Showing papers by "Benoit Coasne published in 2013"

Adsorption, intrusion and freezing in porous silica: the view from the nanoscale

Abstract: This review presents the state of the art of molecular simulation and theory of adsorption, intrusion and freezing in porous silica. Both silica pores of a simple geometry and disordered porous silicas which exhibit morphological and topological disorders are considered. We provide a brief description of the numerical models of porous silicas available in the literature and present the most common molecular simulation and theoretical methods. Adsorption in regular and irregular pores is discussed in the light of classical theories of adsorption and capillary condensation in pores. We also present the different evaporation mechanisms for disordered systems: pore blocking and cavitation. The criticality of fluids confined in pores, which is still the matter of debate, is then discussed. We review theoretical results for intrusion/extrusion and freezing in silica pores and discuss the validity of classical approaches such as the Washburn–Laplace equation and Gibbs–Thomson equation to describe the thermodynamics of intrusion and in-pore freezing. The validity of the most widely used characterization techniques is then discussed. We report some concluding remarks and suggest directions for future work.

Adsorption of carbon dioxide, methane, and their mixtures in porous carbons: effect of surface chemistry, water content, and pore disorder.

Abstract: The adsorption of carbon dioxide, methane, and their mixtures in nanoporous carbons in the presence of water is studied using experiments and molecular simulations. Both the experimental and numerical samples contain polar groups that account for their partially hydrophilicity. For small amounts of adsorbed water, although the shape of the adsorption isotherms remain similar, both the molecular simulations and experiments show a slight decrease in the CO2 and CH4 adsorption amounts. For large amounts of adsorbed water, the experimental data suggest the formation of methane or carbon dioxide clathrates in agreement with previous work. In contrast, the molecular simulations do not account for the formation of such clathrates. Another important difference between the simulated and experimental data concerns the number of water molecules that desorb upon increasing the pressure of carbon dioxide and methane. Although the experimental data indicate that water remains adsorbed upon carbon dioxide and methane ad...

Molecular Simulation of Adsorption and Transport in Hierarchical Porous Materials

Abstract: Adsorption and transport in hierarchical porous solids with micro- (∼1 nm) and mesoporosities (>2 nm) are investigated by molecular simulation. Two models of hierarchical solids are considered: microporous materials in which mesopores are carved out (model A) and mesoporous materials in which microporous nanoparticles are inserted (model B). Adsorption isotherms for model A can be described as a linear combination of the adsorption isotherms for pure mesoporous and microporous solids. In contrast, adsorption in model B departs from adsorption in pure microporous and mesoporous solids; the inserted microporous particles act as defects, which help nucleate the liquid phase within the mesopore and shift capillary condensation toward lower pressures. As far as transport under a pressure gradient is concerned, the flux in hierarchical materials consisting of microporous solids in which mesopores are carved out obeys the Navier–Stokes equation so that Darcy’s law is verified within the mesopore. Moreover, the f...

On the molecular origin of high-pressure effects in nanoconfinement: the role of surface chemistry and roughness.

Abstract: Experiments and simulations both suggest that the pressure experienced by an adsorbed phase confined within a carbon nanoporous material can be several orders of magnitude larger than the bulk phase pressure in equilibrium with the system. To investigate this pressure enhancement, we report a molecular-simulation study of the pressure tensor of argon confined in slit-shaped nanopores with walls of various models, including carbon and silica materials. We show that the pressure is strongly enhanced by confinement, arising from the effect of strongly attractive wall forces; confinement within purely repulsive walls does not lead to such enhanced pressures. Simulations with both the Lennard-Jones and Barker-Fisher-Watts intermolecular potentials for argon-argon interactions give rise to similar results. We also show that an increase in the wall roughness significantly decreases the in-pore pressure due to its influence on the structure of the adsorbate. Finally, we demonstrate that the pressures calculated from the mechanical (direct pressure tensor calculations) and the thermodynamic (volume perturbation method) routes yield almost identical results, suggesting that both methods can be used to calculate the local pressure tensor components in the case of these planar geometries.

Gas Uptake in Solvents Confined in Mesopores: Adsorption versus Enhanced Solubility

Abstract: Three molecular mechanisms for gas uptake in a solvent confined in mesopores are identified. On the one hand, CO2 uptake is an adsorption-driven phenomenon that arises from the strong interaction between the gas molecules and the pore surface. On the other hand, H2 uptake is a confinement-induced enhanced solubility in which solubility is favored in the regions of low solvent density formed by the layering of the solvent. In partially filled pores, adsorption at the gas/liquid solvent interface is a third mechanism that leads to large gas uptakes. This study, which sheds light on previously reported yet unclear oversolubility in pores, provides a guide to design hybrid porous catalysts consisting of a solvent confined in a porous solid.

First-principles molecular dynamics study of glassy GeS 2 : Atomic structure and bonding properties

Abstract: The structure of glassy GeS${}_{2}$ is studied in the framework of density functional theory, by using a fully self-consistent first-principles molecular dynamics (FPMD) scheme. A comparative analysis is performed with previous molecular dynamics data obtained within the Harris functional (HFMD) total energy approach. The calculated total neutron structure factor exhibits an unprecedented agreement with the experimental counterpart. In particular, the height of the first sharp diffraction peak (FSDP) improves considerably upon the HFMD results. Both the Ge and the S subnetworks are affected by a consistent number of miscoordinations, coexisting with the main tetrahedral structural motif. Glassy GeS${}_{2}$ features a short-range order quite similar to the one found in glassy GeSe${}_{2}$, a notable exception being the larger number of edge-sharing connections. An electronic structure localization analysis, based on the Wannier functions formalism, provides evidence of a more enhanced ionic character in glassy GeS${}_{2}$ when compared to glassy GeSe${}_{2}$.

Water self-diffusion at the surface of silica glasses: effect of hydrophilic to hydrophobic transition

Abstract: We study water dynamics at a silica aqueous interface. Both hydrophilic (hydroxylated) surfaces and hydrophobic surfaces (dehydroxylated upon irradiation) have been generated from atomistic simulations. A new method for the calculation of the normal self-diffusion coefficients based on the calculation of mean first passage times is proposed. It uses the Smoluchowski theory of Brownian motion and it takes proper account of the layering of the molecules. In the case of parallel self-diffusion coefficients, a decrease is found compared to the bulk values. It can be described in terms of hydrodynamic boundary conditions induced by the surface confinement. This hydrodynamic explanation is not enough to interpret the case of normal self-diffusion coefficients for which an important diminution is found. Normal self-diffusion coefficients appear to depend strongly on the hydrophilicity of the surface. They tend towards their bulk value only at long distances from the surfaces. The first layer of water molecules i...

Molecular intermittent dynamics of interfacial water: probing adsorption and bulk confinement

Abstract: Numerous natural and manufactured systems such as colloidal suspensions, geological pore networks, catalysts, and nanofluidic devices develop a large and sometimes complex interface strongly influencing the dynamics of the fluid entrapped inside these materials. A coarse grain picture of this molecular dynamics can be considered as an intermittence of adsorption steps and bulk relocations from one point to another point of the interface. Adsorption statistics such as the adsorption time distribution and its first moment reflect the degree of interaction of the molecule with the colloidal interface. Relocation statistics strongly depend on the shape of the pore, the surface forces and the bulk confinement. In this paper, a theoretical analysis of this intermittent dynamics is presented. A direct comparison with molecular dynamics simulations is proposed in the case of liquid water confined inside a hydrophilic substrate (silica slit pore with hydroxylated surfaces) or inside a hydrophobic substrate (carbon nanotube). Analysis of this intermittent dynamics allows quantification of the level of interaction of the vicinal water with the solid interface inside an independent adsorption region in exchange with the confined bulk fluid. The possibility of experimentally probing this dynamics using NMR relaxometry is emphasized.

Adsorption, Intrusion and Freezing in Porous Silica: The View from the Nanoscale

Abstract: This review presents the state of the art of molecular simulation and theory of adsorption, intrusion and freezing in porous silica. Both silica pores of a simple geometry and disordered porous silicas which exhibit morphological and topological disorders are considered. We provide a brief description of the numerical models of porous silicas available in the literature and present the most common molecular simulation and theoretical methods. Adsorption in regular and irregular pores is discussed in the light of classical theories of adsorption and capillary condensation in pores. We also present the different evaporation mechanisms for disordered systems: pore blocking and cavitation. The criticality of fluids confined in pores, which is still the matter of debate, is then discussed. We review theoretical results for intrusion/extrusion and freezing in silica pores and discuss the validity of classical approaches such as the Washburn–Laplace equation and Gibbs–Thomson equation to describe the thermodynamics of intrusion and in-pore freezing. The validity of the most widely used characterization techniques is then discussed. We report some concluding remarks and suggest directions for future work.

Adsorption-based characterization of hierarchical metal–organic frameworks

Abstract: Nitrogen adsorption at 77 K on metal–organic framework (MOF) is investigated by means of molecular simulations. We consider both regular Cu–BTC crystal and a MOF-based hierarchical porous solid consisting of a mesopore carved out of a Cu–BTC crystal. The t-plot method is applied to these solids by using a non-porous Cu–BTC surface as the reference sample. The values of the mesoporous and external surface areas are determined from the t-plot, and the validity of the method for this type of hierarchical solid is discussed.