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.

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.

Intermolecular potential models for water and alkanes describe pure component properties fairly well, but fail to reproduce properties of water-alkane mixtures. Understanding interactions between water and non-polar molecules like alkanes is important not only for the hydrocarbon industry but has implications to biological processes as well. Although non-polar solutes in water have been widely studied, much less work has focused on water in non-polar solvents. In this study we calculate the solubility of water in different alkanes (methane to dodecane) at ambient conditions where the water content in alkanes is very low so that the non-polar water-alkane interactions determine solubility. Only the alkane-rich phase is simulated since the fugacity of water in the water rich phase is calculated from an accurate equation of state. Using the SPC/E model for water and TraPPE model for alkanes along with Lorentz-Berthelot mixing rules for the cross parameters produces a water solubility that is an order of magnitude lower than the experimental value. It is found that an effective water Lennard-Jones energy ε(W)/k = 220 K is required to match the experimental water solubility in TraPPE alkanes. This number is much higher than used in most simulation water models (SPC/E-ε(W)/k = 78.2 K). It is surprising that the interaction energy obtained here is also higher than the water-alkane interaction energy predicted by studies on solubility of alkanes in water. The reason for this high water-alkane interaction energy is not completely understood. Some factors that might contribute to the large interaction energy, such as polarizability of alkanes, octupole moment of methane, and clustering of water at low concentrations in alkanes, are examined. It is found that, though important, these factors do not completely explain the anomalously strong attraction between alkanes and water observed experimentally.

in Wiley Online Library (wileyonlinelibrary.com)The recent global agreement signed in Kigali to limit the use of hydrofluorocarbons (HFCs) as refrigerants, starting by 2019, has promoted an active area of research toward the development of low global warming potential (GWP) new refrigerants. Hydrofluoroolefins (HFOs) have been proposed as a low GWP alternative to third generation HFC refrigerants, but further work on fully characterizing them and their blends with other compounds is still required to fully assess their performance to replace the ones in current use. In this work, the polar and perturbed chain statistical associating fluid theory coupled with the density gradient theory is used to predict the vapor-liquid equilibrium, isobaric heat capacity, speed of sound, and surface tension of selected HFC and HFO-based commercial azeotropic blends as fourth generation low GWP refrigerants, seeking for a predictive tool for these properties.

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The use of hydrofluorocarbons (HFCs) as alternative non-ozone depleting refrigerants for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) has grown during the last couple of decades. Owing to their considerable global warming potential, a global deal has been reached recently to limit the production and consumption of HFCs. For rational design of new refrigerants that are environmentally friendlier, the thermodynamics of current ones need to be well understood first. In this work, we examine the phase behavior of azeotropic refrigerants obtained by mixing HFCs with normal alkanes. The vapor-liquid equilibria (VLE) of these binary systems exhibit positive deviation from Raoult's law in the bulk, and a negative deviation from surface ideality (aneotrope) at the interface. The phase equilibria, second order thermodynamic derivative properties and interfacial properties of these complex systems were studied here using a modified version of the Statistical Associating Fluid Theory (SAFT) combined with Density Gradient Theory (DGT). The model was able to accurately capture the azeotropic nature of the phase equilibria and predict their composition and pressure at temperatures where experimental data are limited. In addition, accurate descriptions of the interfacial tensions were also obtained when compared with available experimental data, predicting the minimum found in surface tension as a function of composition. The molecular-based theory allowed the calculation of interfacial properties for which there is no experimental data available yet. Predictions show that the aneotrope occurs at a lower HFC composition for R-152a and R-134a systems in comparison to R-143a and R-125 systems. According to the calculated density profiles, HFC molecules appear to be preferentially adsorbed at the interface causing the surface tension of the n-alkane rich phase to decrease at low HFC concentrations. At high HFC concentrations, the phenomenon is inverted and n-alkane molecules are preferentially adsorbed causing the surface tension of the HFC rich phase to decrease. Consequently, the aneotrope point can be defined as the state at which the surface activity of both molecules is identical, or the relative adsorption of one component versus the other at the interface becomes zero.

Deviation from ideal solution behavior is due to differences in intermolecular interactions, e.g., molecular size, shape, dispersion, multipolar, and hydrogen bonding interactions. Activity coefficients characterize deviations from ideal solution behavior; however, most activity coefficient models lack the physics to explicitly account for intermolecular forces such as hydrogen bonding and multipolar interactions. At the same time, molecular theories (in the form of equations of state) that explicitly account for hydrogen bonding and multipolar interactions have been developed and validated versus molecular simulation results and experiment. In this work, we present a novel approach to calculate activity coefficients using theoretically based equations of state that explicitly describe these molecular interactions. The suggested approach enables phase equilibrium calculations to be performed at a significantly faster rate in comparison to conventional fugacity coefficient approaches. This is attributed to the fact that the molar volume is treated as an input in the approach; hence, iterations over the volume at a given pressure are not required. The polar and perturbed chain form of the statistical associating fluid theory (Polar PC-SAFT) is used in this paper to develop an activity coefficient model (SAFT-AC) for mixtures where self-association, solvation, and dipolar interactions dominate. Excellent agreement with experimental data was observed for all systems. The latter validates the use of the suggested approach as a powerful means for rapid phase equilibrium calculations in process simulators and PVT modeling software, providing consistency between activity coefficient and fugacity coefficient approaches. Extensions should lead to smart simulation methods where the simulation decides when to switch between activity coefficient and fugacity coefficient approaches depending on system conditions.