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: 4.46). 07/2008; 24(14):7285-93. DOI: 10.1021/la800567g
Source: PubMed


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.

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    • "In this work, we show that, due to molecular discreteness, hysteresis is a natural and unavoidable feature of sorption in nanoporous solids with fixed pore geometries. In Part I [1], we showed that misfit pressures due to discrete molecular forces around heterogeneities in the nanopore geometry generally provide local energy barriers for the passage of the adsorbate-vapor interface, consistent with evidence from molecular dynamics simulations [19] [20] [22] [23] [21]. As the thermodynamic driving force is increased by changing the vapor pressure, the interface remains pinned at the heterogeneity until a sudden " snap-through instability " occurs, analogous to snap-through buckling of a flat arch. "
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    ABSTRACT: Motivated by the puzzle of sorption hysteresis in Portland cement concrete or cement paste, we develop in Part II of this study a general theory of vapor sorption and desorption from nanoporous solids, which attributes hysteresis to hindered molecular condensation with attractive lateral interactions. The classical mean-field theory of van der Waals is applied to predict the dependence of hysteresis on temperature and pore size, using the regular solution model and gradient energy of Cahn and Hilliard. A simple "hierarchical wetting" model for thin nanopores is developed to describe the case of strong wetting by the first monolayer, followed by condensation of nanodroplets and nanobubbles in the bulk. The model predicts a larger hysteresis critical temperature and enhanced hysteresis for molecular condensation across nanopores at high vapor pressure than within monolayers at low vapor pressure. For heterogeneous pores, the theory predicts sorption/desorption sequences similar to those seen in molecular dynamics simulations, where the interfacial energy (or gradient penalty) at nanopore junctions acts as a free energy barrier for snap-through instabilities. The model helps to quantitatively understand recent experimental data for concrete or cement paste wetting and drying cycles and suggests new experiments at different temperatures and humidity sweep rates.
    Full-text · Article · Nov 2011 · Journal of the Mechanics and Physics of Solids
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    • "The recent advent of molecular dynamic (MD) simulations is advancing the knowledge of nanoporous solids and gels or colloidal systems in a profound way [37] [23] [24] [25] [34] [35] [42] [36] [46]. Particularly exciting have been the new results by Rolland Pellenq and co-workers at the Concrete Sustainability Hub in MIT led by Franz-Josef Ulm [14] [15] [16]. "
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    ABSTRACT: The sorption-desorption hysteresis observed in many nanoporous solids, at vapor pressures low enough for the the liquid (capillary) phase of the adsorbate to be absent, has long been vaguely attributed to changes in the nanopore structure, but no mathematically consistent explanation has been presented. The present work takes an analytical approach to account for discrete molecular forces in the nanopore fluid and proposes two related mechanisms that can explain the hysteresis at low vapor pressure without assuming any change in the nanopore structure. The first mechanism, presented in Part I, consists of a series of snap-through instabilities during the filling or emptying of non-uniform nanopores or nanoscale asperities. The instabilities are caused by non-uniqueness in the misfit disjoining pressures engendered by a difference between the nanopore width and an integer multiple of the thickness of a monomolecular adsorption layer. The second mechanism, presented in Part II, consists of molecular coalescence within a partially filled surface, nanopore or nanopore network. This general thermodynamic instability is driven by attractive intermolecular forces within the adsorbate and forms the basis to develop a unified theory of both mechanisms. The ultimate goals of the theory are to predict the fluid transport in nanoporous solids from microscopic first principles, and to determine the pore size distribution and internal surface area from sorption tests.
    Full-text · Article · Aug 2011 · Journal of the Mechanics and Physics of Solids
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    • ", and 10 nm. The silica pore structures have been prepared using a method based on Coasne et al. (2008); Fig. 1 shows the landscape for a pore of 3 nm diameter, as an example. In every case the following simulations were conducted: (1) Configurationalbias Monte Carlo (CBMC) simulations in the grand canonical ensemble were performed to determine the pure component adsorption isotherms; these simulations allow calculation of the thermodynamic factors required for the determination of the Fick diffusivities using Eq. "
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    Full-text · Article · Mar 2009 · Chemical Engineering Science
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