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Understanding fundamental solvation phenomena and mixture thermodynamic properties for organic molecules in ionic liquids is essential to the development of ionic liquids in many application areas. In the present work, molecular simulations were used to compute a wide range of properties for the pure ionic liquid trimethylbutylammonium bis(trifluoromethylsulfonyl)imide as well as mixtures of this ionic liquid with ethanol, 1-propanol, dimethylformamide, and dimethylsulfoxide. A new force field for the ionic liquid was developed and validated by computing ionic liquid surface tension and density as a function of temperature. Force fields for ethanol and propanol were taken from the literature, while new force fields were developed for dimethylformamide and dimethylsulfoxide. These force fields were shown to yield vapor-liquid coexistence curves, vapor pressure curves and critical points in excellent agreement with experimental data. Absorption isotherms, enthalpies of mixing and mixture volumes were then computed and shown to agree well with available literature. The simulations help rationalize the observed trends in solubility and enthalpy of mixing in terms of the relative strength of hydrogen bonding between the solutes and the ionic liquid. It was found that the entropy of absorption plays a very important role in the solvation process. The PCIP-SAFT equation of state was able to fit the experimental data (or simulation results when experiments were unavailable) very accurately with only small adjustable binary interaction parameters.
RESUMO
Transport properties of carbon dioxide and methane are predicted for temperatures between (273.15 and 573.15) K and pressures up to 800 MPa by molecular dynamics simulations. Viscosities and thermal conductivities were obtained through the Green-Kubo formalism, whereas the Einstein relation was used to provide self-diffusion coefficient estimates. The differences in property predictions due to the force field nature and parametrization were investigated by the comparison of seven different CO2 models (two single-site models, three rigid three-site models, and two fully flexible three-site models) and three different CH4 models (two single-site models and one fully flexible five-site model). The simulation results show good agreement with experimental data, except for thermal conductivities at low densities. The molecular structure and force field parameters play an important role in the accuracy of the simulations, which is within the experimental deviations reported for viscosities and self-diffusion coefficients considering the most accurate CO2 and CH4 models studied. On the other hand, the molecular flexibility does not seem to improve accuracy, since the explicit account of vibrational and bending degrees of freedom in the CO2 flexible models leads to slightly less accurate results. Nonetheless, the use of a correctional term to account for vibrational modes in rigid models generally improves estimations of thermal conductivity values. At extreme densities, the caging effect observed with single-site representations of the molecules restrains mobility and leads to an unphysical overestimation of viscosities and, conversely, to the underestimation of self-diffusion coefficients. This result may help to better understand the limits of applicability of such force fields concerning structural and transport properties of dense systems.
RESUMO
Ionic liquids are salts that are liquid near ambient conditions. Interest in these unusual compounds has exploded in the last decade, both at the academic and commercial level. Molecular simulations based on classical potentials have played an important role in helping researchers understand how condensed phase properties of these materials are linked to chemical structure and composition. Simulations have also predicted many properties and unexpected phenomena that have subsequently been confirmed experimentally. The beneficial impact molecular simulations have had on this field is due in large part to excellent timing. Just when computing power and simulation methods matured to the point where complex fluids could be studied in great detail, a new class of materials virtually unknown to experimentalists came on the scene and demanded attention. This topical review explores some of the history of ionic liquid molecular simulations, and then gives examples of the recent use of molecular dynamics and Monte Carlo simulation in understanding the structure of ionic liquids, the sorption of small molecules in ionic liquids, the nature of ionic liquids in the vapor phase and the dynamics of ionic liquids. This review concludes with a discussion of some of the outstanding problems facing the ionic liquid modeling community and how condensed phase molecular simulation experts not presently working on ionic liquids might help advance the field.
