Capillary Condensation Region Research Articles

New approach to the calculation of pistachio powder hysteresis

Abstract Moisture sorption isotherms for pistachio powder were determined by gravimetric method at temperatures of 15, 25, 35 and 40°C. A selected mathematical models were tested to determine the best suitable model to predict isotherm curve. The results show that Caurie model had the most satisfactory goodness of fit. Also, another purpose of this research was to introduce a new methodology to determine the amount of hysteresis at different temperatures by using best predictive model of isotherm curve based on definite integration method. The results demonstrated that maximum hysteresis is related to the multi-layer water (in the range of water activity 0.2-0.6) which corresponds to the capillary condensation region and this phenomenon decreases with increasing temperature.

Impact of Chemical Heterogeneity on the Accuracy of Pore Size Distributions in Disordered Solids

Disordered porous solids, such as heterogeneous catalysts, possess internal void spaces with high degrees of both chemical and geometric heterogeneity. For pore size distributions (PSDs) to be useful void space descriptors for aiding understanding of phenomena such as catalyst activity, their level of accuracy must be known. Due to differences in specific interactions and wetting behavior, the choice of adsorbate can severely impact the accuracy of PSDs derived for disordered materials using gas sorption. Wetting effects in the capillary condensation region have recently been studied extensively by simulation, but there have been no complementary, definitive experimental studies of sorption mechanisms. However, in this work, the novel integrated gas sorption and mercury porosimetry technique has been used to prepare a particular subset of pores, located within the disordered network of a given amorphous material (eg silica or alumina), that had dead-ends with different chemical properties to the walls, and then definitively compare the individual sorption behavior of two different, common adsorbates (nitrogen and argon) within just these pores. The presence of a heavy metal surface was found to lead to a shift to higher pressure in the positions of adsorption and desorption branches and a widening of the hysteresis for argon isotherms for silica pores, but not for alumina pores. In contrast, no such effect was observed for nitrogen. Hence, using argon adsorption rather than nitrogen adsorption was found likely to lead to pore size inaccuracies of at least ∼200% in characterizing silica-supported platinum catalysts, for example. The experimental findings on effects of wetting and pore length were found to be consistent with findings from recent Monte Carlo and MFDFT simulations. Further, analysis of hysteresis widths also suggested that the standard approach using NLDFT would be unsuitable for the type of samples studied here. The results highlight particular issues for the accuracy of PSDs for catalysts consisting of metal nanoparticles supported on oxide materials.

Interaction of Ethanol Vapour with Silane-Modified Surface of Corning Porous Glass CPG-10

The interaction of ethanol vapour at 293 K with modified Corning controlled porous glass (CPG-10) samples of various pre-silylation is reported. Ethanol adsorption on CPG-10 was greatly reduced by silylation to extents largely depending on the silylation extent, type of silane used, and on the concentration and accessibility of residual silanols. The obtained adsorption isotherms, BET surface areas, as well as the observed changes at the various stages during the sorption particularly in the capillary condensation region also showed similar dependence. In the capillary condensation region, in the case of the dimethyl dichlorosilane-modified sample, hysteresis type was changed from H3 to H1. On the triethyl chlorosilane-modified sample an unexpected suppression of hysteresis effects occurred with a consequent transformation of type IV to type II adsorption isotherm.

Adsorption equilibrium and kinetics for capillary condensation of trichloroethylene on MCM-41 and MCM-48

Adsorption equilibrium data of trichloroethylene (TCE) on synthesized MCM-48 were determined at different temperatures (303.15, 308.15, 313.15, 323.15 K) using a gravimetric technique. MCM-41 was chosen for comparison purpose. The adsorption equilibria of TCE on two mesoporous materials show an unusual type IV isotherm. A hybrid isotherm model composed of Langmuir and Sips equations was applied to fit TCE adsorption on mesorpores including its surface adsorption and capillary condensation. The proposed isotherm correlated the measured adsorption data quite well over the whole pressure range. Adsorption kinetics for MCM-41 and MCM-48 was also investigated under two distinct pressure regions representing surface adsorption and capillary condensation, using a diffusion model. It was found that the mass transfer for the region of capillary condensation was considerably slower than that for the surface adsorption region. We also found that the mass transfer in MCM-48 having the cubic mesopore system was faster than that in MCM-41 having the cylindrical mesopore structure.

X-ray Study of Freezing and Melting of Water Confined within SBA-15

To examine the freezing and melting behavior of water in partially filled pores of porous silica, we performed X-ray diffraction measurements of water confined inside the cylindrical pores of SBA-15 with a pore radius of 3.9 nm as a function of temperature and pore filling. The freezing temperature increased continuously with increasing pore filling even in the region of capillary condensation. The results are related to the different states of the pore water depending on the degree of pore filling. On the other hand, the melting of the frozen pore water took place at a well-defined temperature of 256 K, independent of the level of pore filling. The X-ray diffraction patterns show that the freezing of the pore water results in formation of ice microcrystals with almost the same structure and size, irrespective of the different states of the pore water.

Water vapor adsorption of ready-to-cook wheat

Water vapor adsorption of bulgur and hedik samples made from Triticum durum (Gediz 75) and Triticum aestivum (Panda) wheat were investigated. Experiments were conducted at 5, 15, 20, 30, and 40 °C for bulgur and at 20 °C for hedik samples between relative humidity (RH) of 11% and 100%. Bulgur samples became moldy before attaining equilibrium above 5 °C and 81% RH between 4 and 28 d depending on temperature ( T). Hedik samples did not become apparently moldy and always had smaller equilibrium moisture content ( X) than their bulgur counterparts at 20 °C and same RH. Type of wheat governed adsorption of the products. Bulgur and hedik of Gediz wheat always exhibited higher water adsorption capacity than bulgur and hedik of Panda wheat, respectively. The Guggenheim–Anderson–Boer model fitted all samples very well under the given conditions. The monolayer moisture content ( X m) was 9.7% and 9.6% (d.b.) for bulgur and hedik of Gediz wheat, and 7.5% and 6.5% (d.b.) for bulgur and hedik of Panda wheat, respectively. The heat of adsorption ( Q ) of bulgur samples was not affected by temperature and decreased with increasing X. Gediz bulgur had higher Q values than Panda bulgur up to the capillary condensation region. The average Q was obtained as 75, 50, and 45 kJ/mol for Gediz bulgur, and 73, 49, and 45 kJ/mol Panda bulgur for the monolayer, multilayer, and capillary adsorption regions, respectively.

Correlation between two- and three-dimensional condensations in cylindrical mesopores of active carbons

Adsorption interactions in narrow cylindrical capillaries decay with radii approximately as r −1.1 and this slowly decreasing adsorption potential is responsible for adsorption behavior in the capillary condensation region. Filling of mesopores with concave curvatures is considered to be an evolution of the two-dimensional (2D) condensation, which occurs on the mesopore walls at the critical condensation pressure. An adsorption isotherm in the individual mesopore exhibits the feature characteristic of condensation: a vertical jump at a pressure approximately corresponding to the critical pressure of a 2D-condensation. Mesopore filling pressures are found proceeding from statistical mechanical theories of adsorption on the planar surface taking into account lateral adsorbate–adsorbate interactions. The model describes the adsorption branch of the hysteresis loop. The underlying reason of the hysteresis loop is discussed. Analysis of adsorption isotherms for benzene and nitrogen shows that the developed model and the classical Kelvin equation lead to close values of mesopore radii.

Adsorption–Desorption Flow of Condensable Vapors through Mesoporous Media: Network Modeling and Percolation Theory

Flow of condensable vapors in mesoporous media is investigated theoretically and experimentally during adsorption and desorption processes. A typical permeability curve of a condensable vapor is strongly enhanced in the capillary condensation region. This is because additional capillary pressure gradients are imposed on the capillary-condensed pores, which act as “good” conductors compared to the noncondensed pores, which are considered “poor” conductors. The percolation scaling properties that hold for a system of “good” and “poor” conductors are confirmed for the cases examined. As the ratio of gas flow/capillary-enhanced flow decreases, the rise of permeability with pressure becomes sharper. The network connectivity has a strong impact on the maximum permeability value and on the width of the scaling law regions. The contribution of surface flow does not affect the permeability in the peak region, but results in a shrinkage of the scaling law regions. During desorption, a marked hysteresis in the permeability curves is found and it is attributed only to thermodynamic hysteresis. The maximum permeability values in this case are higher and shifted to lower relative pressures.

Relations between Structural Parameters and Adsorption Characterization of Templated Nanoporous Materials with Cubic Symmetry

A systematic approach is proposed to structural characterization of templated nanoporous materials with cubic symmetry by gas adsorption. We hypothesize that regular structures of these materials can be described in terms of triply periodic minimal surfaces (TPMS), similarly to bicontinuous mesophases observed in oil−water, lipid, block copolymer, and other amphiphilic systems. We relate topological characteristics of TPMS to the pore structure parameters evaluated from adsorption measurements, such as the specific surface area, pore volume, mean pore size, and also pore wall thickness. The relations obtained can be used for discrimination of possible TPMS morphologies. The method developed is used for characterization of newly synthesized MCM-48 mesoporous materials by low-temperature nitrogen adsorption. We show that adsorption data fully support the minimal gyroid model of MCM-48 structure (Ia3d space group) established earlier by the X-ray diffraction (XRD) and transmission electron microscopy studies. The mean pore size of MCM-48 can be accurately described by the hydraulic diameter calculated from the capillary condensation region of nitrogen adsorption isotherms by the nonlocal density functional theory method. Moreover, the adsorption method allows one to estimate the pore wall thickness, which cannot be done by XRD. For a series of high-quality MCM-48 materials reported recently in the literature, the calculated mean wall thickness varied from 0.8 to 1.2 nm. The adsorption method developed is recommended as a complement to X-ray diffraction and electron microscopy techniques for characterization of nanoporous materials.

Nitrogen adsorption isotherms on porous powders of alumina and silica coated with manganese oxides

A detailed examination of low temperature gas adsorption indicates differences in the mode of dispersion of manganese oxide on alumina from dispersion on silica. This difference is further reflected by the variation on B.E.T. C values within the two series of samples. Calculations also reveal that physical structure influences the C value, particularly so in the region where the monolayer is being formed, and in the region of capillary condensation. The fraction of surface covered (θ) during the adsorption process has been calculated and compared with results obtained on non-porous materials.

The structure of the pore space of coal in bump-prone and burst-prone seams and the form of the moisture filling this space

To make a detailed study of the sorption pore space of the coal, we can use the method of sorption of vapor. For bodies permeated by a network of pores and capillaries, the form of the adsorption isotherms depends directly on the transverse dimensions of the capillaries (i.e., on their effective radius r/sub eff/). In most cases the adsorption isotherms of porous materials are of the so-called S shape (or approximate to it), with hysteresis over some range of relative vapor pressures p/p/sub s/. In adsorbents with fine pores (r/sub eff/ less than or equal to 10 A), or, on the other hand, in nonporous ones (and also in very coarsely porous ones), there may be no hysteresis. The first steep rise in the isotherm corresponds to monomolecular adsorption in which the first layer of gas molecules forms dipoles tightly bound to the surface. On further rise in pressure the adsorption continues to increase, because new adsorbed layers continue to form until the adsorbed gas (vapor) begins to condense on the surface or in the capillaries as an ordinary liquid. Therefore the second steep rise in the isotherm, characterized by hysteresis, is attributed to capillary condensation in the pores ofmore » the adsorbent. Capillary condensation begins on the polymolecular film; its start coincides with the start of hysteresis. In the region of capillary condensation, the relative pressure p/p/sub s/ above the meniscus of a liquid (gas)in the capillaries is related by Thomson's equation to the characteristics of the liquid (gas) in the capillary and the corresponding radius of the capillary r. With water vapor as the adsorbate, the relative vapor pressure is attained by means of the different relative air humidities above sulfuric acid solutions of various concentrations. Coal samples from various mines were examined and correlated with rock burst occurrences.« less

Sorption and flow of carbon dioxide and some hydrocarbons in a microporous carbon membrane

The flow of He, CO 2, CH 4, C 2H 6, C 3H 8, n-C 4H 10 and neo-C 5H 12 has been investigated using microporous carbon membranes. Physical constraint of the compacted carbon with steel plungers in a steel membrane holder substantially altered sorption equilibria and hysteresis loops in the multilayer and capillary condensation region as compared with unconstrained powder, but made little difference in the region below monolayer coverage. Capillary pressure differences between constrained compact and unconstrained powder showed maxima in the vicinity of monolayer coverage and near coverages where differential diffusion coefficients also went through maxima. This supports a hydrodynamic mechanism for sorbed fluids as monolayer coverage was approached and exceeded. For these larger uptakes flow rates were dominated by contributions arising from the presence of sorbed films with concentration gradients. Permeabilities did not show maxima found with differential diffusion coefficients. Blockage by sorbed films of the gas phase transport of helium was investigated for hydrocarbons using helium as internal indicator. Blockage became nearly complete as sorption approached monolayer coverage.

NMR spin echo studies on mobility and diffusion of benzene adsorbed on silica

Diffusion coefficients of benzene in porous plugs compressed from Aerosil were measured by the spin echo technique with the use of constant and pulsed field gradients. In order to obtain surface self-diffusion coefficients D s ∗ the contribution from molecular mobility in the pore volume has been evaluated. D s ∗ shows a maximum near θ = 1 in agreement with earlier results obtained from rates of adsorption. Differences between the limiting experimental diffusion coefficients in the region of capillary condensation and the bulk liquid are attributed to the tortuosity of the pore system. Activation energies for surface self-diffusion are larger than for T 2.

Metal-catalyzed oxidation in the presence of water in foods.

AbstractMethyl linoleate was oxidized in model systems consisting of either cellulose or casein with which the lipid was dispersed with water containing cobalt salts. The dispersion was extruded into Warburg flasks, frozen and freeze‐dried at 100 µ Hg and with platen temperatures of 80 F. The samples were then humidified over saturated salt solutions to give moisture contents from less than 1 g H2O/100 g solids up to 30 g H2O/100 g solids. The higher moisture contents were obtained by addition of glycerol to the model system during preparation and humidification at 60–75% RH. Chelating agents including EDTA and citric acid in concentrations of 1 to 10 moles per mole of cobalt ion were used in some experiments. Oxidation was followed manometrically and by peroxide analysis. At low water contents, water acts as an antioxidant through hydration of metallic catalysts. As the moisture content increases, water promotes oxidation through its solvent activity. In the region of capillary condensation, antioxidant effect of metal hydration is overshadowed by the prooxidant effect of metal solubilization. The water soluble chelating agents such as EDTA act on metals in aqueous solution and their activity is promoted by increased moisture content.

Kinetics in linoleate oxidation in model systems.

AbstractOxidation of methyl linoleate, trilinoleate and linoleic acid has been studied in model systems based on various solid supports. Oxidation was followed by measurement of oxygen absorption, peroxide values and products of oxidation as a function of moisture equilibrium relative humidity. Effects of various metals, histidine and the antioxidants propyl gallate and butylated hydroxytoluene were studied. The results indicate: (a) at 50 C oxidation of protein increases with increasing moisture content and the protein interacts with peroxides changing the overall oxidation rate; (b) increasing moisture content shows the same inhibitory effect on oxidation of trilinoleate as it does on methyl linoleate; (c) the effectiveness of antioxidants is increased with increasing humidity but some chelating agents complexed with metals become catalytic at the higher moisture content; and (d) at moisture contents in the region of capillary condensation, mobility of reactants is enhanced since the rate of oxidation increases significantly.

Some remarks about capillary condensation and pore structure analysis

Adsorption and desorption isotherms of nitrogen are widely used for the determination of the pore structures of adsorbents, catalysts, and other porous solids. Recently, the present authors published a paper in which they offered two new methods of pore structure analysis. These methods, like the previous ones, employ the capillary condensation region of the isotherms for the analysis—the region in which the adsorption and desorption isotherms differ from each other. The considerations advanced in the present paper attempt to answer the questions ( a) why in most instances the desorption isotherm, but in certain instances the adsorption isotherm, gives better results for pore structure analysis; ( b) what are the advantages of the two new methods over those presently employed; and ( c) why the cylindrical pore shape idealizations used by most investigators frequently give reasonable results.

Surface conductivity and conduction mechanisms on adsorption of vapours on silica

Surface conductivity of a high surface-area porous silica adsorbent has been studied upon adsorption of CH3NO2, C6H6, dioxane, H2O and D2O. For low surface coverages an exponential dependence between conductance and amount of gas adsorbed was found, whereas in the capillary condensation region the exponential dependence changed gradually to a linear one. The temperature dependence of the conductivity, appearance of polarization, and, with D2O, the detection of electrolytic products proved that the conductivity is of an ionic type. Energies of activation for conduction depended on coverage and decreased with the amount adsorbed. Measurements were performed on surfaces modified by heat treatment, deuteration and enrichment in Na+ ions. Possible mechanisms for surface conductivity are suggested.

Inclusion of fluorine compounds in faujasite I. The physical state of the occluded molecules

An account has been given of the intracrystalline sorption of SF 6 , CF 4 , C 2 F 6 , C 3 F 8 , n - C 4 F 10 , C 2 F 4 Cl 2 , perfluoromethyl cyclo hexane, perfluorodimethyl cyclo hexane and perfluoro cyclo butene in crystals of a synthetic near-faujasite. The inclusion of all these molecules except perfluoro cyclo butene was reversible. In the region of capillary condensation in interparticle spaces, hysteresis not associated with inclusion, was observed. Perfluoro cyclo butene underwent a chemical reaction within the zeolite. The equilibrium isotherms have been analyzed, and it has been shown that if the quotient p (l — θ ) / θ is plotted against θ , curves of characteristic form are obtained for all the fluoro-compounds so examined. Reference to previous studies of hydrocarbon inclusion and of zeolitic water in faujasite showed this behaviour to be general for these materials. It can be interpreted for small θ in terms of configurational entropy changes associated with an observed exothermal molecule-molecule interaction (part II) and with changes in heats of inclusion due to these interactions. It appears to be determined, for larger values of θ , either by decrease in vibrational entropy of the included molecules, associated with higher binding energy (Barrer & Sutherland 1956) or by loss of rotational entropy due to closer packing, or to both effects simultaneously.

Adsorption of mixtures of gases

1. The volumetric-gravimetric method was used to study the adsorption of a mixture of water and ethyl chloride vapors on active charcoal with growth of intermediate porosity at a temperature of 75°. 2. The set of isotherms for the adsorption of water at constant values for the adsorption of ethyl chloride can be described in a certain region by the equation proposed by Dubinin and Serpinskii. The set of isotherms for the adsorption of ethyl chloride at constant values for the adsorption of water can be described in the entire region studied by the equation derived by Dubinin, Saverina and Radushkevich for charcoals of the first structural type. 3. The effective density of the water, adsorbed in the micropores of charcoal, is substantially greater than the normal density of water at the same temperature. Transition to sorption in the region of capillary condensation is characterized by a reduction in the water density. 4. Combining the two indicated equations makes it possible to satisfactorily describe the adsorption of a mixture of water and ethyl chloride vapors on active charcoal, employing the parameters of the individual adsorption isotherms of the two compounds and one empirical parameter when water adsorption a1≤2 mmole/g.

Rates of flow through microporous solids

AbstractRates of flow of pure gases, both those with no adsorption and those with appreciable adsorption, were studied as a function of pressure level, pressure drop, and temperature for flow through 1/2‐in.‐diameter cylindrical plugs of activated carbon and of unsintered Vycor glass. Adsorption isotherms for the pure gases on Vycor glass were measured over the range of variables covered in the flow studies. A few measurements were made for bulk liquid flowing through a Vycor plug.Permeabilities, which are proportional to the rate of flow per unit of pressure drop, were satisfactorily correlated for hydrogen, helium, argon, and nitrogen by employing existing gas‐phase flow theory. Permeabilities considerably larger than the values predicted from the nonadsorbed gas correlation, sometimes more than seventeen times as large, were observed for ethylene, propylene, and isobutane flowing through a Vycor plug. For the hydrocarbon‐Vycor systems, permeabilities for vapor flow are as much as sixty times larger than for bulk liquid flow.The unusual flow phenomena for the hydrocarbon‐Vycor systems are attributed to a rapid transport in the adsorbed layer. The total transport is treated as being the sum of gas‐phase and adsorbed‐layer flow. An equation describing adsorbed‐layer movement is derived by utilizing a force balance together with thermodynamic principles. The resulting equation has just one empirical constant, and its use requires adsorption‐isotherm data. It correlates very well the surface flow rates for the major range of the variables covered in this investigation. Rate measurements were made for adsorbed‐layer concentrations ranging from about one tenth of a monolayer up through the capillary condensation region. Deviations in the one constant form of the equation are observed below one tenth of a monolayer. The available literature data on flow in adsorbed layers are reasonably well correlated by the same equation.