Molecular Modeling of Fluoropropene Refrigerants

Different fluoropropenes are currently considered as refrigerants, either as pure compounds or as components in low GWP (global warming potential) refrigerant mixtures. Due to their limited commercial production, experimental data for the thermophysical properties of fluoropropenes and their mixtures are in general rare, which hampers the exploration of their performance in technical applications. In principle, molecular simulation can be used to predict the relevant properties of refrigerants and refrigerant blends, provided that adequate intermolecular potential functions (“force fields”) are available. In our earlier work (Raabe, G.; Maginn, E. J., <i>J. Phys. Chem. B</i> <b>2010</b>, <i>114</i>, 10133–10142), we introduced a transferable force field for fluoropropenes comprising the compounds 3,3,3-trifluoro-1-propene (HFO-1243zf), 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf), and hexafluoro-1-propene (HFO-1216). In this paper, we provide an extension of the force field model to the trans- and cis-1,3,3,3-tetrafluoro-1-propene (HFO-1234ze­(E), HFO-1234ze) and the cis-1,2,3,3,3-pentafluoro-1-propene (HFO-1225ye­(Z)) as well as revised simulation results for HFO-1216. We present Gibbs ensemble simulation results on the vapor pressures, saturated densities, and heats of vaporization of these compounds in comparison with experimental results. The simulation results show that the force field model enables reliable predictions of the properties of the different fluoropropenes and also reproduces well the differing vapor–liquid coexistence and vapor pressure curve of the cis- and trans-isomers of 1,3,3,3-tetrafluoro-1-propene, HFO-1234ze and HFO-1234ze­(E). For these two isomers, we also present molecular dynamics simulation studies on their local structure

Molecular Modeling of Fluoropropene Refrigerants

Different fluoropropenes are currently considered as refrigerants, either as pure compounds or as components in low GWP (global warming potential) refrigerant mixtures. Due to their limited commercial production, experimental data for the thermophysical properties of fluoropropenes and their mixtures are in general rare, which hampers the exploration of their performance in technical applications. In principle, molecular simulation can be used to predict the relevant properties of refrigerants and refrigerant blends, provided that adequate intermolecular potential functions (“force fields”) are available. In our earlier work (Raabe, G.; Maginn, E. J., <i>J. Phys. Chem. B</i> <b>2010</b>, <i>114</i>, 10133–10142), we introduced a transferable force field for fluoropropenes comprising the compounds 3,3,3-trifluoro-1-propene (HFO-1243zf), 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf), and hexafluoro-1-propene (HFO-1216). In this paper, we provide an extension of the force field model to the trans- and cis-1,3,3,3-tetrafluoro-1-propene (HFO-1234ze­(E), HFO-1234ze) and the cis-1,2,3,3,3-pentafluoro-1-propene (HFO-1225ye­(Z)) as well as revised simulation results for HFO-1216. We present Gibbs ensemble simulation results on the vapor pressures, saturated densities, and heats of vaporization of these compounds in comparison with experimental results. The simulation results show that the force field model enables reliable predictions of the properties of the different fluoropropenes and also reproduces well the differing vapor–liquid coexistence and vapor pressure curve of the cis- and trans-isomers of 1,3,3,3-tetrafluoro-1-propene, HFO-1234ze and HFO-1234ze­(E). For these two isomers, we also present molecular dynamics simulation studies on their local structure

Efficient solvation free energy simulations: impact of soft-core potential and a new adaptive λ-spacing method

<p>The calculation of Gibbs free energy of solvation Δ<i>G</i> is commonly applied to predict the solubility of solutes in different solvents. The transition path between the thermodynamic end states of full and non-existent solute–solvent interactions is described by the spacing of intermediate states and the soft-core potential employed to scale the interactions. The choice of both, the soft-core potential and the distribution of intermediate states, has a large impact on the simulated Δ<i>G</i>. In this work, we determine the free energy of hydration for 10 neutral amino acid side chain analogues to analyse the impact of different soft-core potentials on the consistency of results from two different free energy methods, i.e. thermodynamic integration and free energy perturbation. Additionally, we propose a new method to improve the alchemical pathway by adaptive spacing of intermediate states. Our technique is easy to implement and not limited to any simulation package or free energy technique in particular, making it easily adaptable to a variety of workflows. Exploiting the ability to systematically optimise alchemical pathways, we introduce a scheme to define a suitable number of windows based on prescribed configurational space overlap. With this, excessive sampling is avoided and computational costs can be reduced.</p

A Force Field for 3,3,3-Fluoro-1-propenes, Including HFO-1234yf

The European Union (EU) legislation 2006/40/EC bans from January 2011 the cooperative marketing of new car types that use refrigerants in their heating, ventilation, and air conditioning (HVAC) systems with global warming potentials (GWP) higher than 150. Thus, the phase-out of the presently used tetrafluoroethane refrigerant R134a necessitates the adoption of alternative refrigerants. Fluoropropenes such as 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) are currently regarded as promising low GWP refrigerants, but the lack of experimental data on their thermophysical properties hampers independent studies on their performance in HVAC systems or in other technical applications. In principle, molecular modeling can be used to predict the relevant properties of refrigerants, but adequate intermolecular potential functions (“force fields”) are lacking for fluoropropenes. Thus, we developed a transferable force field for fluoropropenes composed of CF3−, −CF=, −CH=, CF2=, and CH2= groups and applied the force field to study 3,3,3 trifluoro-1-propene (HFO-1243zf), 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf), and hexafluoro-1-propene (HFO-1216). We performed Gibbs ensemble simulations on these three fluoropropenes to compute the vapor pressure, saturated densities, and heats of vaporization. In addition, molecular dynamics simulations were conducted to provide predictions for the density, thermal expansivity, isobaric heat capacity, and transport properties of liquid HFO-1234yf in the temperature range from 263.15 to 310 K and pressures up to 2 MPa. Agreement between simulation results and experimental data and/or correlations (when available) was good, thereby validating the predictive ability of the force field

Molecular Modeling of the Vapor−Liquid Equilibrium Properties of the Alternative Refrigerant 2,3,3,3-Tetrafluoro-1-propene (HFO-1234yf)

The European Union legislation 2006/40/EC results in a phase-out of the presently used tetrafluoroethane refrigerant R134a from automotive heating ventilation and air conditioning systems. This necessitates the adoption of alternative refrigerants, and 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) is currently regarded as the most promising alternative refrigerant. However, the lack of experimental data hampers independent studies on its performance in technical applications. We have developed a force field for HFO-1234yf that enables reliable predictions of its thermophysical properties via molecular simulation. The simulation results complement experimental data and provide a molecular-level perspective of the fluid behavior. In this letter we present the force field and its validation using Gibbs ensemble simulations on its vapor liquid equilibria