Understanding the Thermodynamics of Hydrogen Bonding in Alcohol-Containing Mixtures: Self Association

Abstract

The perturbed chain form of the polar statistical associating fluid theory (Polar PC-SAFT) was used to model lower 1-alcohol + n-alkane mixtures. The ability of the equation of state to predict accurate activity coefficients at infinite dilution was demonstrated as a function of temperature. Investigations show that the association term in SAFT plays an important role in capturing the right composition dependence of the activity coefficients in comparison with nonassociating models (UNIQUAC). Results also show that considering long-range polar interactions can significantly improve the fractions of free monomers predicted by PC-SAFT in comparison with spectroscopic data and molecular dynamic (MD) simulations carried out in this work. Furthermore, evidence of hydrogen-bonding cooperativity in 1-alcohol + n-alkane systems is discussed using spectroscopy, simulation, and theory. In general, results demonstrate the theory's predictive power, limitations of first-order perturbation theories, as well as the importance of considering long-range polar interactions for better hydrogen-bonding thermodynamics.

Relation of Wertheim association constants to concentration-based equilibrium constants for mixtures with chain-forming components

“…Because these approaches differ considerably, there is significant scatter in the resulting values for the same alcohol [47]. For CPA and SAFT, the Δ values are typically determined by fitting density and vapor pressure [23,25,54].…”

Extensions of the SAFT model for complex association in the bulk and interface

“…As shown above and in Fouad et al [39], SAFT results depend on the number of association sites chosen and the effect of bond cooperativity is not captured. Wertheim's first order perturbation theory has limitations on modeling associating fluids [4][5][6][7].…”

“…Table 1 show small differences in association energy, we assume that the association energy for all alcohols are approximately equal to that of pure ethanol. Assuming that the mass densities are not too different, SAFT predicts that all of the monomer fraction data should overlap when plotted against the molar concentration of association sites multiplied by the ratio of the Mayer f-function at a certain temperature to the Mayer f-function at 298 K as a reference [39]. As shown in Figure 7, the spectroscopic data for the monomer fractions from methanol to 1-hexanol alcohols in different solvents overlap.…”

“…This section aims to examine the ability of PC-SAFT to accurately predict thermodynamics of mixtures containing associating components. Details of the model will be published shortly [39]. The accuracy of the theory for the extent of hydrogen bonding can be shown by comparing free monomer fractions predicted by the theory to spectroscopic data measured by Asprion et al [40] and von Solms et al [41].…”

“…Next, we examine the significance of bond cooperativity in predicting accurate hydrogen bonding distribution in a 1-alcohol + n-alkane system. To do so, Polar PC-SAFT and MD simulation were used by Fouad et al [39] to predict the fraction of molecules bound in dimers for 1-butanol + n-hexane binary system. Results were also compared to scattered spectroscopic data measured by by Asprion et al [40].…”