Water Sorption Kinetics Research Articles

Water sorption and dehydration kinetics of Tunisian rosemary leaves

Water sorption isotherms of rosemary were investigated at three temperatures: 30, 40 and 50°C, corresponding to drying temperatures, by the static method consisting of the use of different sulphuric acid solutions, over a range of relative humidity from 0.05 to 0.9. Experimental data are used to determine the best models for predicting the desorption equilibrium moisture content of rosemary for known values of temperature and relative humidity. The second part of this study deals with the drying kinetics of rosemary in an infrared dryer. The experimental results are used to determine the characteristic curve of drying.

Kinetics of water sorption on SWS-1L (calcium chloride confined to mesoporous silica gel): Influence of grain size and temperature

Kinetics of water sorption on loose grains of composite sorbent CaCl 2 confined to mesoporous silica (SWS-1L) was measured at T = 33 – 69 ∘ C and P ( H 2 O ) = 8 – 70 mbar over water uptake range 0–0.47 g/g for various particle sizes R p (between 0.355 and 1.4 mm). The measurements were performed in a constant pressure unit based on a CAHN microbalance under isothermal external conditions. The results obtained evidence an enhancement of the sorption rate and apparent diffusion constant with the decrease in the particle size (approximately as R p - 2 at R p > 0.71 mm ). Contribution of thermal effects was found for water sorption on smaller SWS particles (0.355–0.425 mm), which decreases the sorption rate. The apparent water diffusivity was found to depend on the local slope of the SWS water sorption isotherm. The pore diffusivity of water in the temperature range 33–69 °C was calculated from experimental data D e = ( 0.24 ± 0.12 ) × 10 - 6 m 2 / s that is approximately 10 times lower than the Knudsen pore diffusivity estimated for pores of silica KSK. The possible reasons of the diffusivity reduction are discussed.

Kinetics of water sorption by chitosan and its blends with poly(vinyl alcohol)

Gravimetric and spectrophotometric studies of the kinetics of water sorption from air of various water activities by chitosan of different deacetylation degrees (and molecular weights) and chitosan/poly(vinyl alcohol) blends were conducted. Analysis of the presented results has shown a clear effect of the deacetylation degree of chitosan and the presence of poly(vinyl alcohol) in chitosan blends on water sorption rate. The character of sorption curves, calculated values of diffusion coefficients of water vapour and micropore volumes have shown that the higher chitosan deacetylation (and higher M w) the lower the sorption of water vapour. The observed effect is explained by a rising ordering of molecular chains leading to an increase of structural packing in chitosan. Modification of the chitosan structure in miscible blends with poly(vinyl alcohol) has been reflected in their sorptive properties which depend on the weight fraction of poly(vinyl alcohol) and water activity. Good compatibility of the components, resulting from the presence of hydrogen bonds between specific groups (hydroxyl, amide) of chitosan and poly(vinyl alcohol) chains, causes an increase in molecular packing and thus, a decrease of pore volume and a rate of water diffusion.

Water Adsorption Kinetics and Contact Angles of Pharmaceutical Powders

Water sorption kinetics and water contact angles have been characterized for a range of pharmaceutical powders: ambroxol hydrochloride, griseofulvin, N,n-octyl-D-gluconamide, paracetamol, sulfathiazole, and theophylline. The uptake of water by powder samples at saturated vapor pressure was modeled using a pseudo first-order kinetic relationship. Parameters from this model have been correlated with the concentration and reactivity of the active surface sites of the pharmaceutical powders and their contact angles. The study has shown that analysis of water adsorption kinetics can be a powerful technique for characterizing the surface chemistry and wettability of pharmaceutical powders, and is particularly sensitive to their surface modification through excipient adsorption: ethyl(hydroxyethyl)cellulose treatment of griseofulvin and butyryl chloride treatment of sulfathiazole are reported as case studies.

Ionic liquids as solvent and solvent additives for the synthesis of sol–gel materials

The ionic liquid (IL) 1-butyl-3-methylimidazolium chloride was used as a drying control chemical additive in the synthesis of silica sol–gel materials with and without methanol as a co-solvent. The resulting gels were characterized by using thermogravimetric analysis, differential scanning calorimetry, infrared spectroscopy and water sorption kinetics. Calcined gels were analyzed using scanning electron microscopy and nitrogen adsorption isotherms for surface area and pore volume determination. Non-calcined gels were monolithic and showed general cloudiness with lesser degrees observed at higher IL volumes. Calcinations resulted in the formation of powders with increased available surface area as the amount of IL volume was increased. This is consistent with an increase in respective pore volume but a general decrease in average pore size. The resulting materials exhibited conventional structural microdomains, in contrast to periodicity reported when other ionic liquids were used as templates.

Corrosion electrochemical characteristics of red iron oxide pigmented epoxy coatings on aluminum alloys

Red iron oxide pigmented epoxy coatings were prepared on aluminum alloys and were characterized by electrochemical impedance spectroscopy (EIS) immersed in NaCl solution. The evolutions of impedance models of coated metals were obtained by the fitting analysis of experimental data using suitable equivalent electrical circuits (EEC). The results indicated that the composite electrode system could be fitted by three impedance models. At the initial immersion stage, coatings acted as a barrier layer and only performed a simple circuit consisting of a coating resistance ( R c) parallel to a coating capacitance ( C c). After a certain time of exposure, water and (or) oxygen arrived at the metal surface through the coating, leading to the formation of electrochemical corrosion sites at metal interface and thereby the appearance of double-layer capacitance ( C dl) and charge transfer resistance ( R ct). After intensive attacking of metal substrates, the mass transfer of corrosion products was in difficulty. In this case, the diffusion elements were added to the EEC. It was found that due to the presence of inert pigment particles, the mass transfer behaviors were interestingly different from those of the varnish polymer coatings. Electrical parameters were also obtained from the EIS data. ln C c–time curve showed a Case II water sorption kinetics, typical non-Fickian diffusion for water uptake.

Monitoring the drying process of lasagna pasta through a novel sensing device-based method

The desiccation process of lasagna pasta is usually accompanied by bending and breaking of the dry pasta slices, due to self generated internal stress. The diffusive transfer of water through the slice bulk is responsible of these phenomena, leading to excessive product reject. The water sorption equilibrium and kinetics of drying were measured for different pasta mixtures. Measurements have been performed by means of a novel equipment based on a raw alumina substrate (3 × 3 mm sized) equipped with gold interdigitated electrodes, and a strict analogy exists between current and water diffusion data. Moreover, in situ measurements have been performed in an Italian pasta factory with an on line system assembled on the desiccation apparatus. Results of the sorption studies show a Case II diffusion mechanism, with the formation of a glassy shell around the pasta sample during desiccation at low water vapour activities, which prevents the further desiccation of the internal humid core. Thus, an accurate control of the humidity both in the sample and the external environment was performed to avoid stress generation in the material and cracks formation. The measuring device provided an accurate control of the sample humidity and a fast response to external environment modifications during desiccation.

A New Approach to Predict the Water‐transport Properties of Multilayer Films Intended for Food‐packaging Applications

ABSTRACT: A mathematical model that predicts the water‐permeability coefficient of multilayer films intended for food‐packaging applications is presented. A 6‐layer film composed of 5 polyolefinic layers and a layer made of a copolymer of ethylene and vinylalcohol (EVOH) was used to validate the proposed model. Water‐permeability tests were run at 25°C both on each single layer and on the multilayer film; whereas the water‐sorption isotherm and the water‐sorption kinetics were determined only for the EVOH layer. Results suggest that the water‐permeability coefficient of multilayer films can be predicted from the knowledge of the water‐transport properties of each constituent layer.

Water diffusion in a soft contact lens polymer and its tolerance to UV radiation studied by positron lifetime technique

AbstractThe kinetics of water sorption, the topology or the free‐volume changes due to the presence of sorbed water in a soft contact lens polymer, poly(2‐hydroxyethyl methacrylate), were investigated by using the positron lifetime technique. It was found that the ortho‐positronium lifetime increases in the beginning of sorption because of microstructural swelling of the polymer matrix. After reaching a maximum, the lifetime decreases and becomes constant, maybe because of the filling of the free‐volume holes with water molecules. The diffusion process is found to be non‐Fickian. By using the dual‐mode sorption model, the Fickian‐controlled part and the relaxation‐controlled part of diffusion were separated. Further, the positron results seem to indicate the existence of water clusters in the sorbed lens material. The tolerance or stability of the soft lens material to UV radiation seems to be satisfactory as revealed by positron results. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1355–1366, 2004

Novel water sorption behavior of emulsion blends

As waterborne emulsions continue to replace their solvent-based counterparts in various applications, the water resistance of cast films is attracting significant interest. The characterization of the water sorption kinetics and structure–property relationships related to water sorption is, therefore, an important area for investigation. In this study, the water sorption kinetics of emulsion blends were compared with those of their blend components. The investigated blends were at equal weight (dry basis) fractions for each emulsion. The initial water sorption rate for immiscible emulsion blends was found to be significantly higher than composite values of the constituents. This behavior was due to percolation networks in the blends because thermodynamic constraints prevented diffusion across the interface bordering dissimilar particles, leaving a interface enriched with water-sensitive species. The peak water sorption for the immiscible emulsion blends was lower than the composite values because of the ability of the water-sensitive species to rapidly diffuse out of the samples due to the percolation network. This behavior existed for room-temperature-cast samples and persisted as the time and temperature exposure was increased. Atomic force microscopy results clearly showed the potential for percolation networks in the blends. Higher glass-transition emulsion polymers [e.g., poly(vinyl acetate)] exhibited similar behavior, and this indicated poor film formation like that for the immiscible emulsion blends. These results indicated that the degree of film formation was critical with respect to the water sorption characteristics of emulsion films. Immiscible emulsion blends were compared with miscible emulsion blends for which all constituents exhibited excellent film formation (unblended). The immiscible blends exhibited a significant difference in water sorption compared with the miscible blends because of the existence of percolation networks. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 933–939, 2003

Water sorption and electrical/dielectric properties of organic‐inorganic polymer blends

AbstractOrganic‐inorganic polymer blends (OIPB) were obtained by reaction of organic and inorganic oligomers. The organic oligomer was synthesized with 2,4‐toluene diisocyanate (TDI) and oligooxypropylene glycols (OPG) with various molecular weights (MW). The inorganic component was a water solution of sodium silicate. The OIPB obtained are hydrophilic and have great water sorption ability (the relative weight of sorbed water reaches 2000 %). The kinetics of water sorption and the changes of electrical conductivity during sorption were studied. Sorption ability, and mechanical, electrical and dielectric properties of OIPB depend on molecular weight of OPG: conductivity increases with increasing MW, whereas the sorption ability correlates with the mechanical properties. The influence of the inorganic phase content on the electrical and dielectric properties was studied as well.

A general approach to describe the antimicrobial agent release from highly swellable films intended for food packaging applications

A mathematical model able to describe the release kinetics of antimicrobial agents from crosslinked polyvinylalcohol (PVOH) into water is presented. The model was developed by taking into account the diffusion of water molecules into the polymeric film, the counter-diffusion of the incorporated antimicrobial agent from the film into water, and the polymeric matrix swelling kinetic. To validate the model the water sorption kinetics as well as the release kinetics of three antimicrobial agents (i.e., lysozyme, nisin and sodium benzoate, all approved to be used in contact with food) were determined at ambient temperature (25 °C). The three investigated active agents were entrapped in four films of PVOH with a different degree of crosslink. The model was successfully used to fit all the above sets of data, corroborating the validity of the hypothesis made to derive it.

Modeling the Lysozyme Release Kinetics from Antimicrobial Films Intended for Food Packaging Applications

ABSTRACT: The aim of this work is to develop a mathematical model to predict the lysozyme release kinetics from crosslinked polyvinylalcohol (PVOH) into an aqueous solution. The model was developed by taking into account both the diffusion of water molecules into the polymeric film and the counter‐diffusion of the incorporated antimicrobial agent from the film into the aqueous solution. In particular, it was considered that the lysozyme release from a swelling polymer could be regarded as “anomalous diffusion” with moving boundary conditions. The water sorption kinetics of 4 films, differing for the degree of crosslink, were determined at ambient temperature (25 °C). The results obtained by fitting the model to the experimental data are satisfactory.

Modeling the water transport properties of casein-based edible coating

A mathematical model able to predict the water transport properties of casein-based edible coating is presented. The model consists of two parts. The first is aimed to describe the water solubilization process into the investigated film, and it was derived taking into account the presence of both “bound water” and “free water”. The second is intended to describe the water diffusion process, and it was derived considering both the dependence of water diffusion coefficient of local water concentration and the superposition of polymer relaxation to stochastic diffusion. To validate the model both water sorption isotherm and sorption kinetics of the investigated film were determined at 25 °C. The proposed model was successfully fitted to both sorption and diffusion data, corroborating the validity of the hypothesis used to derive it.

Experimental study on the kinetics of water vapor sorption on selective water sorbents, silica gel and alumina under typical operating conditions of sorption heat pumps

A selective water sorbent (SWS) is a composite material consisting of a porous host matrix and a hygroscopic substance (commonly an inorganic salt) impregnated into its pores. This work presents an experimental investigation for the kinetics of water vapor sorption on two host materials; namely mesoporous silica gel and alumina in comparison with the two composites SWS-1L and SWS-1A formed by impregnating these two host matrices with CaCl 2. Moreover, the kinetics of water vapor sorption on microporous silica gel have been also investigated. The measurements have been carried out on 3 g samples of loose pellets on an isothermal wall under three different operating conditions of sorption heat pumps. The results obtained evidence a remarkable increase in the differential water loading of both SWS-sorbents over their host materials. However, and due to the increased diffusion resistance to water sorption resulting from the salt impregnation, the kinetics of water sorption into the host matrices is faster than that into the two SWS-composites. Moreover, SWS-1L is found to be faster than SWS-1A in sorbing water vapor. The differential water loading on microporous silica is about twice that on mesoporous silica and alumina, but the sorption kinetics are a little bit slower.

ENHANCED WATER SORPTION OF A SEMI-INTERPENETRATING POLYMER NETWORK (IPN) OF POLY(2-HYDROXYETHYL METHACRYLATE) (PHEMA) AND POLY(ETHYLENE GLYCOL) (PEG)

ABSTRACT An attempt was made to enhance the water-sorption capacity of polymers of 2-hydroxyethyl methacrylate (HEMA) by preparing its semi-interpenetrating polymer network (IPN) with a hydrophilic polymer such as poly(ethylene glycol) (PEG). The effects of various factors, such as history of the polymer sample, chemical architecture of the IPN, presence of salt ions in the swelling medium, and temperature of the swelling medium, were investigated on the water sorption kinetics of the IPNs. The IPN was characterized by IR spectral analysis and various structural parameters, such as molecular weight between crosslinks (Mc), crosslink density (q) and number of elastically effective chains (Ve), were evaluated. The IPNs were also assessed for their antithrombogenic potential.

Water sorption in oil palm fiber reinforced phenol formaldehyde composites

Abstract Water sorption kinetics in oil palm fiber reinforced phenol formaldehyde (PF) composites and oil palm/glass hybrid fiber reinforced PF composites was investigated. The influence of fiber loading, relative volume fractions of fibers in hybrid composites and fiber surface modifications on the kinetic and thermodynamic parameters of water sorption by the composites were also studied. Water sorption at four different temperatures was analysed and compared. Composite with 10 wt% fiber loading exhibits maximum water uptake. Hybridisation of the oil palm fiber with glass considerably decreased the water sorption by the composite. The concentration dependency of the diffusion coefficient was analysed and discussed.

Comparison of the effects of Mg(OH) 2 and sucrose on the stability of bovine serum albumin encapsulated in injectable poly( d, l-lactide- co-glycolide) implants

Incomplete release and poor stability of encapsulated proteins are common hurdles to overcome when developing poly(lactide- co-glycolide) (PLGA) controlled-release systems. Antacid excipients such as Mg(OH) 2, which increase both microclimate pH and polymer water uptake, have been shown to prevent acid-induced instability of proteins encapsulated in PLGA. The purpose of this study was to delineate the effects of microclimate pH and polymer water content on the stability of encapsulated bovine serum albumin (BSA) by comparing the effects of Mg(OH) 2 with those of another excipient, sucrose, which increases polymer water content without significantly affecting acid-base chemistry of the polymer. These two excipients, when encapsulated in PLGA at appropriate levels (3% Mg(OH) 2 vs. 10% sucrose), were found to cause identical water sorption kinetics, thus allowing the effect of the two microclimate parameters to be determined. In contrast to their similar effects on polymer water sorption, Mg(OH) 2 afforded a much greater stabilization effect on encapsulated BSA than did sucrose, with less than 7% aggregates for 3% Mg(OH) 2 compared to 51% for 10% sucrose and 81% without either excipient after 4 weeks of incubation at 37°C. When the protein stabilization rationale of neutralizing the acidic microenvironment by adding Mg(OH) 2 was applied to the delivery of an important therapeutic protein, tissue plasminogen activator (t-PA), t-PA stability was also improved and the active protein was completely recovered during a one month period of in vitro release. These data demonstrated that although increased water uptake induced by antacid excipients may improve the stability of the encapsulated proteins, the homogeneous acid neutralization effect is unique to antacid excipients such as Mg(OH) 2, which is necessary to maintain the stability of proteins in acidic PLGA specimens.

Kinetics of Water Sorption in NafionThin Films − Quartz Crystal Microbalance Study

Quartz crystal microbalance measurements are used to investigate the water sorption in the cast Nafion films exposed to water vapor. Equilibrium and kinetic sorption data are reported for H- and Na-form Nafion films of a thickness ca. 0.02 μm. Water sorption isotherms obtained agree well with the literature data for Nafion117. However, the water diffusion coefficients evaluated from the initial sorption and desorption rates by using the theory of diffusion in a plane sheet are ca. 7 orders of magnitude lower than those obtained from sorption measurements in ca. 200 μm thick Nafion117 membranes. It is concluded that the kinetics of water sorption (desorption) in the partially hydrated Nafion exposed to water vapor is controlled by the first-order process of water transfer in and out of the nanoscopic hydrophilic regions (ion clusters) for membranes thinner than 10 μm. On the other hand, the linear diffusion is the rate-determining step of water transport in thick Nafion membranes and in fully hydrated thin Nafion films.

A novel spectroscopic approach to investigate transport processes in polymers: the case of water–epoxy system

A novel experimental approach, based on in situ FTIR spectroscopy in the transmission mode, has been developed to monitor sorption–desorption behaviour of small molecules in polymer films. This technique, along with classical gravimetric analysis, has been used to investigate water vapour transport in an epoxy resin. Aim of the investigation was to elucidate the different types of interaction that water molecules form with the macromolecular network and their change as function of the amount of sorbed water. This analysis has been performed at several water vapour activities (0.08, 0.2, 0.4, 0.6, 0.8) at 24°C by using a FTIR cell specifically designed for the in situ monitoring of water sorption in the epoxy film. Particular attention has been paid to realise a very accurate control of water vapour pressure and temperature in the cell. Water sorption kinetics has been also investigated in the same conditions by using an electronic microbalance. Information gathered from the results of both experimental approaches is potentially useful to clarify the plasticising action of sorbed water. To this aim, the amount of water sorbed by itself, is not a reliable predictor of possible physical effects on the matrix, since the plasticising efficiency of water molecules is expected to change with the level of interaction they establish with the polymeric matrix. FTIR based analysis supplies a very useful experimental tool to discriminate among different types of penetrant–polymer molecular interaction. In fact, several spectroscopically distinguishable ‘types’ of sorbed water molecules have been detected in the case of the analysed water–epoxy system and their evolution as a function of the penetrant concentration has been followed.