Joël Puibasset

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.

A grand canonical Monte Carlo simulation study of water adsorption on Vycor-like hydrophilic mesoporous silica at different temperatures

The grand canonical Monte Carlo (GCMC) simulation technique is used to study water adsorption and condensation in a realistic Vycor-like silica mesoporous system at various temperatures. Water–water interactions are described using the SPC model while water–silica interactions are calculated in the framework of the PN-TrAZ model. Thermodynamic quantities (namely water adsorption isotherms and isosteric heat of adsorption curves) have been calculated along with water–water and water–Vycor pair distribution functions. The simulated adsorption isotherm at room temperature compares well with published experimental data. Isosteric heat curves are characteristic of adsorption in a heterogeneous environment. We also show that the BET method for specific surface determination is not valid in the case of water confined in silica mesoporous materials. By analysing water–water and water–Vycor pair correlation functions, we demonstrate the existence of strong distortion compared to bulk water due to the influence of the silica surface. The hydrogen bond is significantly elongated and angle distorted.

Effect of pore morphology and topology on capillary condensation in nanopores : a theoretical and molecular simulation study

We report a theoretical and simulation study of the temperature dependence of adsorption hysteresis for porous matrices having different morphologies and topologies. We used off-lattice Grand Canonical Monte Carlo (GCMC) simulations and two Density Functional Theories (DFT): we used the standard DFT in the non local approximation for cylindrical pores and the coarse-grained lattice DFT developed by Kierlik et al. [7] for disordered porous materials. We aim at gaining some insights on the concept of critical hysteresis temperature defined as the temperature at which the adsorption/desorption isotherm becomes reversible.

Fluid adsorption in linear pores: a molecular simulation study of the influence of heterogeneities on the hysteresis loop and the distribution of metastable states

Porous materials are known to adsorb fluid and can be characterised by measurement of fluid adsorption isotherms. Many nanoporous materials exhibit linear pores such as MCM-41 and porous silicon or alumina. In such systems, data adsorption analysis is considered to be straightforward within the approximation of independent domains. This article, which reviews previous molecular simulation works, aims at showing that the presence of heterogeneities within the pores actually invalidates this hypothesis, with consequences for porosity characterisation. To enlighten the effects, starting from perfect cylinders, the number of heterogeneities is progressively increased, up to large numbers, for which specific simulation tools are used to take into account the interdependence between the domains. The adsorption/desorption isotherms are calculated and correlated to the appearance of an exponentially large number of metastable states.

Influence of temperature on water adsorption / desorption hysteresis loop in disordered mesoporous silica glass by Grand Canonical Monte Carlo simulation method

The water adsorption / desorption properties in a mesoporous silica glass are investigated by way of Grand Canonical Monte Carlo simulations. 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 ability of the PN-TrAZ potential to describe the hydrophilic properties of silica surfaces is shown through the calculation of the adsorption isotherm and isosteric heat of adsorption at 300 K, which compare well to experimental data for Vycor. This study is extended to several temperatures, and the evolution of the hysteresis loop is examined.

Finite-size corrections in simulation of dipolar fluids

Monte Carlo simulations of dipolar fluids are performed at different numbers of particles N = 100-4000. For each size of the cubic cell, the non-spherically symmetric pair distribution function g(r,Ω) is accumulated in terms of projections gmnl(r) onto rotational invariants. The observed N dependence is in very good agreement with the theoretical predictions for the finite-size corrections of different origins: the explicit corrections due to the absence of fluctuations in the number of particles within the canonical simulation and the implicit corrections due to the coupling between the environment around a given particle and that around its images in the neighboring cells. The latter dominates in fluids of strong dipolar coupling characterized by low compressibility and high dielectric constant. The ability to clean with great precision the simulation data from these corrections combined with the use of very powerful anisotropic integral equation techniques means that exact correlation functions both in real and Fourier spaces, Kirkwood-Buff integrals, and bridge functions can be derived from box sizes as small as N ≈ 100, even with existing long-range tails. In the presence of dielectric discontinuity with the external medium surrounding the central box and its replica within the Ewald treatment of the Coulombic interactions, the 1/N dependence of the gmnl(r) is shown to disagree with the, yet well-accepted, prediction of the literature.

On the Thermodynamics and Experimental Control of Twinning in Metal Nanocrystals

This work demonstrates a new strategy for controlling the evolution of twin defects in metal nanocrystals by simply following thermodynamic principles. With Ag nanocrystals supported on amorphous SiO2 as a typical example, we establish that twin defects can be rationally generated by equilibrating nanoparticles of different sizes through heating and then cooling. We validate that Ag nanocrystals with icosahedral, decahedral, and single-crystal structures are favored at sizes below 7 nm, between 7 and 11 nm, and greater than 11 nm, respectively. This trend is then rationalized by computational studies based on density functional theory and molecular dynamics, which show that the excess free energy for the three equilibrium structures correlate strongly with particle size. This work not only highlights the importance of thermodynamic control but also adds another synthetic method to the ever-expanding toolbox used for generating metal nanocrystals with desired properties.

Numerical characterization of the density of metastable states within the hysteresis loop in disordered systems.

An improved approach is proposed to analyze the density of metastable states within any hysteresis loop, such as those observed in magnetic materials or for adsorption in porous materials. Except for a few analytically tractable models, most calculations have to be performed numerically on finite systems. The main points to be addressed thus concern the average over various material samples (the so-called realizations of the disorder), and the finite size analysis to estimate the thermodynamic limit. As an improvement of previously existing methods, it is proposed to introduce the Fourier transform of the density of metastable states (characteristic function). Its logarithm is shown to be additive and can straightforwardly be averaged over disorder. This procedure leads to a new definition of the complexity in finite size, giving the usual quenched complexity in the thermodynamic limit, while being better suited to performing finite size analysis. The calculations are illustrated on a molecular simulation based model for a simple fluid adsorbed in heterogeneous siliceous tubular pores mimicking mesoporous materials like MCM-41 or porous silicon. This approach is expected to be of general interest for hysteresis phenomena, including magnetic materials.

Vapor–Liquid Equilibrium

Every time a fluid is confined at a nanometer scale, the predominance of the fluid–substrate interactions strongly distorts its intrinsic properties. For instance, it is observed that the amount of fluid adsorbed in a nanoporous substrate is not a single-valued function of the chemical potential and may present a hysteresis. Understanding this phenomenon is a fundamental issue since it appears in the most frequently used method to characterize porous materials. The aim of this chapter is to focus on the analog of phase coexistence and the corresponding phase diagram for confined fluids. Lying between theory and experiments, molecular simulation allows accurate calculations of confined fluid properties in very realistic porous models. We focus on heterogeneous tubular pores, which constitute a good model for MCM-41, one of the most widely used molecular sieves. Little is known about the consequences of morphological or chemical heterogeneities in these systems. To keep advantage of the cylindrical symmetry, a new simulation framework is used to perform calculations of important thermodynamic properties of the confined fluid, such as a thermodynamic pressure and coexistence diagram.

Molecular simulation study of the heat capacity of metastable water between 100 and 300 K

ABSTRACT Molecular simulations have been used to study the heat capacity of metastable liquid water at low temperature adsorbed on a smooth surface. These calculations aim at modelling water properties measured by experiments performed on water films adsorbed on Vycor nanoporous silica at low temperature. In particular, the study focuses on the non-monotonous variation of the heat capacity between 100 and 300 K.