Thermodiffusion of CO2 in Water by Nonequilibrium Molecular Dynamics Simulations.

The components of a fluid mixture may segregate due to the Soret effect, a coupling phenomenon in which mass flux can be induced by a thermal gradient. In this work, we evaluate systematically the thermodiffusion of the CO2-H2O mixture, and the influence of the geothermal gradient on CO2 segregation in deep saline aquifers in CO2 storage. The eHeX method, a nonequilibrium molecular dynamics simulation approach, is judiciously selected to simulate the phenomenon. At 350 K, 400 bar, and CO2 mole fraction of 0.02 (aquifer conditions), CO2 accumulates on the cold side, and the thermal diffusion factor is close to 1 in a number of force fields. The lower the temperature, the higher is the separation and the thermal diffusion factor. In colder regions, water self-association is stronger, whereas the CO2-H2O cross-association and the CO2-CO2 interactions enhance at higher temperatures. Thermodiffusion and gravitational segregation have opposite effects on CO2 segregation. At typical subsurface conditions, the Soret effect is more pronounced than gravity segregation, and CO2 concentrates in the top (colder region). Our work sets the stage to model the effect of electrolytes on CO2 segregation in subsurface aquifers and other areas of interest.

Deriving force fields with a multiscale approach: From ab initio calculations to molecular-based equations of state.

Using theoretical and computational tools for predicting thermophysical properties of fluid systems and the soft matter has always been of interest to the physical, chemical, and engineering sciences. Certainly, the ultimate goal is to be able to compute these macroscopic properties from first-principles calculations beginning with the very atomic constitution of matter. In this work, Mie potential parameters were obtained through dimer interaction energy curves derived from ab initio calculations to represent methane and substituted-methane molecules in a spherical one-site coarse-grained model. Bottom-up-based Mie potential parameters of this work were compared with top-down-based ones from the statistical associating fluid theory (SAFT) models for the calculation of thermodynamic properties and critical point by molecular dynamics simulations and SAFT-VR Mie equation of state. Results demonstrated that bottom-up-based Mie potential parameters when averaging the Mie potential parameters of a representative population of conformers provide values close to the top-down-based ones from SAFT models and predict well properties of tetrahedral molecules. This shows the level of consistency embedded in the SAFT-VR Mie family of models and confers the status of a purely predictive equation of state for SAFT-VR Mie when a reasonable model is considered to represent a molecule of interest.

Phase behavior of hard cylinders.

Using isobaric Monte Carlo simulations, we map out the entire phase diagram of a system of hard cylindrical particles of length (L) and diameter (D) using an improved algorithm to identify the overlap condition between two cylinders. Both the prolate L/D > 1 and the oblate L/D < 1 phase diagrams are reported with no solution of continuity. In the prolate L/D > 1 case, we find intermediate nematic N and smectic SmA phases in addition to a low density isotropic I and a high density crystal X phase with I-N-SmA and I-SmA-X triple points. An apparent columnar phase C is shown to be metastable, as in the case of spherocylinders. In the oblate L/D < 1 case, we find stable intermediate cubatic (Cub), nematic (N), and columnar (C) phases with I-N-Cub, N-Cub-C, and I-Cub-C triple points. Comparison with previous numerical and analytical studies is discussed. The present study, accounting for the explicit cylindrical shape, paves the way to more sophisticated models with important biological applications, such as viruses and nucleosomes.

Impact of anion shape on Li+ solvation and on transport properties for lithium-air batteries: a molecular dynamics study.

Lithium-air batteries have emerged as an interesting alternative for advanced energy storage devices. The complexity of such systems imposes great challenges. One of them resides in the selection of the lithium salt/solvent pair. Many electrolyte properties affect the operation of the batteries. Among these, the transport properties and structural features have a special place. Via molecular dynamics simulations, we have calculated solution viscosity, ionic diffusivities and conductivities, as well as structural information, for two different salts in dimethyl sulfoxide (DMSO): lithium hexafluorophosphate - LiPF6, and lithium pyrrolide - LiPyr, at different temperatures and salt molalities. We show that, despite similar ionic transport properties, Li+ solvation in the different salts is significantly different. Therefore, solutions with different solvation properties, which impact the overall battery performance, might present analogous ionic dynamics.

Statistical Mechanical Model for Adsorption Coupled with SAFT-VR Mie Equation of State.

We extend the SAFT-VR Mie equation of state to calculate adsorption isotherms by considering explicitly the residual energy due to the confinement effect. Assuming a square-well potential for the fluid-solid interactions, the structure imposed by the fluid-solid interface is calculated using two different approaches: an empirical expression proposed by Travalloni et al. ( Chem. Eng. Sci. 65 , 3088 - 3099 , 2010 ), and a new theoretical expression derived by applying the mean value theorem. Adopting the SAFT-VR Mie ( Lafitte et al. J. Chem. Phys. , 139 , 154504 , 2013 ) equation of state to describe the fluid-fluid interactions, and solving the phase equilibrium criteria, we calculate adsorption isotherms for light hydrocarbons adsorbed in a carbon molecular sieve and for carbon dioxide, nitrogen, and water adsorbed in a zeolite. Good results are obtained from the model using either approach. Nonetheless, the theoretical expression seems to correlate better the experimental data than the empirical one, possibly implying that a more reliable way to describe the structure ensures a better description of the thermodynamic behavior.

Anisotropic parallel self-diffusion coefficients near the calcite surface: A molecular dynamics study.

Applying classical molecular dynamics simulations, we calculate the parallel self-diffusion coefficients of different fluids (methane, nitrogen, and carbon dioxide) confined between two {101̄4} calcite crystal planes. We have observed that the molecules close to the calcite surface diffuse differently in distinct directions. This anisotropic behavior of the self-diffusion coefficient is investigated for different temperatures and pore sizes. The ion arrangement in the calcite crystal and the strong interactions between the fluid particles and the calcite surface may explain the anisotropy in this transport property.