Article

Adsorption of simple fluid on silica surface and nanopore: effect of surface chemistry and pore shape.

Institut Charles Gerhardt Montpellier, CNRS (UMR 5253) and Université Montpellier 2, Montpellier, France.
Langmuir (Impact Factor: 4.38). 07/2008; 24(14):7285-93. DOI: 10.1021/la800567g
Source: PubMed

ABSTRACT This paper reports a molecular simulation study on the adsorption of simple fluids (argon at 77 K) on hydroxylated silica surfaces and nanopores. The effect of surface chemistry is addressed by considering substrates with either partially or fully hydroxylated surfaces. We also investigate the effect of pore shape on adsorption and capillary condensation by comparing the results for cylindrical and hexagonal nanopores having equivalent sections (i.e., equal section areas). Due to the increase in the polarity of the surface with the density of OH groups, the adsorbed amounts for fully hydroxylated surfaces are found to be larger than those for partially hydroxylated surfaces. Both the adsorption isotherms for the cylindrical and hexagonal pores conform to the typical behavior observed in the experiments for adsorption/condensation in cylindrical nanopores MCM-41. Capillary condensation occurs through an irreversible discontinuous transition between the partially filled and the completely filled configurations, while evaporation occurs through the displacement at equilibrium of a hemispherical meniscus along the pore axis. Our data are also used to discuss the effect of surface chemistry and pore shape on the BET method. The BET surface for fully hydroxylated surfaces is much larger (by 10-20%) than the true geometrical surface. In contrast, the BET surface significantly underestimates the true surface when partially hydroxylated surfaces are considered. These results suggest that the surface chemistry and the choice of the system adsorbate/adsorbent is crucial in determining the surface area of solids using the BET method.

0 Bookmarks
 · 
212 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Diffusion of pure components (hydrogen (H2), argon (Ar), krypton (Kr), methane (C1), ethane (C2), propane (C3), n-butane (nC4), and n-hexane (nC6)) in silica nanopores with diameters of 1, 1.5, 2, 3, 4, 5.8, 7.6, and 10 nm were investigated using molecular dynamics (MD). The Maxwell–Stefan (M–S) diffusivity (Đi,s) and self-diffusivities (Di,self,s) were determined for pore loadings ranging to 10 molecules nm−3. The MD simulations show that zero-loading diffusivity Đi,s(0) is consistently lower, by up to a factor of 10, than the values anticipated by the classical Knudsen formula; the differences increase with increasing adsorption strength. Only when the adsorption is negligible does the Đi(0) approach the Knudsen diffusivity value.MD simulations of diffusion in binary mixtures C1–H2, C1–Ar, C1–C2, C1–C3, C1–nC4, C1–nC6, C2–nC4, C2–nC6, and nC4–nC6 in the different pores were also performed to determine the three parameters Đ1,s, Đ2,s, and Đ12, arising in the M–S formulation for binary mixture diffusion. The Đi,s in the mixture were found to be practically the same as the values obtained for unary diffusion, when compared at the same total pore loading. Also, the Đi,s of any component was practically the same, irrespective of the partner molecules in the mixture. Furthermore the intermolecular species interaction parameter Đ12, could be identified with the binary M–S diffusivity in a fluid mixture at the same concentration as within the silica nanopore. The obtained results underline the overwhelming advantages of the M–S theory for mixture diffusion in nanopores.Our study underlines the limitations of the commonly used dusty-gas approach to pore diffusion in which Knudsen and surface diffusion mechanisms are considered to be additive.
    Chemical Engineering Science. 01/2009;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We construct an atomistic silica pore model mimicking templated mesoporous silica MCM-41, which has molecular-level surface roughness, with the aid of the electron density profile (EDP) of MCM-41 obtained from X-ray diffraction data. Then, we present the GCMC simulations of argon adsorption on our atomistic silica pore models for two different MCM-41 samples at 75, 80, and 87 K, and the results are compared with the experimental adsorption data. We demonstrate that accurate molecular modeling of the pore structure of MCM-41 by using the experimental EDP allows the prediction of experimental capillary evaporation pressures at all investigated temperatures. The experimental desorption branches of the two MCM-41 samples are in good agreement with equilibrium vapor–liquid transition pressures from the simulations, which suggests that the experimental desorption branch for the open-ended cylindrical pores is in thermodynamic equilibrium.
    Adsorption 04/2013; 19(2-4). · 1.55 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    Chemical Society Reviews 01/2013; · 24.89 Impact Factor

Full-text (2 Sources)

Download
68 Downloads
Available from
May 19, 2014