Water Desorption Isotherms Research Articles

Water sorption and desorption isotherms of some naturally occurring zeolites

Water sorption/desorption isotherms of some naturally occurring zeolites (more appropriately, zeolitic tuffs), such as chabazite, clinoptilolite, erionite, mordenite, and phillipsite, as measured by a volumetric method are reported. As expected, all the zeolites showed the Brunauer Type I isotherms with upward deviation at higher pressures, the latter as a result of multilayer sorption and liquid condensation in mesopores. The desorption branch of the isotherms exhibited small hystereses in all the samples. The micropore-filling capacities of the different zeolites were calculated from the Dubinin equation in order to delineate the effects of mesopores, impurities, crystallinity, and particle size. The Dubinin micropore-filling capacities were more or less found to be coincident to the Langmuir monolayer capacity.

Interpretation of water desorption isotherms of lysozyme

The interrelationship between water desorption isotherms and the humidity dependences of the Young's modulus and volume strains of different lysozyme crystals and films has been analysed. The strain of protein samples is shown to be dependent on their elastic properties and to be induced by Laplace's pressure which can be calculated from the Thomson (Kelvin) relation. The energy of the mechanical deformation of protein samples expended for repacking and deformation of protein molecules considerably contributes to the integral change of free energy upon dehydration. Solid protein samples should be considered as deformable porous materials whose water sorption isotherms are determined not only by the number and properties of individual hydration centres but also by the macroscopic properties of protein solids.

Sorption of water vapor by Siblon fibre

Water vapor sorption and desorption isotherms for Siblon high-modulus viscose fibre have been obtained at 20°C. It has been shown that Siblon has a higher absorptive power for water vapor as compared with other viscose fibres, textile yarn, and cotton cellulose.

EFFECT OF WATER ACTIVITY ON TEXTURE PROFILE PARAMETERS OF APPLE FLESH

ABSTRACTFresh apple with a water activity of approximately 0.99 was partially dehydrated, then equilibrated to nine water activity levels down to Aw 0.01. The water desorption isotherm, for apple is given. Instrumental texture profile analysis was performed at each Aw level. As the Aw decreased, hardness increased slowly, then rapidly at Aw 0.12 then decreased again at Aw 0.01; deformation first increased sharply then steadily decreased; springiness increased to Aw 0.33 then decreased; cohesiveness increased down to Aw 0.65, held steady to Aw 0.33 then decreased; gumminess and chewiness increased down to Aw 0.12 then decreased. Fracturability was present only at the extremes (Aw 0.99 and 0.01) and was absent at all other water activity, levels. Complex changes in textural properties occur as apple is dehydrated and most of these changes occur most rapidly near the BET monolayer level of water activity.

A Kinetic Method for Determining Desorption Isotherms of Water on Clays

AbstractA method of using Knudsen cells to measure the vapor pressure of water in clay‐water gels is described. The vapor pressures obtained at each mass ratio of water to clay using Knudsen cells are similar to those obtained by equilibration with saturated salt solutions and may be used in the same way.

Adsorption of water on copper and cuprous oxide

Water adsorption and desorption isotherms are reported for evaporated Cu and Cu2O using a quartz crystal oscillator method with an ultimate sensitivity of 10−8 g cm−2. Cu2O adsorbs more water than Cu at a given relative humidity (RH). Higher adsorbed water content on Cu2O a a given RH partly accounts for the breakdown of its passivity against sulfidation at elevated RH as reported earlier by the author. Isotherms are fitted to a BET-type (type II) model. From the BET parameters, the monolayer capacity and the heat of adsorption have been computed.

Adsorption and desorption of water on Au by the quartz‐crystal‐oscillator method

Initial water adsorption and desorption isotherms (Type II) are reported for Au‐coated quartz‐crystal substrates at temperatures of 15.5°, 23°, 30°, and 39 ° using a quartz‐crystal oscillator with an ultimate sensitivity of ∠10−8 g/cm2. Hysteresis between adsorption and desorption is most probably related to Au porosity and thermal shock. Isotherms are shown to qualitatively fit the BET model for adsorption at low relative pressures (referenced to the condensation pressure P0). From the computed BET parameters, the heat of adsorption is found to be ∠1 kcal/mole and the monolayer capacity at 23 °C is ∠0.2 μg/cm2.

Die Hysterese der Adsorptions‐ und Desorptionsisothermen an porösen Silicagelen und Aluminiumoxiden

AbstractIn selected groups of amorphous silica gels and finely crystalline aluminium oxides, hysteresis loops between the adsorption‐ and desorption isotherms of water were determined. The total volumes of the pore systems were analysed with the aid of the differential quotients of the hysteresis loops. In the group of silica gels relationships will be demonstrated between the precipitation of polysilicid acids and the distribution of the pore volume in the xerogels. In the group of aluminium oxides the influence of sintering at high temperatures on the distribution of the pore volume will be described.The hysteresis loops and especially the differential quotients distinctly differentiate specific properties of the used materials with their variables pore systems.An apparatus was used which recorded weights of water (= volumes and volume differences) with high sensitivity over the entire range p/po = 0 to 0,95. The appearance of hysteresis according to the fundamental theory was a sign that in this range adsorption and desorption of the liquid phase water took place in mesopores.

Kinetics of water vapor sorption in a model freeze‐dried food

AbstractSorption kinetics experiments were performed at 37 C with microcrystalline cellulose held between dryness and 51% water activity in the absence of other gases. Effective diffusion coefficients (<10−4 cm2/s) were greater than any previously reported for similar experiments. A model based on heat and mass transfer properties of the sample is proposed. The vapor space permeability calculated by application of this model is related to the structure of the microcrystalline cellulose as determined from water desorption isotherm analysis, mercury intrusion porosimetry, and steady state permeability techniques. Microcrystalline cellulose, like some freeze‐dried foods, contains micropores and macropores. During sorption, the macropores are important in determining the balance between the internal and environmental control during the first half of sorption. The micropores, which account for less than 1% of the void volume of the porous matrix but approximately 40% of the surface area, control the mass transfer properties of the sample during the latter stages of sorption. Because of the great difference between their mass transfer properties, the macropores approach local equilibrium faster than the micropores; this phenomenon suggests that changes in the effective diffusion coefficient and permeability as sorption proceeds are related to the structure of the sample rather than to the moisture content itself.

Structure and properties of polyamides

Sorption and desorption isotherms of water at 27 °C have been determined for a series ofα-substituted nylons 3 (methyl to butyl and phenyl derivatives), which at room temperature are deep below the respective glass transition temperatures. It follows from the rate of establishment of sorption equilibria as well as from sudden changes of the sorption isotherms, that in all samples, without respect to their different structure, a change of the mechanism takes place when vapour activity passes through a value of about 0.25. From the point of view of the amount of water (C1) sorbed on the accessible amide group with water activity equal 1 (accessibility having been determined by means of deuteriation), the methyl, dimethyl and phenyl derivative sorb like non-substituted nylons 2 to 6 (C1=3/2), the other samples with longer aliphatic substituents sorb like nylon 8 and higher nylons (C1=1/2).

On the Structure and Properties of Polyamides. XXVII. The Mechanism of Water Sorption in Polyamides

AbstractThe sorption and the desorption isotherms of water are described for several crystalline polyamides (nylons 2, 3, 6, 7, 8, 11), amorphous polyamides (poly‐C‐methylcaprolactam, terpolymer of caprolactam, aminoheptanoic acid and aminoundecanoic acid) and model amides (N‐ethyl acetamide, Y‐ethyl butyramide, N‐butyl capronamide) at 27°C., with amides also at 40 and 55°C. On the basis of our own as well as reported data (on sorption isotherms, heat balance of sorption, sorption heats, infrared absorption, nuclear magnetic resonance, mechanical properties of polyamide‐water systems), we propose the following mechanism for the sorption of water in polyamides (capillary condensation is not considered): In nylon 6, three molecules of water are sorbed on two neighboring amide groups in an accessible region. The first molecule forms a double H‐bonds between the CO groups (firmly bound water) and the other two molecules form H‐bonds between the CO and the NH groups (loosely bound water). It turns out that even in nylons 2 to 6, water is probably sorbed by this mechanism whereas in nylons 8 to 12 only one molecule of water is sorbed on two amide groups, probably as a firmly bound water. Nylon 7 stands somewhere between the two groups.

A Comparison of Pore Size Distributions of Activated Carbons Calculated from Nitrogen and Water Desorption Isotherms

A Comparison of Pore Size Distributions of Activated Carbons Calculated from Nitrogen and Water Desorption Isotherms