Pore-lattice deformations in ordered mesoporous matrices: experimental studies and theoretical analysis

Abstract

The sorption of fluids in mesoporous silica is an important physical phenomenon with a wide range of applications. Traditionally, mesoporous materials have been considered as inert scaffolds for the sorption and condensation reaction of the fluid. Here we present in situ small angle X-ray diffraction experiments providing evidence for a sorption strain induced in the solid that manifests itself as a change in the lattice parameter of the ordered mesopore array as the pores gradually adsorb fluid material. The experimental data are analyzed by means of Monte Carlo simulations carried out in a grand canonical ensemble describing a fluid confined by deformable substrates. We show that-in agreement with experimental data-sorption of a nonpolar fluid causes the pores to expand initially, to shrink abruptly when capillary condensation sets in, and to expand again as more liquid-like fluid is adsorbed subsequently. We show that the pore pressure can be extracted from a thermodynamic analysis of sorption isotherms in the liquid-like regime and that this information can be used for an estimation of the Young's modulus of the porous silica material. In addition, our Monte Carlo simulations indicate that the phase behavior of confined fluids is considerably changed by the deformability of the confining solid. This is reflected by a change of the location of phase boundaries at sufficiently subcritical temperatures.

Capillary condensation in deformable mesopores: wetting versus nanomechanics

“…Typical lattice parameters, pore sizes, as well as the pore volume fractions of these materials are listed in Table 2.1 of Ref. 21. For the sake of concreteness we focus only on experiments using pentane as the adsorbate.…”

“…To make contact with a specific experimental situation we focus on sorption experiments [21] employing ordered mesoporous silica of the type MCM-41 [14]. These materials consist of grains of roughly 1 µm size, containing cylindrical nanopores on a well ordered twodimensional hexagonal pore lattice with pore diameter of the order of 3-4 nm and narrow size distributions.…”

“…To establish contact between theory and experiment in this manuscript we estimate both the diameter σ of a hypothetical pentane molecule in a dense liquid phase and a realistic distance s z0 of the 6 Table 2.1 of Ref. 21) which we adopt for sake of concreteness and to enable a direct comparison between experimental and theoretical data discussed in this work. Finally, noticing that both wall atoms and fluid molecules are assumed to be of the same size and on account of repulsive forces between them there is a distance of about σ at each of the solid substrates that is inaccessible to fluid molecules.…”

Adsorption-induced deformation of nanoporous materials—A review

“…A number of papers employed molecular modeling techniques for predictions of adsorption-induced strains in various nanoporous materials: Monte Carlo simulations 40,[81][82][83][84][85][86][87][88][89][90][91][92] or classical density functional theory (DFT). [93][94][95][96] At the same time, several purely analytical theories were also proposed.…”

“…81 In addition to qualitatively reproducing the observed adsorption-induced deformation, they showed that network flexibility significantly alters the adsorption and desorption isotherms as compared to a rigid network. Schoen and coworkers 83,84 performed a Monte Carlo simulation study for a model system, Lennard-Jones (LJ) fluid adsorption in a slit pore, where the solid atoms were allowed to elastically depart from their equilibrium sites, by a harmonic potential. Such a system is able to be strained normal to the pore walls, changing the width of the slit pore.…”