Water Vapor Sorption Kinetics Research Articles

Experimental study on the kinetics of water vapor sorption on the inner surface of silica nano-porous materials

An experiment of kinetic water vapor adsorption of inner surface of silica nano-porous materials is performed by using the thermal gravimetrical method in a relative humidity range from 0.15 to 0.9 at different temperature levels of 25, 35, 45, 55, 65 °C, respectively. The temperature and humidity environment was supplied by a hygrothermostat. This experiment work shows that the water vapor adsorption equilibrium on silica nano-porous materials belongs to Type IV according to Brunauer’s classification and could be well correlated by BET (Stephen Brunauer, Paul Hugh Emmett and Edward Teller) equation. The exponential-decay-2 model proposed in this work could fit the full kinetic adsorption process while the pseudo models failed. Under the same relative humidity, although the water vapor concentration increases significantly with temperature, the saturated water uptake decreases slightly with the increase of temperature for the adverse effect of temperature on adsorption.

Kinetics of water vapor sorption on porous glass as a method of porometry

Equilibrium low temperature nitrogen adsorption and a new method based on the kinetics of water vapor sorption at room temperature are used for a comparative study of pore morphology of high-silica porous glass. The values of the total pore volume, the specific surfaces, the effective mesopore diameters, and the fraction of each mode in the total mesopore volume are established using both techniques. It is found that the rate of water vapor sorption at constant pressure grows in inverse ratio to the diameter of cylindrical menisci in pores. The time dependence of the volume of adsorbed water is linear, while the characteristic time of filling for each pore mode varies directly with the square of the effective pore mode diameter. The proposed new kinetic porometry technique based on analyzing water vapor adsorption kinetics at room temperature is quite simple as it requires no complicated vacuum equipment and allows the simultaneous examination of large numbers of porous material samples.

The water vapour sorption behaviour of acetylated birch wood: how acetylation affects the sorption isotherm and accessible hydroxyl content

The water vapour sorption isotherms and sorption kinetics of birch (Betula pendula L) acetylated to different levels have been determined using a dynamic vapour sorption (DVS) apparatus. A DVS instrument was also used to determine the accessible hydroxyl content in the wood samples using deuterium exchange. The results are reported in terms of the reduced equilibrium moisture content (EMCR), in which the moisture content per unit mass of wood substance is used for the calculation. As the level of acetylation of the wood samples increased there was a corresponding reduction in EMCR of the wood samples, which was accompanied by a decrease in hysteresis in the same order. The sorption kinetics were also determined using the DVS and analysed using the parallel exponential kinetics model, in which the sorption kinetics curve is composed of two processes (labelled fast and slow). Using this analysis, it is possible to calculate two pseudo-isotherms associated with the two processes. The sorption isotherm is a composite of the sorption isotherms associated with the fast process water and the slow process water and there are significant differences in behaviour between the two. It is suggested in this paper that the fast process is related to diffusion limited kinetics, whereas the slow process is a relaxation-limited phenomenon. The reduction in accessible OH content due to acetylation was well correlated with the weight gain due to acetylation, although the relationship did not exactly correspond with that theoretically determined.

Moisture sorption characteristics of Greek durum wheat semolina

The moisture sorption isotherms of Greek durum wheat semolina were determined at 25, 30, 35, and 40°C using the gravimetric (static) and the Novasina hygrometric methods. Good agreement was found between sorption isotherms for semolina obtained with the Novasina and the static gravimetric methods. Sorption isotherms were of the characteristic sigmoid shape (BET type II). Sorption data showed that the moisture content increased while water activity was increasing at a constant temperature, and decreased while temperature was increasing at a constant relative humidity. The water vapor sorption kinetics of semolina was also studied using the static gravimetric method. With the temperature and relative humidity rising, the rate of water sorption also increased. The increase, though, was not stable. Initially, there was a steep rise, but while moisture contents reached equilibrium the rate was substantially decreased. The experimental data were successfully fitted to 7 of widely used isotherm models (B.E.T., G.A.B., Freundlich, Halsey, Henderson, Chung and Pfost, and Oswin). Isosteric heat of sorption (ΔHs) of the studied substrate (semolina) was also determined using the modified Clausius–Clapeyron equation. The steep increase (absolute values) of ΔHs at low moisture contents indicate that water molecules interact more strongly at lower moisture contents.

Sorption and wetting properties of pectin edible films

The water vapour sorption kinetics and isotherms of pectin films prepared by the casting method were determined. The measurement of water vapour sorption kinetics was conducted using a saturated sodium chloride solution to obtain constant relative humidity of the environment (75.3%). The measurement was carried out at the temperature of 25°C over a 24 h period. The water vapour adsorption rate was the highest in the first hours of the process. The exponential equation fitted well the experimental data of water vapour adsorption with time. Glycerol concentration in the analysed films affected the increasing water vapour adsorption. The water vapour sorption isotherms were analysed using the saturated salt solutions with water activity from 0.113 to 0.901 for 3 months at 25°C. The sorption isotherms curves had a sigmoidal shape for all films. Glycerol content affected water vapour adsorption during 3 months. Peleg’s equation was appropriate for the mathematical description of the sorption isotherms. The microstructure of pectin films showed different internal arrangement as a function of the film composition.  

Effect of modified starch or maltodextrin incorporation on the barrier and mechanical properties, moisture sensitivity and appearance of soy protein isolate-based edible films

Abstract This work aimed to study the effect of starch (acetylated di-starch phosphate and starch acetate) and maltodextrin (DE 10.2 and DE 15.6) on the properties of soy protein isolate (SPI) films. Films were cast from heated (70 °C for 20 min) alkaline (pH 10) aqueous solutions of soy protein isolate (8%) containing glycerol (50% of SPI) as a plasticizer and starch or maltodextrin (20% of SPI). For all types of films, water vapor sorption kinetics during 24 h at 25 °C and 75% relative humidity (RH), diffusion coefficient of water vapor, water vapor permeability at 25 °C for two relative humidity differentials (40–75 and 40–100%), tensile strength and elongation at break, color and microstructure were determined after conditioning film specimens at 25 °C and 53% RH for 7 days. Addition of starch and maltodextrin had a significant effect on water vapor adsorption of soy films and increase of diffusion coefficient from 1.73 to 2.60 m2 s− 1. Water vapor permeability increased as relative humidity differentials increased and observed results can be correlated with the plasticizing effect of water. All films demonstrated low values of tensile strength (~ 1.14–1.96 MPa) and elongation at break (~ 25.6–34.7%). All films obtained were opaque and homogenous. Industrial relevance Edible films based on proteins are among the new products being developed in order to reduce the use of plastic packaging polymers for food applications. There is a real need to find methods for production of soy films with properties accepted by the industry. One of the solutions is preparation of composite films based on protein and polysaccharide components. This work deals with the physico-chemical behavior of two types of carbohydrate polymers (modified starch and maltodextrins) on the physical–chemical properties of edible films based on soy protein isolate. Moisture sorption and kinetics, mechanical and barrier properties have been studied and related to structure. This paper suggests that physico-chemical properties of soy protein isolate films can be modified by the incorporation of modified starch and maltodextrins.

Development and characterization of composite edible films based on sodium alginate and pectin

The objective of this study was to characterize physical properties including thickness, colour, water vapour sorption kinetics and isotherms, water vapour permeability, tensile strength, elongation at break, and microstructure of composite films prepared by casting sodium alginate and low methoxy pectin at proportions of: 100–0%, 75–25%, 50–50%, 25–75%, and 0–100%. Combination of both polysaccharides gave continuous, homogenous and transparent films. All of analyzed films reached their state of equilibrium within the 24h of adsorption time. Water sorption isotherms for all films had a sigmoidal shape and were not influenced by the film composition. Sorption kinetics and isotherms indicated hydrophilic character of investigated films. Statistically significant (p<0.001) correlations were found between colour, water vapour permeability, tensile strength, elongation at break, and chemical composition (alginate and pectin) of composite films. A different internal arrangement was observed as a function of film composition.

Kinetics of saturated water vapor sorption on soot particles

In order to study the condensation activity of carboncontaining aerosols, the kinetics of sorption of saturated water vapors was investigated on model soot samples modified by the deposition of organic and inorganic substances corresponding to different classes of compounds contained in solid state products of natural fuel combustion. The values of limiting water sorption were determined and rate constants were cal� culated. It was established that the limiting sorption on graphitized thermal soot (GTS) modified by acids and salts (e.g., phenol, benzoic, phthalic, trimellitic, and sulfuric acids; sodium benzoate; and ammonium sul� fate) per modifier molecule rose by two orders of magnitude, relative to the initial carbon matrix, and ranged from ≈2 to 200 molecules. It was found that hydrophilic centers covalently bound to the surface of furnace black (FB) and its oxidized form are shielded to a considerable degree as a result of modification by hydro� phobic substances (hexadecane, octacosane, anthracene, and stearic acid). We conclude that hexadecane modification of GTS containing hydrophilic centers not bound to its surface (sulfuric acid and ammonium sulfate molecules) have no effect on the sorption of saturated water vapors.

Influence of earlywood/latewood and ring position upon water vapour sorption properties of Sitka spruce

Variation in the water vapour sorption kinetics of the stemwood of Sitka spruce (Picea sitchensis (Bong.) Carr.) has been studied, with respect to possible variations between earlywood and latewood and between different annual ring positions from pith to bark. The sorption kinetics was analysed using the parallel exponential kinetics (PEK) model that has recently been introduced by this laboratory for studies with wood. Significant differences in kinetic parameters were found between ring positions, corresponding to differences between juvenile wood and mature wood that contrast with previously reported work in the literature. Differences were also found in the sorption kinetics of earlywood and latewood in agreement with the literature. Thus far, there has been no satisfactory explanation as to the physical interpretation of the PEK model and in this paper it is argued that the kinetic behaviour is consistent with what would be expected for sorption with a viscoelastic material. Analysis of the PEK data using the Kelvin–Voigt model has proved that it is possible to obtain values for the wood cell wall stiffness and viscosity that are in broad agreement with previously published values in the literature.

The influence of photodegradation and weathering on the water vapour sorption kinetic behaviour of scots pine earlywood and latewood

The effect of weathering or accelerated photodegradation on the water vapour sorption kinetics of Scots pine ( Pinus sylvestris) has been studied. The sorption kinetics were analysed using the parallel exponential kinetics (PEK) model and the parameters obtained used as input into a Kelvin–Voigt thereby providing values for the wood cell wall stiffness and viscosity. Differences in kinetic behaviour were found between earlywood and latewood samples, and between samples which had been photodegraded or weathered when compared to unexposed samples. The values for the cell wall stiffness are in general agreement with values previously published in the literature.

Water sorption by anhydrous ionic liquids

The kinetics of water vapour sorption by several anhydrous hydrophobic and hydrophilic ionic liquids (ILs) were gravimetrically determined at 25 °C and two levels of humidity, namely 43 and 81%. A simple equation was used to fit the data. The kinetic parameters obtained from the different ILs were compared and the differences were related to the IL structures. Results showed that even hydrophobic ILs absorb water at an unexpected speed.

Water vapor sorption kinetics of wood modified with glutaraldehyde

AbstractThe modifying effects of Scots pine (Pinus sylvestris L.) wood with the crosslinking agent glutaraldehyde (GA) on the water vapor sorption kinetics were studied by curve fitting the experimental isotherm sorption data obtained using a Dynamic Vapor Sorption apparatus using the parallel exponential kinetics model (PEK model) and the Hailwood–Horrobin model (H–H model) for the isotherm. Both the H–H model and the PEK model provided good fits to the experimental data. Modification of wood with GA reduced both the time to equilibrium and the equilibrium moisture content (EMC). According to the PEK and H–H models, the reduction in EMC was mainly due to the decrease of moisture content (MC) associated with the slow sorption processes and polylayer water. The fast sorption processes and monolayer water were little affected by GA modification. Compared to the untreated control, the estimated total water absorbed by wood treated to a WPG of 20.9% at 100% RH decreased by 52.2%, by extrapolating the fitted curves derived from the H–H model. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Sorption induced relaxations during water diffusion in S-PEEK

This paper presents an analysis of the sorption kinetics of water vapor and liquid water in the glassy polymer sulfonated poly(ether ether ketone) (S-PEEK). Sorption isotherms are determined experimentally using a gravimetric sorption balance, and the relative contributions of Fickian diffusion and relaxational phenomena are quantified as a function of the water concentration in the polymer using the model of Hopfenberg and Berens.Analysis of the sorption isotherms and determination of the sorption kinetics prove the occurrence of both Fickian sorption behavior and relaxational phenomena already at very low water concentrations in the polymer. With increasing water concentration, the relative importance of relaxation phenomena increases, whereas the relative contribution of Fickian diffusion decreases.Based on the water vapor sorption kinetics only, the Fickian diffusion coefficient increases over two orders of magnitude with increasing water vapor concentration. Taking also the diffusion kinetics from liquid water sorption experiments into account reveals a change of even three orders of magnitude of the Fickian diffusion coefficient when the water concentration in the polymer increases.

Dynamics of water sorption on a single adsorbent grain caused by a large pressure jump: Modeling of coupled heat and mass transfer

Here we present a mathematical model of coupled heat and mass transfer in the course of water adsorption on a single adsorbent grain, which is in thermal contact with an isothermal metal plate. Adsorption is caused by a large jump of vapour pressure over the grain. The radial profiles of the grain temperature and the water concentrations in the adsorbed and gas phases were calculated as a function of time. Strong interrelationship between the shapes of the water sorption isotherm and the radial uptake profile was revealed. The best fit to the experimental data of [B. Dawoud, Yu. I. Aristov, Experimental study on the kinetics of water vapour sorption on selective water sorbents, silica–gels and alumina under typical operating conditions of sorption heat pumps, Int. J. Heat Mass Transfer 46 (2) (2003) 273–281] corresponds to the water diffusivity D e = 6.0 × 10 −6 m 2/s and the coefficient of the heat transfer α p = 70 W/(m 2 K) which are larger than those measured in quasi-equilibrium experiments.

Statics and kinetics of water vapor sorption of small loblolly pine samples

A better understanding of the sorption behavior of different wood structures could be useful in protecting wood against wood deterioration and fungal attack. The purpose of this study was to investigate the effect of differences among earlywood, latewood, and tree ring location within the stem cross-section of loblolly pine (Pinus taeda) on the sorption kinetics and statics of water vapor under ambient conditions. The water vapor sorption of earlywood and latewood in different tree rings was recorded using a dynamic contact angle analyzer under relative humidity changes from 11 to 89%, as provided by saturated salt solutions. Earlywood had higher sorption rates and diffusion coefficients than latewood, while outer tree rings had higher sorption rates and diffusion coefficients than inner tree rings. The sorption isotherms of earlywood, latewood, and different tree ring locations within the stem cross-section were fitted very well by a Hailwood–Horrobin model.

Effect of capillary condensation on water sorption by composite calcium chloride in a porous matrix sorbents

To develop a model of water sorption from air by composite salt in a porous matrix sorbents, the effect of capillary condensation was studied [1]. It was shown that, at high humidity of incoming air, it is necessary to take into account the capillary condensation of water in small pores of the matrix because the capillary condensation significantly affects the concentration and temperature profiles in the sorbent bed and leads to intense moisture accumulation at the bed inlet. The model was used to study the kinetics of water vapor sorption in a flow adsorber with a fixed bed of calcium chloride in porous aluminum oxide and calcium chloride in a porous carbon-carbon composite Sibunit sorbents.

Kinetics of water sorption in flax and PET fibers

Composite materials are usually reinforced by synthetic fibers as carbon, glass etc…. Because of their good mechanical properties and low density, natural fibers are being considered more preferentially as reinforcement. The application of natural fibers as reinforcements in composite materials requires a strong adhesion between the fiber and the matrix. The poor resistance towards water absorption is one of the drawbacks of natural fibers which makes it more important to understand the dynamic properties of penetration of water molecules through these heterogeneous materials. Water vapour sorption kinetics in natural flax fibers have been performed at 25 °C by using an electronic microbalance (IGA, Hiden). By using the Fickian model for a complete cylinder water diffusion coefficients have been determined and calculated at short times (first half-sorption) and long times (second half-sorption) of kinetic curve and for different water activities. The values obtained for D 1 and D 2 are rather similar on the all range of water activity. Generally, water diffusivity increases and then decreases with water activity. The increase of diffusivity at low water activities may be explained on the basis of the dual mode sorption (Langmuir and Henry sorption’s combination) whereas the decrease for the higher activities can be attributed to the immobilization of sorbed water molecules due to the water clustering.

Kinetics of water sorption on a CaCl2-in-silica-gel-pores sorbent: The effects of the pellet size and temperature

Kinetics of water vapor sorption on the CaCl2-in-KSK-pores composite (SWS-1L) have been studied at T = 33–69°C and vapor pressures of 8–70 mbar for pellet sizes of 2R pel = 0.355–0.425, 0.71–0.85, and 1.2–1.4 mm. Sorption has been measured under isothermal conditions on a thermobalance by abruptly raising the vapor pressure in the measurement cell by a small value and then maintaining the new pressure. In the initial portion of the kinetic curves, the amount of sorbed water (Δm) increases in proportion to the sorption time (t) to the power 1/2. From the slope of the Δm versus t 1/2 curve, it is possible to derive the sorption rate constant k D = D eff/R 2 pel and the effective diffusivity D eff. The latter is independent of R pel for 2R pel ≥ 0.71 mm. The rate of water sorption on smaller (0.355-to 0.425-mm) pellets grows less rapidly, apparently because of the effect of the heat of sorption. The effective diffusivity is determined by the local slope of the water vapor sorption isotherm for SWS-1L. Applying an appropriate correction enables one to calculate the effective diffusivity for water vapor in the sorbent pores, which appears to be D e = (0.35 ± 0.17) × 10−6 m2/s. This value is approximately 10 times smaller than the Knudsen water diffusion coefficient calculated for a single cylindrical pore with a size equal to the average pore size of the composite. Two possible causes of this discrepancy are discussed, specifically, an increase in the pore tortuosity because of the presence of the salt and the interaction between water and the salt.

Moisture sorption/desorption behavior of various manmade cellulosic fibers

AbstractThe kinetics of dynamic water vapor sorption and desorption on viscose, modal, cotton, wool, down, and polyester fibers and lyocell knit fabrics were investigated according to the parallel exponential kinetics (PEK) model. The total equilibrium moisture regain (Minf(total)) in all the materials decreased with increasing temperature. However, the partial equilibrium fast sorption, determined by PEK simulation at 60% relative humidity (RH) and 36°C, was larger than that at 20°C, whereas the partial equilibrium slow sorption was smaller. The characteristic times in fast sorption (τ1) and in slow sorption (τ2) for lyocell were reduced when the conditions were changed from 60% RH and 20°C to 36°C, whereas those for the other fibers increased. Lyocell exhibited the highest Minf(total) value and the lowest τ1 and τ2 values, and this suggested high equilibrium moisture content and fast moisture uptake/release, that is, high moisture accessibility for lyocell. The relationships between the moisture regain, hysteresis, water retention capacity, and Brunauer–Emmett–Teller surface volume in the materials were also examined. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1621–1625, 2005

Sorption and Transport of Water Vapor in Amphiphilic Block Copolymer Films

Water vapor sorption in films consisting of poly(ethylene oxide)‐poly(propylene oxide)‐poly(ethylene oxide) (PEO‐PPO‐PEO) amphiphilic block copolymers has been measured gravimetrically at air relative humidity in the range 38–94% at 24°C. The water sorption isotherm was obtained and compared to data for water vapor desorption (drying) for the same system. The kinetics of water vapor sorption were determined. The percentage of water gain is initially a linear function of square root of time, indicating Fickian diffusion in the block copolymer films. Diffusion models were used to fit the experimental results and to extract water diffusion coefficient values in the block copolymer film, the internal structure of which was initially semicrystalline (due to PEO) and, above ∼12% water, lamellar lyotropic liquid crystalline.