Roland J.-M. Pellenq

Capillary condensation in a disordered mesoporous medium: a grand canonical Monte Carlo study

The adsorption of rare gases in a disordered mesoporous silica glass has been studied by means of grand canonical Monte Carlo (GCMC) simulation. A series of porous samples has been obtained by using an off-lattice 3D reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered porous materials such as Vycor. The off-lattice functional of 115m2g−1 Vycor is applied to a simulation box containing silicon and oxygen atoms of cubic cristoballite with a homothetic reduction in order to obtain porous samples with mean pore size around 35 Å and specific surface around 220 m2 g−1. A realistic surface chemistry is then obtained by saturating all the dangling oxygen bonds with hydrogen. Some topological properties of the different 3D reconstructions of the Vycor-like material are analysed using chord length distributions and small angle scattering data. The GCMC Ar, Kr and Xe adsorption/desorption isotherms are calculated at different temperatures. At sufficiently low temperatures, they exhibit a capillary condensation transition with an adsorption branch having a finite slope accompanied by a hysteresis loop upon desorption. It has been shown on a set of simulated argon isotherms, that evolution with temperature of the GCMC results is similar to experiment. At the temperature at which the hysteresis loop disappears, it was found that the compressibility of the dense liquid-like phase at the maximum of the so-called hysteretic coexistence curves increases significantly. In the low pressure and temperature domain, different adsorption scenarios can be interpreted on the basis of a Zisman-type of criterion for wetting. The BET surface area is shown to be strongly related to this criterion. At higher pressure, it was found that the pore size distribution obtained by using the standard BJH analysis applied to both simulated adsorption and desorption data qualitatively reproduces the main features of the chord length distribution.

A simple method for calculating dispersion coefficients for isolated and condensed-phase species

A simple method is developed for the derivation of two-body and three-body dispersion coefficients for isolated and in-crystal atoms from a knowledge of their dipole polarizability and effective number of electrons. The method is checked by comparison with a large body of published theoretical results.

Electrostatic Attraction and/or Repulsion Between Charged Colloids: A (NVT) Monte-Carlo Study

Abstract We have performed canonical Monte-Carlo simulation of the distribution of counter-ions between two uniformly charged colloids of different geometries (infinite slabs, discoids and spheres). We have calculated the net force (or the pressure) between the colloids as a function of the interparticle separation in order to determine their stability. Simulations were performed within the primitive model which describes short-ranged excluded volume effects and long-ranged electrostatic interactions. Long-ranged behavior of the Coulomb potential has been handle by different numerical procedures: Ewald summations, hyperspheres method or external self-consistent fields approximation. In all cases, the net force between a pair of colloids results from the balance between the electrostatic attraction and the contact repulsion exerted by the condensed counter-ions. In the case of two infinite slabs, both contributions are (in absolute value) of the same order of magnitude; the resulting net force depends on the so-called electrostatic coupling which is the ratio of the counter-ion/surface electrostatic term at contact divided by the thermal energy. At high coupling (high surface density, polyvalent ions and/or low dielectric constant), we have demonstrated the existence of an attractive domain responsible for the cohesion of various lamellar materials (calcic-clay, cement, organic dispersion…). At low coupling (monovalent counterions in water), we have only detected a monotonous swelling behavior (repulsion) of the charged interfaces. We discuss these results on the basis of ionic correlations within the double-layers of condensed counter-ions. In addition to the infinite-slab case, we present results for a pair of discoid and spherical colloids in order to evaluate finite-size effects. By contrast to infinite interfaces, the electrostatic attraction is found to be negligible for a pair of parallel discoids neutralized by monovalent counter-ions (weak coupling conditions); the net force is then repulsive and driven by the contact force. A net attraction is also found in the case of divalent counter-ions (strong coupling conditions). No attractive regime is found in the case of interacting spheres neutralized by monovalent counter-ions.

A Grand Canonical Monte-Carlo Simulation Study of Xenon Adsorption in a Vycor-like Porous Matrix

We have performed atomistic Grand Canonical Monte-Carlo (GCMC) simulations of adsorption of xenon in a Vycor-like matrix at 195 K. The disordered mesoporous network is obtained by applying a numerical 3D off-lattice reconstruction procedure to a simulation box originally containing silicon and oxygen atoms of a non-porous silica solid. In order to reduce the computational cost, we have applied a homothetic decrease of the simulation box dimensions which preserves the morphology and the topology of the pore network (the average pore dimension is then around 30 Å). The surface chemistry is obtained in a realistic fashion by saturating all dangling bonds with hydrogen atoms. Small angle scattering spectra calculated on different numerical samples have evidenced a departure from Porods law due to surface roughness. The simulated isotherms calculated on such disordered connected porous networks, show the capillary condensation phenomenon. The shape of the adsorption curves differs from that obtained for simple pore geometries. The analysis of the adsorbed quantity distribution indicates partial molecular-film formation depending on the local surface curvature and roughness.

A comparison of water adsorption on ordered and disordered silica substrates

The adsorption properties of water adsorbed on various silica substrates are investigated by way of Grand Canonical Monte Carlo simulations (GCMC). The SPC and PN-TrAZ potential are used to describe water–water and water–silica interactions. The numerical sample of mesoporous silica glass (pore size: 3.6 nm) was obtained by off-lattice reconstruction, known to reproduce in a realistic way the geometrical complexity of high specific surface Vycor. The chemistry of the surface is made realistic by hydroxylation. The simulated adsorption isotherm and isosteric differential enthalpy of adsorption compare well to experimental data for Vycor, showing the ability of the PN-TrAZ potential to describe the hydrophilic properties of silica surfaces. This study was extended to several crystallographic faces of cristobalite. Their adsorption properties differ widely from each other. It is shown that the hydrophilic properties are not simply related to surface hydroxyl density but are also related to the local structure of the silica surface. In spite of these large variations, it is possible to reproduce the adsorption isotherm of the mesoporous disordered sample by applying a natural averaging procedure over the different crystallographic faces of cristobalite.

Adsorption/Condensation of Xenon in a Disordered Silica Glass Having a Mixed (Micro a Mixed and Meso) Porosity

Abstract We have studied adsorption of xenon in a mixed (micro and meso) porosity silica controlled porous glass (CPG) by means of grand canonical Monte-Carlo (GCMC) simulation. A numerical sample of the CPG adsorbent has been obtained by using an off-lattice reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered mesoporous materials. The off-lattice functional of (100m2/g)-Vycor is applied to a simulation box containing silicon and oxygen atoms of orthorhombic silicalite zeolite with an homothetic reduction of factor 2.5 so as to obtain a CPG sample exhibiting both micro and meso porosity. A realistic surface chemistry is then obtained by saturating all oxygen dangling bonds in the mesoporosity with hydrogen. The Xe adsorption/desorption isotherms is calculated at 195 K. It is shown that in the particular case of xenon, the difference of energetics between zeolitic micropores and CPG mesopores lead to two distinct adsorption processes which occur consecutively. As a consequence, both the microporosity and the mesoporosity can be calculated independently.

Physisorption in nanopores of various sizes and shapes : A Grand Canonical Monte Carlo simulation study

Abstract We have performed atomistic Grand Canonical Monte Carlo simulations (GCMC) of adsorption of argon at 77xa0K in silica nanopores of different size and shape in order to assess the concept of t-plot (thickness of the adsorbed film with increasing pressure) used in phenomenological models for capillary condensation and mesoporous solids characterisation. Results obtained for cylindrical pores of different sizes are compared to the case of a non porous substrate. The film adsorbed in pore of diameter 10xa0nm has the same thickness than that obtained on the planar substrate. By contrast, as pores get smaller, we show the existence of a confinement effect on the t-plot. A FHH type of law (with a pore-size depending exponent) can be used to model this confinement effect. We have also investigated the effect of surface roughness. Results compared to the case of a smooth cylindrical pore indicates that surface roughness increases significantly the adsorbed amount and leads to an apparent film thickness thicker than for the smooth substrate. Roughness at larger length scale (pore constriction) is shown to have also a strong influence on capillary condensation which occurs at lower pressure than for the equivalent unconstricted pore; the corresponding apparent t-plot is largely affected by the presence of the constriction. Finally, simulations performed in two ellipsoidal and a hexagonal pores are reported. For the largest ellipsoidal pore and the hexagonal pore, we show that the gas/adsorbate interface keeps memory of the pore morphology at low pressures and, then, adopts a cylindrical shape as the pressure is increased. By contrast, in the smallest ellipsoidal pore, which also presents the most asymmetrical shape, the interface remains asymmetrical over the entire pressure range prior to the capillary condensation. For some of the pore geometries studied in this work, we discuss the validity of the BET model.

A Structural Study of Dehydration/Rehydration of Tobermorite, a Model Cement Compound

We have investigated the structure and the dehydration/rehydration process of Tobermorite-like Calcium Silicates Hydrates (CSH) by using High Resolution Transmission Electronic Microscopy (HRTEM) and a new in-situ temperature and pressure controlled X-Ray Diffraction set up. CSH samples were synthesized with a Calcium/Silicium ratio 0.9 following a procedure which allows to produce model compounds of cement: a calcio-silicate layered structure with an interlamellar spacing of 14xa0A. Atomic scale resolved TEM data confirm the layered structure of Tobermoritic CSH: in particular we were able to show directly for the first time the silicate chain alignment and measure the so-called dreierketten pattern within the silicate chains. A newly developed X-Ray diffraction cell which allows to control temperature and pressure is used to study the dehydration/rehydration of Tobermorite and Ca-Montmorillonite. In the particular case of Tobermorite, the variations of the interlayer distance, as measured with the new in-situ X-Ray cell at atmospheric pressure and upon increasing temperature, are very close to those obtained with a standard ex-situ X-Ray apparatus coupled to a Controlled Rate Thermal Analysis heating technique (which allows for quasi-equilibrium conditions). This validates the (T,P) in-situ X-Ray cell: in agreement with literature, we have clearly identified two structural transitions upon heating under atmospheric pressure : (14xa0A -11xa0A) and (11xa0A −xa09.5xa0A) at Txa0=xa0330xa0K and 510xa0K respectively. Furthermore, we demonstrate that the first structural transition of Tobermorite upon dehydration is irreversible by contrast to that observed for Ca-Montmorillonite clay.

THE ADSORPTION OF ARGON ON ZNO AT 77K

We have studied the adsorption of argon onto ZnO surfaces at 77 K by means of quasi-equilibrium adsorption volumetry coupled with high resolution microcalorimetry and Grand Canonical Monte-Carlo (GCMC) simulations. The adsorbate/surface adsorption potential function (PN type) used in the simulations, was determined on the basis of ab initio calculations (corrected for dispersion interactions). The first aspect of this work was to test the ability of a standard solid-state Hartree—Fock technique coupled with a perturbative semi-empirical approach in deriving a reliable adsorption potential function. The dispersion part of the adsorbate/surface interatomic potential was derived by using perturbation theory-based equations while the repulsive and induction interactions were derived from periodic Hartree—Fock (CRYSTAL92) calculations. GCMC simulations based on this adsorption potential allow one to calculate adsorption isotherms and isosteric heat versus loading curves as well as singlet distribution function...

Molecular Engineering of the Cohesionin Neat and Hybrid Cement Hydrates

On the basis of recent molecular simulation or experimental studies, we discuss two possible strategies for improving the mechanical properties of cementitious materials by modifying the bonding scheme in the hydrates at molecular level. We focus on the calcium silicate hydrates (C-S-H). A first strategy would be based on the strengthening of the cohesion forces acting between the individual C-S-H lamellae or between their crystallites. Monte Carlo simulations in the primitive model framework and ab initio atomistic calculations suggest that the cohesion of C-S-H is mainly due to a combination of sub-nano range ionic-covalent forces and meso-range ionic correlation forces. Both types of forces may be modified, at least in theory, by changing the nature of the interstitial ions, their hydration state, or the charge density on the C-S-H lamellae. A second strategy, akin to a bio-mimetic or tissue engineering approach, would be to hybridize the hydrates by grafting organic moieties on the mineral lamellae. We show that this is easily achieved by controlled hydrolysis of mixtures of organo-silane precursors. The outcome may be a material with improved fracture energy.