Anne Boutin

Molecular simulation studies of water physisorption in zeolites.

We report a series of Grand Canonical Monte Carlo simulations of water adsorption in NaY and NaX faujasite, as well as in silicalite-1. Computed adsorption isotherms and heats of adsorption were in good agreement with the available experiments. The existence of cyclic water hexamers in NaX located in the 12-ring windows, recently disclosed by neutron diffraction experiments (Hunger et al., J. Phys. Chem. B, 2006, 110, 342-353) was reproduced in our simulations. Interestingly enough, such cyclic hexamer clusters were also observed in the case of NaY, in which no stabilizing cation is present in the 12-ring window. We also report cation redistribution upon water adsorption for sodium faujasite with varying cation contents (Si ratio Al ratio in the range 1.53-3). A simple and transferable forcefield was used, that enabled to reproduce the different aspects of water physisorption in stable zeolites. The high pressure water condensation in hydrophobic silicalite-1 was reproduced without any parameter readjustment. The method and forcefield used here should be useful for engineering oriented applications such as the prediction of multi-component mixture adsorptive separations in various stable zeolites. It allows to address the issue of the effect of the small amounts of water that are almost inevitably present in zeolite-based separation processes.

Direct Monte Carlo simulations of the equilibrium properties of n-pentane liquid–vapor interface

Direct MC calculations have been carried out to study the liquid–vapor equilibrium of n-pentane. We have used the local long range correction to the configurational energy within the Metropolis scheme and an algorithm allowing us to select randomly with equal probability two different maximum displacements. The thermal equilibrium conditions are checked by calculating the profiles of the configurational temperature in the vapor and liquid phases. We also check the mechanical equilibrium by calculating the profiles of the normal and tangential pressure components. The normal pressure profile is constant through the interface and in both phases on the conditions that the two parts of the long range corrections to the normal pressure area included in the calculations. The critical densities and temperatures are well predicted and the vapor pressures agree satisfactory with the experimental values within larger statistical fluctuations. The long range corrections to the surface tension are calculated using va...

Monte Carlo versus molecular dynamics simulations in heterogeneous systems: an application to the n-pentane liquid-vapor interface.

The Monte Carlo (MC) and molecular dynamics (MD) methodologies are now well established for computing equilibrium properties in homogeneous fluids. This is not yet the case for the direct simulation of two-phase systems, which exhibit nonuniformity of the density distribution across the interface. We have performed direct MC and MD simulations of the liquid-gas interface of n-pentane using a standard force-field model. We obtained density and pressure components profiles along the direction normal to the interface that can be very different, depending on the truncation and long range correction strategies. We discuss the influence on predicted properties of different potential truncation schemes implemented in both MC and MD simulations. We show that the MD and MC profiles can be made in agreement by using a Lennard-Jones potential truncated via a polynomial function that makes the first and second derivatives of the potential continuous at the cutoff distance. In this case however, the predicted thermodynamic properties (phase envelope, surface tension) deviate from experiments, because of the changes made in the potential. A further readjustment of the potential parameters is needed if one wants to use this method. We conclude that a straightforward use of bulk phase force fields in MD simulations may lead to some physical inconsistencies when computing interfacial properties.

From molecular clusters to bulk matter. I. Structure and thermodynamics of small CO2, N2, and SF6 clusters

The thermodynamics and structural properties of small molecular XN clusters (X=N2, CO2, and SF6) are investigated using molecular dynamics simulations. In this paper we compare the behavior of carbon dioxide, nitrogen, and sulfur hexafluoride for a given cluster size of N=13. Evidence is provided for “dynamical coexistence” between solidlike and liquidlike forms of the cluster, in a finite energy range, in the case of (CO2)13 and (N2)13 but not (SF6)13. In addition (N2)13 exibits a solid–solid phase transition characterized by the release of the molecular orientational degree of freedom. A systematic use of the dynamic quenching method enables us to interpret these different behaviors in terms of the energy distribution of minima in the potential energy surface of the systems. A comparison of the strain energies of these clusters, using a model recently proposed by Wales and co-workers, enables us to understand why different molecular clusters exhibit different crossover points from icosahedral to bulk pr...

Vapour-Liquid Phase Equilibria of n-alkanes by Direct Monte Carlo Simulations

Abstract We report results of direct Monte Carlo simulations of n-pentane and n-decane at the liquidvapour interface for a number of temperatures. The intermolecular interactions are modeled using the last version of the anisotropic united atom model (AUA4). We have used the local long range correction energy and an algorithm allowing to select randomly with equal probability two different displacements. The liquid and vapour densities are in excellent agreement with experimental data and with those previously calculated using the GEMC method.

Molecular Simulation of Vapour-Liquid Coexistence Curves for Hydrogen Sulfide-Alkane and Carbon Dioxide-Alkane Mixtures

Abstract Gibbs ensemble Monte Carlo simulations, combined with the configurational-bias technique applied to alkane chains, were performed to calculate hydrogen sulfide-alkane and carbon dioxide-alkane liquid-vapour phase equilibria for several n-alkane molecules (propane, pentane and decane). Recently proposed effective pair potentials that describe accurately the pure components coexistence curves were used in the simulations. It is shown that the use of these force fields together with the Lorentz-Berthelot mixing rules yield a description of the coexistence curves of these binary mixtures that is in good agreement with the experimental data is most cases. Some deviations from the experimental results were observed in the range of high H2S mole fraction and high pressure. Further work is needed in order to improve the level of accuracy of potential models for fluid mixtures of non polar and multipolar molecules.

Thermodynamic study of water confinement in hydrophobic zeolites by Monte Carlo simulations

We report a grand canonical Monte Carlo simulation study of water condensation in four different hydrophobic, all-silica, zeolites. In all cases, water condensation takes place above the saturation vapour pressure through a first-order like phase transition, with a hysteresis loop. This is at odds with the common belief that conventional phase transitions cannot take place in microporous solids such as zeolites. Forced fluid intrusion experiments have long been interpreted in terms of irreversibilities. What we show here is that the most important features of this process can be understood in terms of equilibrium thermodynamics considerations. Finally, a strong depletion of confined water is predicted in these nanoporous solids.

Gibbs ensemble simulations of vapour—liquid phase equilibria of cyclic alkanes

A new set of united atom Lennard-Jones interaction parameters for cyclic alkanes relevant to petrochemical research is proposed from fitting of simulation results to liquid coexistence densities and standard liquid densities. This parameter set leads to a critical temperature for cyclohexane which is only 4% below the experimental value. The critical temperatures for cyclopentane and cyclooctane are underestimated by roughly 7% and 5%, respectively. However, systematic deviations from experimental data are found for the liquid coexistence density as well as for the vapour pressure.

Monte Carlo simulations of squalane in the Gibbs ensemble

Abstract We investigate the liquid–vapour coexistence curve of 2,6,10,15,19,23-hexamethyltetracosane (squalane) near the critical point with a new Lennard–Jones parameter set and compare our results to existing simulation data as well as to recent experimental vapour pressure data. Comparison of the liquid–vapour coexistence curve to previous simulation data reveals that this new force field, which includes tail corrections to the truncation of the non-bonded interactions increases the liquid density. We determine the critical temperature to 829 K and 825 K (with roughly 1% error) for two different system sizes, 72 and 108 molecules, and the critical density to 0.211 g/cm 3 and 0.228 g/cm 3 , respectively. We extrapolate experimental vapour pressure data by use of Antoines law to the temperature range covered by simulation and yield good agreement between simulation and experiment. We note that the vapour pressure in simulation is essentially governed by the ideal vapour pressure.

The temperature-size phase diagram of large SF6 clusters by computer simulation

Abstract We report the full temperature-size phase diagram for closed-shell sulfur hexafluoride (SF6) clusters by molecular dynamics simulation. A bulk-like phase behaviour is observed in the size range N=19 to the thermodynamic limit. The solid-solid transition temperature is roughly unchanged from the thermodynamic limit to a size of ≈ 1000 molecules and decreases as clusters get smaller. A triple point is observed at T ≈ 60 K for N ≈ 30. The main features of the diagram can be explained in terms of the simple thermodynamic model of transition temperature depression based on surface tension difference between the phases.