The phase diagram of water at high pressures as obtained by computer simulations of the TIP4P/2005 model: the appearance of a plastic crystal phase

In this work the high pressure region of the phase diagram of water has been studied by computer simulation by using the TIP4P/2005 model of water. Free energy calculations were performed for ices VII and VIII and for the fluid phase to determine the melting curve of these ices. In addition molecular dynamics simulations were performed at high temperatures (440K) observing the spontaneous freezing of the liquid into a solid phase at pressures of about 80000 bar. The analysis of the structure obtained lead to the conclusion that a plastic crystal phase was formed. In the plastic crystal phase the oxygen atoms were arranged forming a body center cubic structure, as in ice VII, but the water molecules were able to rotate almost freely. Free energy calculations were performed for this new phase, and it was found that for TIP4P/2005 this plastic crystal phase is thermodynamically stable with respect to ices VII and VIII for temperatures higher than about 400K, although the precise value depends on the pressure. By using Gibbs Duhem simulations, all coexistence lines were determined, and the phase diagram of the TIP4P/2005 model was obtained, including ices VIII and VII and the new plastic crystal phase. The TIP4P/2005 model is able to describe qualitatively the phase diagram of water. It would be of interest to study if such a plastic crystal phase does indeed exist for real water. The nearly spherical shape of water makes possible the formation of a plastic crystal phase at high temperatures. The formation of a plastic crystal phase at high temperatures (with a bcc arrangements of oxygen atoms) is fast from a kinetic point of view occurring in about 2ns. This is in contrast to the nucleation of ice Ih which requires simulations of the order of hundreds of ns

Everything you always wanted to know about SDPD⋆ (⋆but were afraid to ask)

An overview of the smoothed dissipative particle dynamics (SDPD) method is presented in a format that tries to quickly answer questions that often arise among users and newcomers. It is hoped that the status of SDPD is clarified as a mesoscopic particle model and its potentials and limitations are highlighted, as compared with other methods

Molecular Dynamics Simulation Study of Organic Solvents Confined in PIM-1 and P84 Polyimide Membranes

International audienceOrganic solvent nanofiltration (OSN) has recently proved to be a promising separation process thanks to the development of membrane materials with suitable resistance toward organic solvents. Among those materials, P84 polyimide membranes are currently the most used in OSN while PIM-1 membranes have recently attracted attention due to their high permeance in apolar solvents and alcohols. Both P84 and PIM-1 membranes have nanosized free volumes, and their separation performance is finely connected to polymer/solvent interactions. Consequently, modeling OSN membranes at the molecular scale is highly desirable in order to rationalize experimental observations and gain a deeper insight into the molecular mechanisms ruling solvent and solute permeation. A prerequisite for understanding solvent transport through OSN membranes is therefore to characterize the membrane/solvent interactions at the molecular level. For that purpose, we carried out molecular simulations of three different solvents, acetone, methanol, and toluene in contact with P84 and PIM-1 membranes. The solvent uptake by both membranes was found to be correlated to the degree of confinement of the solvent, the polymer swelling ability and polymer/solvent interactions. The translational dynamics of the solvent molecules in the PIM-1 membrane was found to be correlated with the solvent viscosity due to the relatively large pores of this membrane. That was not the case with the P84 membrane, which has a much denser structure than the PIM-1 membrane and for which it was observed that the translational dynamics of the confined solvent molecules was directly correlated to the affinity between the P84 polymer and the solvent

Interactions between methanol/toluene binary mixtures and an organic solvent nanofiltration PIM-1 membrane

International audienceIn this work, a molecular scale study of the interactions between a polymer with intrinsic microporosity (PIM-1) membrane and toluene, methanol and their mixtures was performed by means of molecular dynamics simulations. From the radial distribution functions we highlight specific interactions between the hydroxyl group of methanol and the aromatic ring of toluene as well as with the PIM-1 membrane. Moreover, the presence of nitrogen atoms on the PIM-1 backbone makes it possible the formation of hydrogen bonds like interaction (close contact of 2.5 Å) between the methanol molecules and the PIM-1 membrane, thus leading to interfacial anchoring of methanol at the polymer surface and moving toluene molecules away from the surface. However some toluene molecules are located around 3.5 Å due to π-stacking interaction between the aromatic rings of the PIM-1 membrane and those of the confined TOL molecules. At short range, the confined methanol molecules interact with each other like their bulk-phase counterparts whereas long range correlations highlight a existence of confined methanol aggregates. The specific interactions between toluene, methanol and the PIM-1 membrane result in a linear increase of the membrane swelling with the mole fraction of toluene in the binary mixtures. © 2022 Elsevier B.V

Coadsorption of CO2, CH4 and their binary mixture in NaY: a combination of molecular simulation with gravimetry-manometry measurements and microcalorimetry

International audienceThe adsorption of pure carbon dioxide and methane, and their equimolar mixture were explored in a model zeolite NaY by combining grand canonical Monte Carlo simulations and an original experimental approach based on gravimetry/manometry and manometry/microcalorimetry devices. Both experimental and calculated adsorption isotherms and enthalpy profiles obtained for the single gas and a 50/50 binary mixture are provided for pressures up to 30 bar. The gravimetry/manometry apparatus allows the determination of single gas and mixture isotherms whereas the manometry/microcalorimetry system allows a direct access to the enthalpies of adsorption enabling also a validation of the computational work. The aim here is to fully characterize the interactions of the gas mixture in NaY which are thus compared to those for the single gas adsorption. In this way, the computational studies allow an understanding of the complex adsorption phenomena in play. These calculations which are based on different interatomic potentials available in the literature to describe the adsorbate/adsorbent and adsorbate/adsorbate interactions, consist first of selecting the most convenient set of potential parameters able to reproduce well the adsorption data for the pure components. These reliable force fields are thus transferred to the study of the equimolar CO2/CH4 gas mixture. In agreement with the experiments, our simulations show a high selectivity of CO2 over CH4 in NaY for the whole range of pressure. We then report a complete exploration of the preferential adsorption sites for both adsorbates in the gas mixture which are compared to that observed for the pure component adsorption. The originality and the strength of this paper is thus to combine novel experimental tools and realistic models to provide a detailed picture of the microscopic adsorption and co-adsorption mechanisms in NaY