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

Abstract

The thermodynamics of hydrogen bonding in 1-alcohol + water binary mixtures is studied using molecular dynamic (MD) simulation and the polar and perturbed chain form of the statistical associating fluid theory (polar PC-SAFT). The fraction of free monomers in pure saturated liquid water is computed using both TIP4P/2005 and iAMOEBA simulation water models. Results are compared to spectroscopic data available in the literature as well as to polar PC-SAFT. Polar PC-SAFT models hydrogen bonds using single bondable association sites representing electron donors and electron acceptors. The distribution of hydrogen bonds in pure alcohols is computed using the OPLS-AA force field. Results are compared to Monte Carlo (MC) simulations available in the literature as well as to polar PC-SAFT. The analysis shows that hydrogen bonding in pure alcohols is best predicted using a two-site model within the SAFT framework. On the other hand, molecular simulations show that increasing the concentration of water in the mixture increases the average number of hydrogen bonds formed by an alcohol molecule. As a result, a transition in association scheme occurs at high water concentrations where hydrogen bonding is better captured within the SAFT framework using a three-site alcohol model. The knowledge gained in understanding hydrogen bonding is applied to model vapor-liquid equilibrium (VLE) and liquid-liquid equilibrium (LLE) of mixtures using polar PC-SAFT. Predictions are in good agreement with experimental data, establishing the predictive power of the equation of state.

A molecular equation of state for alcohols which includes steric hindrance in hydrogen bonding

“…In Fig. 4 we compare TPT1 and TPT2 predictions to MD simulations (same reference 28 and methodology as Fig. 3 for ethanol, with water modeled with the i-AMOEBA 43 The MD calculation of ( ) in Fig.…”

“…26 There has been much debate in the literature on whether alcohols should be modeled with two (one donor and one acceptor in a 2B scheme) or three association sites. 27,28 Recently, Fouad et al 28 demonstrated that within the polar PC-SAFT equation of state, the 2B scheme gave better agreement with hydrogen bonding distributions in pure alcohols. However, it was simultaneously shown that the 3C model gave better agreement for hydrogen bonding distributions for alcohol-water mixtures, as well as well as improved phase behavior predictions with water.…”

“…Comparison of MD simulations (circles)28 and theory predictions: TPT2-solid curve, TPT1-dashed curve of the fraction of ethanol molecules bonded k times in a saturated liquid…”

“…Comparison of model predictions (dashed curve-TPT1, solid curve-TPT2) to MD simulation results (circles)28 for the average number of bonds per ethanol molecule in a water/ethanol binary mixture at T = 298 K (left) and T = 373 K (right) Same asFig. 4, except for all results scaled by the average number of hydrogen bonds in pure ethanol(1) …”

A doubly associated reference perturbation theory for water

“…The exact interpretation of Luck's data is difficult. Application of TPT to water has met with mixed results 7,15,16,23,28,29 in the description of Lucks data. It has been shown 16,28 that…”