Analysis of criticality and isotherm reversibility in regular mesoporous materials

The reversibility of the adsorption isotherms of various gases has been studied on MCM-41 materials of various diameters, and their mixtures have been prepared by mechanical mixing in 1:1 weight ratio. As expected the results of the latter indicated a pore size distribution having two peaks. The isotherms were completely reversible with two capillary condensation steps for mixed samples whose component pore diameters were lower than 3.8 nm. The samples with one component having pore diameter above 3.8 nm indicated hysteresis associated with the larger pores, with the hysteresis loop found to close before the capillary evaporation from the smaller pore occurs. Empirically two critical sizes, one for absence of the condensation transition (D-CP) and one for absence of hysteresis (D-CH), have been recently identified, but quantitative study of the underlying phenomena has hitherto not been conducted for MCM-41. Various literature models, as well as a new model of the authors, were tested for explaining reversibility of the gas adsorption isotherms for MCM-41. It was found that the model of the authors utilizing the well-known tensile stress hypothesis is the most satisfactory among these alternatives

Capillary coexistence and criticality in mesopores: Modification of the Kelvin theory

The classical model of capillary equilibrium in cylindrical pores is modified here by the introduction of molecular concepts and the solid fluid interaction potential. The new approach accurately predicts capillary coexistence and criticality, with results quantitatively matching those from density functional theory for nitrogen adsorption, while also predicting condensation pressures in agreement with reported experimental findings for MCM-41. The larger critical pore size for nitrogen adsorption in these materials, however, suggests a modification of the potential function parameters, evaluated here from data for hydroxylated silica

Characterization of surface roughness of MCM-41 using methods of fractal analysis

We report here the characterization of surface roughness of the model mesoporous molecular sieve MCM-41, at various scales of resolution. This material comprises several levels of structure-that of the mesopores, the crystallites, the grains, and the particles-spanning four decades of resolution, each having its independent surface properties at its characteristic length scale. The apparent fractal dimension of this material, of various pore diameters, synthesized in our laboratory has been determined at various scales with the help of various characterization techniques such as adsorption, mercury porosimetry, small-angle X-ray and neutron scattering (SAXS and SANS). A new method for the estimation of fractal dimension from a gas adsorption isotherm is proposed which considers the effect of solid-fluid interactions and the meniscus curvature, neglected in earlier methods of fractal analysis. On the basis of the results, the MCM-41 structure is found to have a fractal dimension of 2 at molecular resolutions of 3-7 Angstrom and is therefore highly smooth at this scale. At lower resolutions corresponding to the mesopore diameter (20-50 Angstrom), it possesses an apparent fractal dimension of about 3, suggesting a higher roughness. While not suggestive of a fractal structure for the narrow pore size distribution of MCM-41, the roughness indicates the presence of constrictions in the mesopore channels. At still lower resolutions (80-250 Angstrom and 0.1-0.4 mu m), the structure is also found to be rough. It is also found that MCM-41 has a higher fractal dimension than HMS, and that vanadium incorporation into the structure of MCM-41 increases its roughness

Experimental and theoretical investigations of adsorption hysteris and criticality in MCM-41: studies with O₂, Ar, and CO₂

MCM-41 materials of six different pore diameters were prepared and characterized using X-ray diffraction, transmission electron microscopy, helium pycnometry, small-angle neutron scattering, and gas adsorption (argon at 77.4 and 87.4 K, nitrogen and oxygen at 77.4 K, and carbon dioxide at 194.6 K). A recent molecular continuum model of the authors, previously used for adsorption of nitrogen at 77.4 K, was applied here for adsorption of argon, oxygen, and carbon dioxide. While model predictions of single-pore adsorption isotherms for argon and oxygen are in satisfactory agreement with experimental data, significant deviation was found for carbon dioxide, most likely due to its high quadrupole moment. Predictions of critical pore diameter, below which reversible condensation occurs: were possible by the model and found to be consistent with experimental estimates, for the adsorption of the various gases. On the other hand, existing models such as the Barrett-Joyner-Halenda (BJH), Saito-Foley, and Dubinin-Astakhov models were found to be inadequate, either predicting an incorrect pore diameter or not correlating the isotherms adequately. The wall structure of MCM-41 appears to be close to that of amorphous silica, as inferred from our skeletal density measurements

Adsorption of benzene and ethanol on MCM-41 material

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Not AvailableBackground/aims: Thrombospondins (TSPs) are large multi-modular proteins, identified as natural angiogenesis inhibitors that exert their activity by binding to CD36 and CD47 receptors. The anti-angiogenic effect of TSPs in luteal regression of water buffalo has not been addressed. The present study characterized the expression pattern and localization of TSPs and their receptors in ovarian corpus luteum during different stages of development in buffalo. This study also elucidated the effect of exogenous Thrombospondin1 (TSP1) or the knocking out of the endogenous protein on luteal cell viability and function. Further, the in vitro transcriptional interaction of TSP1 with hormones, LH, PGF2α and angiogenic growth factors, VEGF and FGF2 were also evaluated. Methods: First, the CLs were classified into four groups based on macroscopic observation and progesterone concentration. mRNA expression of examined factors was measured by qPCR, localization by immunoblotting and immunohistochemistry. TSP1 was knocked out (KO) in cultured luteal cells isolated from late luteal stage CLs (day 1116) by CRISPR/Cas9 mediated gene editing technology in order to functionally validate the TSP1 gene. Isolated cells from late stage CLs were also stimulated with different doses of TSP1, LH, PGF2α, VEGF and FGF2 for various time intervals to determine transcriptional regulation of thrombospondins. Results: mRNA expression of TSPs and their receptors were found to be significantly higher in late and regressed stage of CL as compared to other groups which was consistent with the findings of immunoblotting and immunolocalization experiments. It was observed that TSP1 induced apoptosis, down regulated angiogenic growth factors, VEGF and FGF2 and attenuated progesterone production in cultured luteal cells. However, knocking out of endogenous TSP1 with CRISPR/Cas9 system improved the viability of luteal cells, progesterone synthesis and upregulated the expression of VEGF and FGF2 in the KO luteal cells. PGF2α induced the upregulation of TSPs and Caspase 3 transcripts, whereas treatment with LH and angiogenic growth factors (VEGF and FGF2) down regulated the TSP system in luteal cells. Conclusion: Collectively, these data provide evidence that thrombospondins along with their receptors are expressed at varying levels in different stages of CL progression with maximum expression during the late and regressing stages. These results are consistent with the hypothesis that thrombospondins stimulated by PGF2α plays an essential modulatory role in bringing about structural and functional luteolysis in buffalo.Not Availabl