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Interfacial Properties of Tetrahydrofuran and Carbon Dioxide Mixture from Computer Simulation C

Algaba, Jesús, Garrido, José Matías, Míguez, José Manuel, Mejía, Andrés, Moreno-Ventas Bravo, A. Ignacio, Blas, Felipe J.
Journal of physical chemistry 2018 v.122 no.28 pp. 16142-16153
Monte Carlo method, adsorption, asymmetry, carbon dioxide, computer simulation, liquid-air interface, liquids, models, molecular dynamics, prediction, surface tension, temperature, tetrahydrofuran
We determined the interfacial properties of the tetrahydrofuran + carbon dioxide (THF + CO₂) binary mixture from direct simulation of the vapor–liquid interface. We consider two different models of THF based on the transferable parameters for phase equilibria-united atom (TraPPE-UA) version approach. In the first case, we use the original (flexible) TraPPE-UA model force field for the ether [J. Phys. Chem. B 2012, 115, 11234]. The second model is a planar, rigid, and approximated TraPPE-UA model recently proposed by us [J. Chem. Phys. 2016, 144, 144702]. It is demonstrated that the sophisticated flexible model does not have an overall advantage in comparison with the simplified planar model. Indeed, both models are able to predict the phase behavior and interfacial tension of pure THF with high accuracy in a wide range of temperatures and pressures. It is noticed that the planar model is faster for simulation because the internal degrees of freedom are frozen (neither bending nor torsional intramolecular interactions need to be evaluated). Most of the simulations were performed in the molecular dynamics canonical ensemble, and the vapor–liquid interfacial tension is evaluated from the normal and tangential components of the pressure tensor according to the mechanical virial route. We also used the Gibbs ensemble Monte Carlo simulation technique to determine the phase behavior of the mixture under selected conditions to compare the results from both methodologies. In addition to the interfacial tension, we also obtained density profiles and pressure–density and pressure–composition slices of the phase diagram of the mixture at different temperatures and pressures. Simulation results obtained from both models are able to accurately predict the vapor–liquid phase envelope of the system, which is in good agreement with the experimental results. We also compare the predictions for interfacial tension, as obtained from simulation results for the two models, with the experimental data. Agreement between computer simulation predictions and experiments for the interfacial tension of THF + CO₂ mixtures, as a function of pressure, is excellent in nearly all cases. In addition to that, the density profiles associated with carbon dioxide exhibit a relative maximum related to preferential adsorption at the liquid interface of THF. This accumulation in the interface is probably related to the asymmetry in the size of the components of the mixture.