Relationship between Pore-size Distribution and Flexibility of Adsorbent Materials: Statistical Mechanics and Future Material Characterization Techniques

Mechanical Characterization of Hierarchical Structured Porous Silica by in Situ Dilatometry Measurements during Gas Adsorption

Mechanical properties of hierarchically structured nanoporous materials are determined by the solid phase stiffness and the pore network morphology. We analyze the mechanical stiffness of hierarchically structured silica monoliths synthesized via a sol-gel process, which possess a macroporous scaffold built of interconnected struts with hexagonally ordered cylindrical mesopores. We consider samples with and without microporosity within the mesopore walls and analyze them on the macroscopic level as well as on the microscopic level of the mesopores. Untreated as-prepared samples still containing some organic components and the respective calcined and sintered counterparts of varying microporosity are investigated. To determine Young's moduli on the level of the macroscopic monoliths, we apply ultrasonic run time measurements, while Young's moduli of the mesopore walls are obtained by analysis of the in situ strain isotherms during N₂ adsorption at 77 K. For the latter, we extended our previously reported theoretical approach for this type of materials by incorporating the micropore effects, which are clearly not negligible in the calcined and most of the sintered samples. The comparison of the macro- and microscopic Young's moduli reveals that both properties follow essentially the same trends, that is, calcination and sintering increase the mechanical stiffness on both levels. Consequently, stiffening of the monolithic samples can be primarily attributed to stiffening of the backbone material which is consistent with the fact that the morphology on the mesopore level is mainly preserved with the post-treatments applied.

Molecular Simulations Shed Light on Potential Uses of Ultrasound in Nitrogen Adsorption Experiments

Nitrogen adsorption is one of the main characterization techniques for nanoporous materials. The experimental adsorption isotherm provides information about the surface area and pore size distribution (PSD) for a sample. In this work we show that additional insight into PSD can be gained when the speed of sound propagation through a sample is measured during nitrogen adsorption experiment. We analyzed published experimental data on ultrasound propagation through a nanoporous Vycor glass sample during nitrogen adsorption experiment. Next, we calculated the change of the longitudinal and shear moduli of the sample as a function of relative vapor pressure. From this, we show that the shear modulus of the sample does not change upon filling the pores, evidencing that adsorbed nitrogen at 77 K has zero shear modulus, similarly to a bulk liquid. The longitudinal modulus of the sample behaves differently: it changes abruptly at the capillary condensation and keeps gradually increasing thereafter. We performed Monte Carlo molecular simulations to predict the compressibility of adsorbed nitrogen and then calculated the longitudinal modulus of the nitrogen-saturated Vycor using the Gassmann equation. Our theoretical predictions nicely matched the longitudinal modulus derived from the experimental data. Additionally, we performed molecular simulations to model nitrogen adsorbed in silica pores of sizes ranging from 2 to 8 nm. We found that the isothermal elastic modulus of adsorbed nitrogen depends linearly on the inverse pore size. This dependence, along with the proposed recipe for probing the modulus of adsorbed nitrogen, sets up the grounds for extracting additional information about the porous samples, when the nitrogen adsorption is combined with ultrasonic experiments.

Liquid Adsorption of Organic Compounds on Hematite α-Fe2O3 Using ReaxFF

ReaxFF-based molecular dynamics simulations are used in this work to study the effect of the polarity of adsorbed molecules in the liquid phase on the structure and polarization of hematite (α-Fe₂O₃). We compared the adsorption of organic molecules with different polarities on a rigid hematite surface and on a flexible and polarizable surface. We show that the displacements of surface atoms and surface polarization in a flexible hematite model are proportional to the adsorbed molecule's polarity. The increase in electrostatic interactions resulting from charge transfer in the outermost solid atoms in a flexible hematite model results in better-defined adsorbed layers that are less ordered than those obtained assuming a rigid solid. These results suggest that care must be taken when parametrizing empirical transferable force fields because the calculated charges on a solid slab in vacuum may not be representative of a real system, especially when the solid is in contact with a polar liquid.