Interfacial anomaly in low global warming potential refrigerant blends as predicted by molecular dynamics simulations.

Understanding the phase behavior and accurately predicting the thermophysical, interfacial and transport properties of low global warming, fourth generation refrigerants is essential for designing and evaluating refrigeration cycle performances and determining the optimal refrigerant or blends for a selected application. In this paper, we have used molecular dynamics simulations to study the vapour-liquid interface of fourth generation refrigerants including 2,3,3,3-tetrafluoropropene (HFO-1234yf), trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)), methylpropane (isobutane, HC-600a) and binary mixtures containing HFO-1234yf + HC-600a and HFO-1234ze(E) + HC-600a as new alternatives to third generation refrigerants. We provide predictions on their vapour-liquid equilibrium and interfacial properties (such as density profiles, interface thickness and surface tension) derived from the simulations. The results are compared to the experimental data, when available, and calculations made using the statistical associating fluid theory (SAFT). It is found that the mixtures of HFO-1234yf + HC-600a and HFO-1234ze(E) + HC-600a present azeotropic and aneotropic behavior. Molecular dynamics simulations corroborate the aneotrope already predicted by SAFT for these mixtures, highlighting the robustness of using molecular modeling techniques to investigate the performance of low GWP refrigerants and their blends as complementary tools to obtain the required data for the optimization of these systems. Insights into the molecular behavior at compositions before the aneotrope, at the aneotrope and after the aneotrope are provided based on radial distribution functions. It is shown that HC-600a and HFO molecules tend to stay closer to the same type of molecules and accumulate at different sides of the liquid region to act like pure components at the aneotropic composition.

The phase and interfacial properties of azeotropic refrigerants: the prediction of aneotropes from molecular theory.

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.

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

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.

Median nerve motor entry points in the forearm - clinical application

Superficial forearm flexors receive primary and secondary branches from the median nerve (mn). These branches vary in number, size and motor entry points (m) into the muscles. Knowledge of these points is essential for maximal compound muscle action potential (cmap) recording from these muscles. Spasticity of these flexors is treated using botulinum toxin (bt) injection or selective partial neurotomy (spn) of the nerve branches to the spastic muscles. Twenty human cadaveric forearms were dissected. The location of the motor entry points of the median nerve to the superficial forearm flexor muscles was expressed as a distance from the medial (me) and lateral (le) epicondyles of the humerus. Fifty apparently healthy volunteers (25 males and 25 females) underwent cmap recording from the superficial forearm flexors. Thirty patients (15 males and 15 females) with spastic hyperflexion of the wrist and fingers underwent bt injection or spn. Pronator teres (pt) had 2-4 m, flexor carpi radialis (fcr) had 1-3 m and flexor digitorum superficialis (fds) had 3-8 m. Variable shapes of the cmap were recorded from them (monophasic, biphasic or multiphasic). Based on the anatomical results, bt injection was done at 5 points (p1-p5); pt was injected at p1, fcr was injected at p2, and fds is a large muscle and was injected at p3, p4, p5 (proximal, middle, distal), giving good results in 85% of cases; spn was done in severe cases refractory to bt injection with excellent to good results in 80% of cases. The patterns of branching of mn differ from the classically described patterns. Therefore, revising the innervation patterns of the superficial forearm flexors is mandatory, since the variations observed are more diverse than has been described. Identification of the branches and the motor entry points of mn are essential for cmap recording from the superficial forearm flexors, bt injection, spn and tendon transfer

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

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.

Isolating the non-polar contributions to the intermolecular potential for water-alkane interactions.

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.