Water Vapor Adsorption-desorption Isotherms Research Articles

A hydrostable mesoporous γ-Al2O3 membrane modified with Si–C–H organic-inorganic hybrid derived from polycarbosilane

Excellent hydrophobicity of polycarbosilane (PCS)-derived Si–C–H organic-inorganic hybrid was characterized by measuring water vapor adsorption-desorption isotherms at room temperature. Modification with the PCS-derived Si–C–H successfully improved hydrostability in terms of stable gas permeation property of sol gel-derived hydrophilic mesoporous γ-Al2O3 membrane formed on a macroporous α-Al2O3 support: even under saturated water vapor partial pressure at 50 °C, the modified membrane exhibited high gas permeances of helium, hydrogen and nitrogen ranging from 1.2 to 3.0 × 10−6 mol m−2 s−1·Pa−1. The improved gas permeability under the humid condition achieved for the Si–C–H-modified mesoporous γ-Al2O3 membrane in this study was discussed aiming to prepare a gas permeable hydrophobic mesoporous intermediate layer essential for developing H2-permselective ceramic-based multilayered composite membranes with applications to novel solar hydrogen production system.

Effect of Both the Phase Composition and Modification Methods on Structural-Adsorption Parameters of Dispersed Silicas

Tripoli from two Ukrainian deposits was studied in its natural and modified forms. The investigation of natural and modified tripoli involves the identification of their phase compositions through X-ray diffraction and the analysis of their water vapor adsorption-desorption isotherms. The obtained results are evidence of changes in the structural-adsorption parameters of tripoli as a result of modification. Their treatment in boiling water or acid causes apparent alterations of contents of the main phases and sizes of their crystallites, whereas their calcination causes not only the dehydroxylation of surfaces and the agglomeration of phases, but even phase transformation in the case of carbonate tripoli. After analyzing water vapor adsorption-desorption isotherms of natural and modified tripolis, some correlations between their adsorption parameters, phase compositions, main phase contents and crystallite sizes have been found.

Water vapor transport properties of regenerated cellulose and nanofibrillated cellulose films

In this study, water vapor transport properties of nano-fibrillated cellulose (NFC) and regenerated cellulose films, derived from different sources, were investigated and compared with the transport properties of unmodified paper. Gravimetric-IGA experimental system was used to measure the kinetics and water vapor permeabilities (WVPs) of the samples. Water vapor adsorption–desorption isotherms were measured for different cellulose films using Belsorp instrument. Monolayer moisture content was determined by fitting the adsorption isotherm with the Guggenheim–Anderson–De Boer equation (GAB model). The monolayer moisture content of the regenerated cellulose and NFC films were found to be more than two times larger than the monolayer moisture content obtained for unmodified paper sample (made from bleached Kraft pulp ~4.12wt%), at saturation. This indicates a large number of hydrophilic sites available on the surface of regenerated cellulose and NFC films. The water transmission rates (WVTR’s) of regenerated cellulose and NFC films were similar, but lower compared to the unmodified paper measured at a wide range of relative humidities. Furthermore, water vapor permeabilities (WVP’s) of NFC and regenerated cellulose films were found to be concentration dependent exhibiting lower values up to RH=25%, but then showing a rapidly increasing trends above RH=25%. WVP’s of the cellulose films revealed an Arrhenius type of dependency with temperature, indicating an activated process.Water vapor diffusivities were also measured from the uptake rate measurements for a wide range of relative humidities. The diffusivities generally showed dependency on the moisture content of the sample. The effective diffusion coefficients (Deff) of NFC and regenerated cellulose films are 1–2 orders of magnitude lower in comparison with the unmodified paper, depending on the moisture content. The mechanisms of water vapor transport through the cellulose films were also evaluated from the uptake rate experiments. The results showed that the external surface resistance was a more dominant mechanism to mass transfer in the NFC and regenerated cellulose films. On the other hand, water vapor uptakes on unmodified paper seem to be consistent with internal diffusion control mechanism, except at the high moisture contents. At the high relative humidity or moisture contents of the films, both the external surface resistance and diffusion control the transport in transparent cellulose films and paper samples.

Changes in Water Adsorption Desorption Isotherms of Cement Mortars Caused by Carbonation Coupled with Humidification-Drying Cycles

The objective of this work was to examine the influence of carbonation on the water vapor adsorption desorption isotherms of cement materials. Two types of Portland mortars, which were CEM I and CEM II, were carbonated at 20°C, 65% relative humidity and 20% of CO2concentration. The pore size distributions were determined from the water adsorption method. We also studied the coupling effect between carbonation and humidification-drying cycles. The results showed a reduction in microporosity and a small increase in the mesoporosity. The pores clogging due to formation of calcium carbonate was highlighted by the reduction of the quantity of adsorbed water and the decrease in the hysteresis isotherms. The results of this study also indicated that the humidification-drying cycles coincide only from the second cycle because of a difficult evacuation of water during desorption, even at low humidity.

Characterizing pore structure of cement blend pastes using water vapor sorption analysis

Pore structure is an essential factor that influences the mechanical behavior and durability of cement-based porous materials with or without added binders. An empirical model for water vapor sorption isotherms was employed to evaluate the pore structure of hardened cement pastes incorporating granulated blast furnace slag and silica fume. The model is an extension of the Brunauer–Emmett–Teller multilayer adsorption theory. Assuming cylindrical-shaped pores and an adsorbed liquid-like layer between the pore surface and gas phase, pore size distributions of the blended cement pastes were estimated. Calculated pore size distribution curves were compared with those measured by mercury intrusion porosimetry. Added granulated blast furnace slag and silica fume had minor effects on the monolayer adsorption capacity, but reduced the energy of the first and subsequent adsorption layers. The adsorbed liquid-like layer generated sharper pore size distribution peaks that were shifted to the mesoporous region. The pore size distributions were comparable with those determined by the mercury extrusion branch, but differed from those obtained by the mercury intrusion branch. Hysteresis of the water vapor adsorption–desorption isotherms and mercury intrusion–extrusion curves was due to the entrapment of a non-wetting phase in the porous system, further promoted by residual mercury in the pores following mercury extrusion.

Preparation of Hydrothermally Solidified Mesoporous Materials from Diatomaceous Earth for Moisture Control Application

Mesoporous materials were prepared by hydrothermal treatment of a powder compact consisting of natural or calcined diatomaceous earth and slaked lime. Tobermorite formed after the hydrothermal process, leading to strength improvement. The structure tangled with each tobermorite particle, causing the formation of mesopores. The bending strength and mesopore volume of the hydrothermally solidified materials derived from calcined diatomaceous earth was much higher than those derived from natural diatomaceous earth. Impurities in the natural diatomaceous earth suppressed the crystallization and growth of tobermorite, which plays an important role in strength improvement and mesopore formation. Water vapor adsorption–desorption isotherms showed that the amount of adsorbed moisture on the two kinds of hydrothermally solidified materials was almost the same.

Methods of evaluating pore morphology in hybrid organic–inorganic porous materials

Hybrid materials were prepared by immobilizing ionic liquid (IL), based on grafting methylimidazolium cation on the internal surface of porous Vycor ®. The extent of the initial silane grafting, expressed as the fraction of the occupied Vycor ® silanol groups was determined by the analysis water vapor adsorption–desorption isotherms. Chloroform was shown to be the most appropriate solvent, giving silanol occupation degree of the order of 80%. Moreover, the pretreatment of Vycor ® at 430 K under high vacuum conditions, enhanced the monolayer grafting and this can be attributed to avoidance of silane polymerization on the Vycor ® surface and expanded the degree of chloroform coverage up to 96%. Combination of nitrogen physisorption with SAXS led to the elucidation of the morphology of the internal surface after grafting. It was concluded that the adsorbed water on the Vycor ® surface, in combination with the presence of residual water in the solvent, facilitated the formation of bulky, but not continuous organosilane entities, which were generated by the silane polymerization. The grafted samples were further treated with 1-methylimidazole at 358 K for 48 hours following ion exchange with the PF 6 −. A new methodology was developed aiming at the determination of the w/w% percentage of IL loading on the grafted materials. The method is based on appropriate analysis of the transient data of CO 2 adsorption isotherms and the results were in good agreement with the analysis of the TGA measurements.

Effect of Acid Treatment and Alkali Treatment on Nanopore Properties of Selected Minerals

AbstractBentonite, biotite, illite, kaolin, muscovite, vermiculite and zeolite were acidified or alkalized with HCl orNaOH of concentrations 0.0, 0.1, 1.0 and 5.0 mole dm−3 at room temperature for 2 weeks and converted into Ca homoionic forms. Low-temperature nitrogen and room-temperature water-vapor adsorption-desorption isotherms were used to characterize the mineral pores of radii between 1 and 30 nm. Nanopore volumes, size distributions, average radii and fractal dimensions were calculated. Values calculated from the nitrogen isotherms differed from those derived from water-vapor data. With an increase of the acid-treatment concentration, the pore volumes measured using both adsorption techniques increased markedly for all minerals. The pore radii measured from nitrogen isotherms appeared to decrease for all minerals except zeolite, while the pore radius calculated from water-vapor data increased in most cases. The fractal dimension measured from water vapor isotherms decreased in all cases indicating smoothing of the mineral surfaces and decrease in pore complexity. No well defined trends in any of the pore parameters listed above were noted under alkaline treatment. In the reaction of each mineral with acid and alkali treatments, the individual character of the mineral and the presence of impurities seems important.

Surface and micropore properties of saline soil profiles

Based on water vapor adsorption–desorption isotherms, the specific surface and its energetic and geometric properties were estimated for twelve profiles of saline soils of different salinity. The contents of clay fraction and organic matter governed the CEC and the surface area of the soils. The contribution of clay fraction to the CEC was over four times smaller than that of the organic matter, whereas the contribution of clay to surface area was around three times larger. The average water vapor adsorption energy decreased logarithmically with the increase of the sodium adsorption ratio and linearly with the increase of pH. The amount of soil micropores increased with the increase in surface area. The surface fractal dimension appeared to positively correlate with soil CEC.

Surface Acidity and Hydrophilicity of Coprecipitated Al2O3–SiO2 Xerogels Prepared from Aluminium Nitrate Nonahydrate and Tetraethylorthosilicate

Amorphous aluminosilicate xerogels with various chemical compositions were prepared by coprecipitation, and their surface acidity and hydrophilicity were investigated by NH3 gas temperature programed desorption (TPD), water vapor adsorption–desorption isotherms, and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR). The xerogels were synthesized by adding conc. NH4OH to an ethanol solution of calculated amounts of aluminium nitrate nonahydrate and tetraethylorthosilicate, and calcined at 300°C for 4 h. All the NH3 TPD spectra of the xerogels showed similar asymmetric peak profiles at around 200°C tailing to the higher temperature side. The amount of acidity evaluated from the peak area of the TPD spectra showed a maximum at around 10 mol% Al2O3 composition. The change as a function of composition showed a good correlation with the total amount of four and five coordinated Al atoms in the xerogels deduced from the 27Al MAS NMR spectra. The water vapor adsorption isotherms of the xerogels were all of type IV irrespective of the composition. The maximum amounts of water vapor adsorbed by these xerogels were about 600–700 ml(STP)/g and were relatively high compared with those for various other adsorbents reported so far. Since the thickness of the adsorbed water vapor layer of the xerogels in the low relative pressure region increased with increasing Al2O3 content, the surface of the xerogels is considered to become more hydrophilic with increasing Al2O3 content of the xerogels.

Water Vapor Adsorption Properties of Ti-Containing Mesoporous Silica.

Materials enabling intelligent humidity control were synthesized using fumed silica, tetraethylorthotitanate and quaternary alkylammonium surfactant (decyl-, dodecyl-, tetradecyl and hexadecyltrimethylammonium chlorides) as a liquid-crystal template under hydrothermal conditions. X-ray diffraction profiles and transmission electron micrographs indicated that a homogeneous hexagonal structure of SiO2-TiO2 was formed from those products. The mesostructured products had high BET surface areas from 540 to 863m2/g, with uniform pore diameters of 1.5-3nm. These physical quantities were controlled by varying the size of the organic template. Water vapor adsorption isotherms for these materials experienced a sharp increase in adsorption when the relative water vapor pressure (P/P0) was in the range of 0.4-0.6. Over this range of pressures, the maximum amount of adsorbed water was between 20 and 50mass%. The isotherms were classified as type V in the IUPAC notation, showed an unusually large hysteresis. The large hysteresis could be explained by the difference in contact angle between adsorption and desorption. For both the 2nd and 3rd. water vapor adsorption-desorption isotherms, any morphological change was not observed in the isotherms, meaning that these materials have high durability in their humidity control ability. A small shift of the broad Q4 29Si-MAS/NMR spectra indicated that a wider distribution of the Si-O-Si angles in the SiO4 tetrahedra when titanium was incorporated into the silica matrix.

Water Uptake and Swelling Pressure in a Bentonite-Based Backfill

ABSTRACTThe partial specific enthalpy and the partial specific entropy for the water uptake of two bentonites was investigated by means of the water vapor adsorption-desorption isotherms and the heat of immersion with liquid water. From these experiments, and as confirmed by X-ray measurements, it is evident that the interlayer water shows a high degree of order. Calculations of the swelling pressure from the adsorption isotherms show good agreement with measured swelling pressures.The uptake of liquid water by the compacted bentonites under conditions of restrained swelling can be described as a non-steady state diffusion with constant surface concentration. The water uptake of two bentonites in their natural sodium and calcium forms has been investigated. The measurements were carried out in a swelling pressure apparatus with water supply from one or both sides, and in a device modelling the repository conditions with radial water supply. In this modelling experiment, the water uptake was measured and the enclosed air determined. In addition, the swelling pressure was observed and compared to earlier test results.

Pore structure analysis by oxygen adsorption. I. t-curves and methods of analysis

Two new approaches to pore structure analysis were developed by Brunauer and his coworkers: a method for the analysis of micropores, and a method for the analysis of wider pores in which capillary condensation occurs. The two methods were then applied to the analysis of a variety of porous adsorbents, using nitrogen and water vapor adsorption-desorption isotherms, and the pore structure results were compared. The present work was undertaken primarily to find out whether a given adsorbate at two different temperatures gives the same results for the same adsorbent. Oxygen was selected as the adsorbate partly because two temperatures for the isotherms, 77.3 and 90 K, were conveniently available, partly in order to compare the oxygen results with the nitrogen and water vapor results. The prerequisite for the analysis is to have appropriate t-curves, i.e., curves which give the statistical thickness of the film adsorbed on nonporous adsorbents as a function of the relative pressure. Using four nonporous adsorbents, t-curves were determined for oxygen at 77.3 and 90K. Besides presenting the t-curves, the paper also describes the methods of analysis employed at the two temperatures.

Pore structure analysis by water vapor adsorption. III. Analysis of hydrated calcium silicates and portland cements

The pore volume and surface distributions of three hardened pastes of hydrated dicalcium silicate, prepared with water to solid weight ratios, w/s, of 0.45, 0.57, and 0.70, of two hardened pastes of hydrated tricalcium silicate, with w s of 0.45 and 0.57, and of three hardened pastes of a hydrated portland cement, with w s of 0.40, 0.45, and 0.70, were determined by water vapor adsorption-desorption isotherms. In the analyses, the two new methods developed by Brunauer and his coworkers were used: the “MP-method” for the micropores and the “corrected modelless” method for the wider pores. Seven of the eight pastes were previously analyzed by nitrogen adsorption-desorption isotherms, using the same methods. The pore systems of the pastes were only partly accessible to nitrogen; the micropores were not accessible at all, and even the wider pores were only partly accessible. The results indicate the presence of many “ink-bottle” pores, i.e., pores with wide bodies and constricted entrances. In the hydration of both calcium silicates, the products are a calcium silicate hydrate, called tobermorite gel, and calcium hydroxide. The former is a colloidal substance, and it is responsible for the pore properties of the pastes. The pore structures of the pastes of the two silicates were found to be somewhat different. An average portland cement contains about 50% tricalcium silicate and about 25% dicalcium silicate by weight. The tobermorite gel produced in the hydration of portland cement is responsible for the pore properties of cement pastes. These properties were found to be intermediate between those of the two calcium silicates, but closer to those of the tricalcium silicate pastes. The other hydration products of portland cement appear to have little effect, if any, on the pore properties of the tobermorite gel. Tobermorite gel is a layer crystal, similar to some of the clay minerals. Recently, the hypothesis has been advanced that the difference between water vapor and nitrogen adsorption is due to the fact that the former can enter between the layers of the gel, but the latter cannot. The pore structure results presented in the paper refute this hypothesis. The paper also offers a new and more illustrative method for the presentation of the pore structure data than the customary differential curves.

Pore structure analysis by water vapor adsorption. II. Analysis of five silica gels

Complete pore structure analyses were performed for five silica gels, applying the micropore analysis method and the “corrected modelless” method, developed by Brunauer and his coworkers, to water vapor adsorption-desorption isotherms. The same five silica gels were analyzed by the same methods, using nitrogen adsorption-desorption isotherms. The cumulative pore volume and pore surface results obeyed the criteria of correct analysis both for water vapor and nitrogen, and the results obtained by the two adsorbates were in good agreement. This was possible because the entire pore system was accessible to both adsorbates. The pore volume distribution curves, however, were different for the two adsorbates, though not very different. The differences indicate that the pore structure curves are not functions of the adsorbent alone but of both the adsorbent and adsorbate. Complete micropore analysis cannot be performed when the filling of a part of the micropores occurs simultaneously with capillary condensation in some of the larger pores. A method is offered for circumventing this difficulty.