Showing papers by "Benoit Coasne published in 2005"

Temperature Effect on Adsorption/Desorption Isotherms for a Simple Fluid Confined within Various Nanopores

Abstract: We report a Grand Canonical Monte Carlo study of the temperature dependence of adsorption/desorption hysteresis for porous matrices having different morphologies and topologies. We aim at gaining some insights on the concept of critical hysteresis temperature, T cc , defined as the temperature at which the hysteresis loop disappears. Simulated T cc for cylindrical, ellipsoidal, and constricted pores follow the experimental scaling law established for MCM-41 silica materials. In contrast, T cc for Vycor samples with a largest pore size ~2.5 nm and 5.0 nm obey a different relationship, in qualitative agreement with experiments.

Domain theory for capillary condensation hysteresis

Abstract: We discuss how the original domain theory for capillary condensation hysteresis D. H. Everett, The Solid– Gas Interface, Vol. 2 Marcel Dekker, New York, 1967, pp. 1055–1113 must be modified to account for the presence of the film adsorbed at the pore surface. We show that the original predictions scanning behavior, congruence are not valid unless the existence of the adsorbed film is neglected or the dependence of its thickness on the pressure is neglected. We also calculate the scanning curves and subloops that are expected for an assembly of pores having either a regular or irregular nonconstant section. These predictions over the scanning behavior within capillary condensation hysteresis can be used to check whether real materials are made up of independent pores or not. Our results are discussed in the light of experiments and density functional theory calculations for adsorption in porous media. I. INTRODUCTION Adsorption isotherms in mesoporous materials pore size in the range 2–1 0 nm usually exhibit a sharp increase of the adsorbed amount at a pressure below the bulk saturation pressure of the fluid. Such an increase corresponds to the capillary condensation of the fluid confined within the porous solid. In most systems, this phenomenon is accompanied with a large and reproducible hysteresis loop. 1–3 Experimental hysteresis loops are either symmetrical with quasiparallel adsorption/desorption branches type H1 or asymmetrical with a desorption branch much steeper than the adsorption branch type H2. 4 It is generally believed that the shape of the hysteresis loop is related to the absence or presence of connected pores in the porous material. The following International Union of Pure and Applied Chemists IUPAC classification has been proposed. 4 Type H1 hysteresis is usually interpreted as the signature of a material made up of unconnected pores. In this case, theoretical works based on density functional theory DFT, 5,6 lattice gas models, 7 as well as molecular simulations 8,9 suggest that the hysteresis loop is a van der Waals loop of the confined system, i.e., an intrinsic property of the confined fluid. Such an interpretation is usually invoked to explain symmetrical hysteresis loops that are observed for MCM-41 and SBA-15

Molecular modeling of freezing of simple fluids confined within carbon nanotubes.

Abstract: We report Monte Carlo simulation results for freezing of Lennard-Jones carbon tetrachloride confined within model multiwalled carbon nanotubes of different diameters. The structure and thermodynamic stability of the confined phases, as well as the transition temperatures, were determined from parallel tempering grand canonical Monte Carlo simulations and free-energy calculations. The simulations show that the adsorbate forms concentric molecular layers that solidify into defective quasi-two-dimensional hexagonal crystals. Freezing in such concentric layers occurs via intermediate phases that show remnants of hexatic behavior, similar to the freezing mechanism observed for slit pores in previous works. The adsorbate molecules in the inner regions of the pore also exhibit changes in their properties upon reduction of temperature. The structural changes in the different regions of adsorbate occur at temperatures above or below the bulk freezing point, depending on pore diameter and distance of the adsorbate ...

Freezing and melting of azeotropic mixtures confined in nanopores: experiment and molecular simulation.

Abstract: The paper reports on a qualitative comparison between experimental measurements and molecular simulations of the freezing and melting of azeotropic mixtures confined in nanoporous materials. Dielectric relaxation spectroscopy was used to determine the experimental solid/liquid phase diagram of CCl4/C6H12 mixtures confined in activated carbon fibres. Grand Canonical Monte Carlo simulations combined with the parallel tempering technique were used to model the freezing of the azeotropic Lennard–Jones mixture Ar/CH4 in a graphite slit pore. The structure of the crystal phase in the simulations is investigated by means of positional and bond-orientational pair correlation functions and appropriate bond-order parameters. Both simulations and experiments show that the phase diagram of the confined mixture is of the same type as that for the bulk, but the solid/liquid coexistence lines are located at higher temperatures. The effect of confinement and of the wall/fluid interaction on the location of the azeotrope ...

Freezing of Mixtures Confined in a Slit Nanopore

Abstract: We report a Grand Canonical Monte Carlo study of the freezing/melting of Lennard-Jones A/B mixtures confined in a slit pore (H = 1.44 nm). The fluid/fluid interactions are chosen to model A = Ar and B = Kr. Fluid/wall interaction parameters are chosen so that the ratio of the wall/fluid to the fluid/fluid interactions for Kr and Ar is larger and smaller than 1, respectively. We find that the phase diagram of the confined mixture is of the same type than that for the bulk. The freezing temperature of confined mixtures rich in Kr is larger than the bulk. In contrast, we observe a decrease of the freezing temperature for mixtures rich in Ar. The confined crystal has a hexagonal structure (triangular symmetry), except for pure Ar where a square structure is observed.

Effect of Confinement on Freezing of CCl4 in Cylindrical Pores

Abstract: We report experimental and molecular simulation results on the freezing and melting of fluids confined within cylindrical pores. Dielectric relaxation spectroscopy was used to determine the experimental melting points of carbon tetrachloride confined within multi-walled carbon nanotubes. Molecular simulations in the grand canonical ensemble and free energy calculations were performed for the same system to determine the structure and thermodynamic stability of the confined phases, as well as the temperatures and the order of the phase transitions. Both simulations and experiments show evidence of a rich phase behavior in confinement, with a number of inhomogeneous phases that are stable over extended temperature ranges. Multiple transition temperatures both above and below the bulk melting point were obtained from experiments and simulations, with good agreement between both series of results.