Showing papers by "Benoit Coasne published in 2006"

Effects of confinement on freezing and melting.

Abstract: We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials) The most commonly used molecular simulation, theoretical and experimental methods are first presented We also provide a brief description of the most widely used porous materials The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surface-driven and confinement-driven phases are then discussed We also address how confinement affects the glass transition

Adsorption of Simple Gases in MCM-41 Materials: The Role of Surface Roughness

Abstract: This paper reports the development and testing of atomistic models of silica MCM-41 pores. Model A is a regular cylindrical pore having a constant section. Model B has a surface disorder that reproduces the morphological features of a pore obtained from an on-lattice simulation that mimics the synthesis process of MCM-41 materials. Both models are generated using a similar procedure, which consists of carving the pore out of an atomistic silica block. The differences between the two models are analyzed in terms of small angle neutron scattering spectra as well as adsorption isotherms and isosteric heat curves for Ar at 87 K and Xe at 195 K. As expected for capillary condensation in regular nanopores, the Ar and Xe adsorption/desorption cycles for model A exhibit a large hysteresis loop having a symmetrical shape, i.e., with parallel adsorption and desorption branches. The features of the adsorption isotherms for model B strongly depart from those observed for model A. Both the Ar and Xe adsorption branches for model B correspond to a quasicontinuous pore filling that involves coexistence within the pore of liquid bridges and gas nanobubbles. As in the case of model A, the Ar adsorption isotherm for model B exhibits a significant hysteresis loop; however, the shape of the loop is asymmetrical with a desorption branch much steeper than the adsorption branch. In contrast, the adsorption/desorption cycle for Xe in model B is quasicontinuous and quasireversible. Comparison with adsorption and neutron scattering experiments suggests that model B is too rough at the molecular scale but reproduces reasonably the surface disorder of real MCM-41 at larger length scales. In contrast, model A is smooth at small length scales in agreement with experiments but seems to be too ordered at larger length scales.

Gas adsorption in mesoporous micelle-templated silicas: MCM-41, MCM-48, and SBA-15.

Abstract: This paper reports a molecular simulation and experimental study on the adsorption and condensation of simple fluids in mesoporous micelle-templated silicas MCM-41, MCM-48, and SBA-15. MCM-41 is described as a regular cylindrical silica nanopore, while SBA-15 is assumed to be made up of cylindrical nanopores that are connected through lateral channels. The 3D-connected topology of MCM-48 is described using a gyroid periodic minimal surface. Argon adsorption at 77 K is calculated for the three materials using Grand Canonical Monte Carlo simulations. Qualitative comparison with experiments for nitrogen adsorption in mesoporous micelle-templated silicas is made. The adsorption isotherm for SBA-15 resembles that for MCM-41. In particular, capillary condensation and evaporation are not affected by the presence of the connecting lateral channels. In contrast, the argon adsorption isotherm for MCM-48 departs from that for MCM-41 having the same pore size. While condensation in MCM-41 is a one-step process, filling of MCM-48 involves two successive jumps in the adsorbed amounts which correspond to condensation in different domains of the porosity. The condensation pressure for MCM-48 is larger than that for MCM-41. We attribute this result to the morphology of the MCM-48 surface (made up of both concave and convex regions) that differs from that for MCM-41 (concave only). Our results suggest that the pore connectivity affects pore filling when the size of the connections is comparable to that of the nanopores.

Adsorption, structure and dynamics of fluids in ordered and disordered models of porous carbons

Abstract: Grand Canonical Monte Carlo and Molecular Dynamics simulations are used to investigate the adsorption and dynamics of argon in ordered and disordered models of porous carbons. The ordered porous carbon (model A) is a regular slit pore made up of graphene sheets. The disordered porous carbon (model B) is a structural model that reproduces the morphological (pore shape) and topological (pore connectivity) disorders of saccharose-based porous carbons. Three pore widths, H = 7, 11, and 15 A, are selected for model A; they correspond to the smaller, mean, and larger pore sizes of model B. The filling pressures for the graphite slit pores are lower than those for the disordered porous carbon. It is also found that model A is not able to capture the behaviour of the isosteric heat of adsorption of model B. For all pressures, the confined phase in model A is composed of well-defined layers, which crystallize into hexagonal 2D crystals after complete filling of the pores. In contrast, the structure of argon in the...

Freezing of fluids confined in a disordered nanoporous structure.

Abstract: Freezing of a simple fluid in a disordered nanoporous carbon is studied using molecular simulations. Only partial crystallization occurs, and the confined phase is composed of crystalline and amorphous nanodomains. This freezing behavior departs strongly from that for nanopores of simple geometry. We present a method for analyzing the freezing in such disordered materials in terms of a transition in the average size and number of crystalline clusters. The results provide a basis for the interpretation of experiments on freezing in such materials, particularly 1H-NMR and scattering experiments.

Adsorption and Structure of Argon in Activated Porous Carbons

Abstract: Molecular simulations are used to investigate the adsorption and structure of argon in ordered and disordered models of porous carbons. The ordered porous carbon (model A) is an assembly of regular slit pores of different sizes, while the disordered porous carbon (model B) is a structural model that reproduces the complex pore shape and pore connectivity of saccharose-based porous carbons. The same pore size distribution is used for models A and B so that we are able to estimate, for similar confinement effects, how the disorder of the porous material affects the adsorption and structure of the confined fluid. Adsorption of argon at 77.4 K in the two models is studied using Grand Canonical Monte Carlo simulations. The structure of the confined fluid is analyzed using crystalline bond order parameters and positional or bond orientational pair correlation functions. The filling pressure for the assembly of slit pores is much lower than that for the disordered porous carbon. It is also found that the isoster...