systems with this new model is reduced by about 10% compared to the results from previous models. Also, the agreement between the predicted and measured partition coefficients over a wide range of values is improved as a result of better activity coefficient predictions at high dilution by inclusion of the dispersive mixture interaction in the model. The accuracy in the vapor−liquid equilibrium (VLE) predictions is comparable to, or better than, the previous model that was developed for phase equilibria calculations only. The new model also provides parameters for use with the Amsterdam Density Functional (ADF) in addition to DMol 3 .

Solvation quantities from a COSMO-RS equation of state

“…Yet and in spite these, COSMO-RS can form a good basis for the development of a predictive equation of state model. The above are not absolutely necessary in approaches where the excess Gibbs energy expression from COSMO-RS suffices for use in appropriate mixing rules for equation of state and related developments [12][13][14][15]40]. Simply, there is still much room for improvement in this already highly valuable COSMO-RS model.…”

“…COSMO-RS [2-9, [11][12][13][14], considers three major contributions to the intermolecular interaction energy arising from the corresponding London -dispersion, electrostatic, and strong-specific or hydrogen-bonding forces. In a sense, the electrostatic interactions encompass the Keesom dipoledipole and the Debye dipole -induced-dipole interactions, which we will collectively call polar interactions.…”

“…Any contact of two interacting segments m and n with charge densities σ m and σ n , respectively, is characterized by a pair interaction energy ε pair (σ m , σ n ). In COSMO-RS, the probability to find in the system volume a segment with charge density σ m is denoted as p (σ m (14) If n ij is the number of contacts between segments of type i and j, the total potential energy of our system is given by…”

“…which is entirely equivalent to the corresponding COSMO-RS or COSMO-SAC equation [2-9, [11][12][13][14]:…”

“…The important feature of Klamt's version of Quasi-Chemical theory is that it provided with a very efficient numerical algorithm for its solution in the case of complex multicomponent / multigroup systems. The recognition of the Quasi-Chemical groupcontribution character of COSMO-RS has opened the way to its multifaceted extension, including equation-of-state developments [9,[11][12][13][14][15]. These developments were, doubtlessly, a significant step forward in solution thermodynamics as it brings hard-core quantum mechanics and ab-initio calculations sufficiently distilled and refined in order to be used in a groupcontribution thermodynamic framework.…”