P.A. Russo

Adsorption of n-Pentane on Mesoporous Silica and Adsorbent Deformation

Development of quantitative theory of adsorption-induced deformation is important, e.g., for enhanced coalbed methane recovery by CO2 injection. It is also promising for the interpretation of experimental measurements of elastic properties of porous solids. We study deformation of mesoporous silica by n-pentane adsorption. The shape of experimental strain isotherms for this system differs from the shape predicted by thermodynamic theory of adsorption-induced deformation. We show that this difference can be attributed to the difference of disjoining pressure isotherm, responsible for the solid-fluid interactions. We suggest the disjoining pressure isotherm suitable for n-pentane adsorption on silica and derive the parameters for this isotherm from experimental data of n-pentane adsorption on nonporous silica. We use this isotherm in the formalism of macroscopic theory of adsorption-induced deformation of mesoporous materials, thus extending this theory for the case of weak solid-fluid interactions. We employ the extended theory to calculate solvation pressure and strain isotherms for SBA-15 and MCM-41 silica and compare it with experimental data obtained from small-angle X-ray scattering. Theoretical predictions for MCM-41 are in good agreement with the experiment, but for SBA-15 they are only qualitative. This deviation suggests that the elastic modulus of SBA-15 may change during pore filling.

Hydrocarbons adsorption on templated mesoporous materials: effect of the pore size, geometry and surface chemistry

Adsorption–desorption isotherms of aromatic (mesitylene and toluene) and aliphatic (methylcyclohexane, neopentane and n-pentane) hydrocarbons were measured on ordered mesoporous materials, including MCM-41, MCM-48, SBA-15, SBA-16 and MCF silicas, a periodic mesoporous organosilica and a CMK-3 carbon, in order to evaluate the effect of the adsorbent characteristics on the organic compounds adsorption behaviour. A clear separation between aliphatic and aromatic hydrocarbons is observed at low p/po for the materials having pores accessible by narrow openings. The presence of narrow pore openings causes an increase in the volume adsorbed of mesitylene, toluene, methylcyclohexane and n-pentane prior to capillary condensation that does not occur for neopentane. The increase of the hydrophobicity and change of the surface structure, resulting from the incorporation of chloromethyl groups on the silica walls, causes the p/po at which the aromatic hydrocarbons condense to increase while the introduction of aromatic rings into the pore walls has a less significant effect on the condensation pressures. However, at low p/po, all the hydrocarbons have higher affinity for the periodic mesoporous organosilica surface than for the pure silica or silica with chloromethyl groups surfaces, while these lower the affinity of the aromatic hydrocarbons. Good estimates of pore size (Dp) are obtained using the classical Kelvin equation from the mesitylene, toluene and methylcyclohexane adsorption data, for spheroidal pores of at least ∼20 nm. With n-pentane this occurs for pores of ∼30 nm while neopentane underestimates Dp values even for pores as large as these, although to a lesser extent than nitrogen.

Ordered mesoporous silica materials for protein adsorption

Lysozyme and BSA were used, as model proteins of considerably different dimensions, in order to evaluate the influence of the distinct pore structural characteristics of three types of ordered mesoporous silica materials (MCF, SBA-15 and MCM-41) on protein adsorption. Characterisation by X ray diffraction and nitrogen adsorption at 77K revealed the typical pore structural features of each type of material. The maximum of the pore size distributions indicated that the width of the windows of MCF (2) (mesitylene/P123 of 2) was larger than the pore diameter of the unidirectional tubular pores of SBA-15. All the materials presented similar small external surface areas but high pore volumes, with that of MCF (2) being the highest. The adsorption of lysozyme at pH=8 increased in the order MCM-41<< SBA-15< MCF (2), and the uptakes were well above those of BSA at pH=5. Although BSA is not completely excluded from the mesopores of SBA-15 and MCF (2), as happens with MCM-41, the adsorption occurs to a very limited extent. The overall behaviour of these SBA-15 and MCF (2) samples was not significantly different and both revealed potential for the separation of these proteins.

Volatile organic compound adsorption on a nonporous silica surface: how do different probe molecules sense the same surface?

In this work, we compare experimental results to molecular simulation results of volatile organic compound (VOC) adsorption on nonporous silica. We adopted an effective model for the rough solid surface, obtained by a temperature annealing scheme, plus an experimental/simulation nitrogen adsorption tuning process over the silica energetic oxygen parameter. The measurement/prediction of selected VOCs, specifically, n-pentane and methylcyclohexane, is presented in terms of adsorption isotherms, with an emphasis on the angle distribution analysis of the three studied probe molecules with respect to the same modeled surface.

Nitrogen adsorption studies on non-porous silica: the annealing effect over surface non-bridging oxygen atoms

This contribution compares experimental nitrogen adsorption isotherms with molecular simulation results on effective non-porous silica models. A Molecular Dynamic (MD) annealing temperature scheme was adopted for the formulation of three samples of the solid surface. By means of Grand Canonical Monte Carlo (GCMC) simulations, nitrogen adsorption isotherms were obtained for each sample, with charged and uncharged nitrogen molecules, and compared with relevant experimental data. Following comparison of the experimental and simulated nitrogen results, a tuning process over the energetic parameter of the oxygen atoms was performed for the final proposed model. Specifically, the results are presented in terms of adsorption isotherms with an emphasis on the annealing effect over relevant model parameters, such as the surface concentration and the distribution of non-bridging oxygen atoms.

Calculation of Pore Size Distributions in Low‐k Films

Porosimetry is a key technology for the characterization of porous low‐k dielectric films. A critical appraisal of the models used for quantitative interpretation of vapor adsorption isotherms obtained by ellipsometric and x‐ray reflectivity measurements shows that conventional macroscopic methods based on the Kelvin equation are inadequate on the nanoscale. We have developed an advanced molecular model of toluene adsorption in nanopores and the method for pore size distribution calculations based on this mode. The method has been verified against reference ordered porous silicas and applied to selected low‐k films. Significant deviations of the Kelvin equation were found for pores < 4 nm. The new method provides a unified framework for reliable calculations of micro‐ and mesopore size distributions. The results are generally consistent with available data of small angle neutron scattering porosimetry.