Water Sorption Process Research Articles

Structural effects on heat capacity, moisture absorption and thermal expansion of epoxy-novolac polymers

Today epoxy novolac resins are of great interest for the aerospace industry. Thermal properties of the polymer matrix, as well as the effect of moisture on the characteristics of polymer composite materials (PCM) under thermomechanical impacts, are critical tasks in creating dimensionally stable structures operating in a wide temperature range. Particularly, thermal and moisture expansion as well as heat capacity are important parameters for evaluating of polymer matrices and modeling their properties. In this work, polymers based on a number of epoxy novolac resins with aromatic amine hardener were studied. The values of the crosslink density were experimentally determined, and a comparative analysis of thermal expansion and heat capacity for epoxy polymers with different crosslink density was carried out. The dependence of thermophysical properties on polymer crosslinking density is shown, both in glass and highly elastic states. Additionally, this study analyzes the process of water sorption by polymers and the coefficient of moisture expansion. The most significant dependence of the diffusion coefficient was observed with coefficient of moisture expansion.

In situ Electrical Impedance Tomography for Visualizing Water Transportation in Hygroscopic Aerogels.

The global water crisis demands immediate attention, and atmospheric water harvesting (AWH) provides a viable alternative. However, studying the real-time subtle relationship between water absorption, diffusion, and internal structure for hygroscopic materials is challenging. Herein, a dynamic visualization technique is proposed that utilizes an in situ electrical impedance tomography (EIT) system and a precise reconstruction algorithm to achieve real-time monitoring of the water sorption process within aerogels from an internal microstructural perspective. These results can be inferred that composites' pore sizes affecting the kinetics of their moisture absorption. In addition, the diffusion path of moisture absorption and the distribution of stored moisture inside aerogels exhibit intrinsic self-selective behavior, where the fiber skeleton of the aerogel plays a crucial role. In summary, this work proposes a generic EIT-based technique for the in situ and dynamic monitoring of the hygroscopic process, pointing to an entirely new approach regarding research on AWH materials.

Covalent Organic Frameworks for Water Harvesting from Air.

Despite approximately 70 % of the earth being covered by water, water shortage has emerged as an urgent social challenge. Sorbent-based atmospheric water harvesting stands out as a potent approach to alleviate the situation, particularly in arid regions. This method requires adsorbents with ample working capacity, rapid kinetics, low energy costs, and long-term stability under operating conditions. Covalent organic frameworks (COFs) are a novel class of crystalline porous materials and offer distinct advantages due to their high specific surface area, structural diversity, and robustness. These properties enable the rational design and customization of their water-harvesting capabilities. Herein, the basic concepts about the water sorption process within COFs, including the parameters that qualitatively or quantitatively describe their water isotherms and the mechanism are summarized. Then, the recent methods used to prepare COFs-based water harvesters are reviewed, emphasizing the structural diversity of COFs and presenting the common empirical understandings of these endeavors. Finally, challenges and research concepts are proposed to help develop next-generation COFs-based water harvesters.

Kinetic models for water sorption and a hydrolysis reaction with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH)

Biodegradable polymers, including polyhydroxyalkanoate (PHA), have attracted attention because plastic waste is entering the ocean and causing problems. Controlling the rate of PHA degradation is essential for expanding its application, and fundamental studies on the reaction mechanisms are needed. The purpose of this study is to experimentally verify the mechanisms of water sorption and hydrolysis reactions involving poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), a type of PHA, and construct a model. In this study, the water sorption process was measured by near-infrared (NIR) spectroscopy and Fourier transform infrared spectroscopic (FTIR) imaging. From the results, we found that water sorption on PHBH proceeds by non-Fickian diffusion, two kinetic processes involving adsorption and diffusion. The reaction rate of PHBH hydrolysis was determined by gel permeation chromatography (GPC) analysis, and a model was developed to predict PHBH degradation during hydrolysis from the experimental results. Furthermore, we proposed a reaction-diffusion model that combines water sorption and hydrolysis reactions. As a result, the degradation of PHBH plates with different thicknesses can be predicted for this first time.

Probing Water Diffusion Inside Crystals of AlPO‐5 by PFG NMR and IRM for Heat Storage Applications

AbstractDiffusion of water in the aluminophosphates AlPO‐5 is studied using a combination of pulsed field gradient NMR and IR microimaging. The concentration profiles measured by the latter allowed to visualize the process of water sorption and propagation into the crystals of AlPO‐5. The self‐diffusion coefficients obtained by NMR were one order of magnitude higher values compared to corrected ones from the IR microimaging study. The concentration profiles allowed to visualize the process of water sorption and propagation into the crystals of AlPO‐5. The obtained results are compared with experimental and simulation data for the water/AlPO‐5 system for the literature and discussed in the context of its potential use for heat storage applications.

Development of High Temperature Water Sorbents Based on Zeolites, Dolomite, Lanthanum Oxide and Coke

Methanation is gaining attention as it produces green methane from CO2 and H2, through Power-to-Gas technology. This process could be improved by in situ water sorption. The main difficulty for this process intensification is to find effective water sorbents at useful reaction temperatures (275-400 °C). The present work comprises the study of the water sorption capacity of different materials at 25-400 °C. The sorption capacity of the most studied solid sorbents (zeolites 3A & 4A) was compared to other materials such as dolomite, La2O3 and cokes. In trying to improve their stability and sorption capacity at high temperatures, all these materials were modified with alkaline-earth metals (Ba, Ca & Mg). Lanthana-Ba and dolomite sorbents were the most promising materials, reaching water sorption values of 120 and 102 mgH2O/gsorbent, respectively, even at 300 °C, i.e., values 10-times higher than the achieved ones with zeolites 3A or 4A under the same operating conditions. At these high temperatures, around 300 °C, the water sorption process was concluded to be closer to chemisorption than to physisorption.

Hybrid salt-enriched micro-sorbents for atmospheric water sorption

Water shortage severely impacts drought-stricken regions, with estimates indicating that almost half a billion people are affected yearly. Composites of Salt and Porous Matrix (CSPMs) are promising functional materials for water vapor sorption. Here, CSPMs were synthesized by loading SAPO-34 porous crystals with highly hygroscopic salts, namely LiCl and CaCl2, individually (mono-salt systems) or combined (binary salt systems) to enhance water sorption capacity and cyclability. The LiCl and CaCl2 content in the impregnation solution impacted the sorption behavior and equilibrium capacity of the resulting composites. Physicochemical, morphological, textural, and sorption properties were evaluated showing that the confinement of binary salts yielded the highest water uptake (0.88 gw/gads at 25 °C and 90 % RH), which was four times higher than that of the parent SAPO-34. The shape of the obtained water vapor isotherms revealed that the salts introduced into the porous structure led to significant changes in the sorption mechanism, with SAPO-34 following a Langmuir behavior (type I isotherm) and the composites a type II isotherm with associated multilayer formation due to the presence of the salts. Kinetic studies also revealed that the materials follow a PSO model dominated by water-surface interactions. Embedding different salts into the same hosting pores to support atmospheric water harvesting was therefore found to enhance capacity and cyclability compared to single inorganic porous structures toward more efficient water sorption processes.

Structural transitions during the water sorption process in two layered metal hydrogen-bonded organic frameworks and the effect of the H-bond strength between the layers

The strength of the interactions between the layers of 2D porous materials defines the characteristics of their sorption isotherms.

Behaviour of PMMA Resin Composites Incorporated with Nanoparticles or Fibre following Prolonged Water Storage.

When PMMA denture base acrylics are exposed to oral environments for prolonged periods, the denture base absorbs water, which has a negative influence on the denture material and the degree to which the denture base will be clinically effective. This study assessed the water sorption, desorption, and hygroscopic expansion processes within PMMA denture-base resins reinforced with nanoparticles or fibre in comparison to the non-reinforced PMMA. The surfaces of the fillers were modified using a silane coupling agent (y-MPS) before mixing with PMMA. Group C consisted of specimens of pure PMMA whereas groups Z, T, and E consisted of PMMA specimens reinforced with ZrO2, TiO2 nanoparticles, or E-glass fibre, respectively. The reinforced groups were subdivided into four subgroups according to the percentage filler added to the PMMA resin by weight (1.5%, 3.0%, 5.0%, or 7.0%). Five specimens in disc shape (25 ± 1 mm × 2.0 ± 0.2 mm) were tested for each group. To assess water sorption and hygroscopic expansion, specimens from each group were individually immersed in water at 37 ± 1 °C for 180 days. The samples were then desorbed for 28 days at 37 ± 1 °C, to measure solubility. Water sorption and solubility were calculated using an electronic balance in accordance with ISO Standard 20795-1, and hygroscopic expansion was measured using a laser micrometre. Statistical analysis was undertaken at a p ≤ 0.05 significance level using a one-way ANOVA followed by Tukey post-hoc tests. The results demonstrated that the values of sorption (Wsp), mass sorption (Ms%), and % expansion within the tested groups reached equilibrium within 180 days. A noticeable difference was observed in groups Z and E for (Wsp)/(Ms%) compared to the Group C, but this was not significant. However, the difference between Group C and Group T for these measurements was significant. Non-significant differences also existed between each respective reinforced group and the control group in terms of hygroscopic expansion % values. During the 28-day desorption period, there were no differences in the values of solubility (Wsl)/mass desorption (Md%) between Group C and each of the reinforced tested groups. The findings indicate that the inclusion of ZrO2 nanoparticles or E-glass fibres does not increase the water solubility/sorption of the PMMA. However, modifying the PMMA with TiO2 did significantly increase the water sorption level.

Structural modifications occurring during the water sorption processes on a potential material for autonomous indoor humidity control

Structural modifications occurring during the water sorption processes on a potential material for autonomous indoor humidity control

Water Sorption Kinetics of Starch Noodles with Different Cross‐Sectional Shapes

AbstractThe water sorption process of three types of starch noodles with different cross‐sectional shapes is studied in a temperature range of 30°C–70°C. The process could be expressed by a rate equation based on a linear driving‐force approximation, for all the noodles and at any temperature. The activation energy for water penetration is practically negligible for all the noodles, indicating that there is no energy barrier for water migration. For two noodles with approximately square and rectangular cross‐sections, the equilibrium moisture content shows different degrees of temperature dependence at temperatures higher and lower than the gelatinization temperature of starch. The enthalpy changes for water sorption in the high‐ and low‐temperature regions are greater than 20 kJ mol‐1 and lesser than 10 kJ mol‐1, respectively. It is suggested that the rate‐limiting step of water sorption for gluten‐free noodles was movement of water through the laminar fluid film on the noodle surface. The validity of this suggestion is confirmed by the fact that the water sorption rate increased as the shaking speed during water sorption increased. These findings indicate that increasing the specific surface area is effective in preparing gluten‐free starch noodles with a high water sorption rate.

Influence of Fluorine Doping of Activated Carbon Fibers on Their Water Vapor Adsorption Characteristics.

The characterization of fluorinated carbon fibers by water sorption has been broadly investigated in this work. In brief, a pitch-based activated carbon fiber (ACF) was submitted to a fluorination process under different conditions of partial pressure (F2:N2 ratio) and temperature. This led to samples with varied fluorine content and C-F type bonding. The effect of the fluorination treatment on the textural properties of the ACF was studied by means of nitrogen and carbon dioxide adsorption at −196 and 0°C, respectively, while the changes induced in the surface chemistry of the materials were analyzed by XPS. Also, the affinity and stability of the materials toward water was evaluated by single and cycling isotherms. The obtained results show that a mild fluorination not only can preserve most of the textural properties of the parent ACF, but enhance the water uptake at the first stages of the water sorption process, together with a shift in the upswing of the water isotherms toward lower relative humidities. This indicates that fluorination under certain conditions can actually enhance the surface hydrophilicity of carbon materials with specific properties. On the contrary, higher partial pressures led to highly fluorinated fibers with lower porosity and more hydrophobic character. Moreover, they presented a lower chemical stability as demonstrated by a change in the shape of the water isotherms after two consecutive measurements. The kinetics of water sorption in the ACFs provided further insights into the different sorption phenomena involved. Hence, water sorption can definitely help to tailor the water affinity, stability and performance of fluorinated porous carbon materials under humid conditions.

Effect of immidazolium-based green solvents on the moisture absorption and thickness swelling behavior of wood flour/polyethylene composites

The water transport kinetics and moisture-induced thickness swelling behavior of wood-plastic composites (WPCs) in the presence of two imidazolium-based ionic liquids (ILs) were studied. The wood flour chemically treated with ILs were compounded through melt mixing of high-density polyethylene and coupling agent, and finally, the test specimens were produced by compression molding. Hygroscopic rates of the composites were evaluated by immersing them in the water at room temperature and monitoring moisture sorption and thickness changes for several weeks. Besides, the water diffusion coefficients and swelling rate parameters were measured for all composites. It was found that the water absorption and thickness swelling of the WPC specimens reduced as a result of ILs treatment. The SEM analysis revealed that the presence of ILs can improve the quality of adhesion between the polymer matrix and the cellulosic materials, to decrease the gaps in the interfacial region and block the hydrophilic groups. Studying the water sorption process in the specimens showed that the process follows kinetics and mechanisms described by Fick’s law. The highest water diffusion coefficients and swelling rate parameters were found in composites with untreated wood.

The Impact of Reactive Ionic Liquids Addition on the Physicochemical and Sorption Properties of Poly(Vinyl Alcohol)-Based Films.

A new type of hybrid polymeric-based film containing 1-(1,3-diethoxy-1,3-dioxopropan-2-ylo)-3-methylimidazolium bromide (RIL1_Br) and 1-(2-etoxy-2-oxoethyl)-3-methylimidazolium bromide (RIL2_Br) reactive ionic liquids was elaborated. Poly(vinyl alcohol) (PVA)-based films with 9–33 wt % of RILs were subsequently characterized using Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA) and TGA-FTIR. PVA-RIL films were also studied in tensile tests, contact angle and sorption measurements. RIL incorporation enhanced thermal and mechanical stability of PVA membranes due to the hydrogen bonds between RILs and polymer chains. Membrane swelling behavior in water (H2O), ethanol (EtOH), and propan-2-ol (IPA) and the kinetics of water sorption process revealed that PVA-RILs membranes possess the highest affinity towards water. It was pointed out that both the RIL type and the RIL amount in the polymer matrix have significant influence on the membrane swelling behavior and the water sorption kinetics.

Salt hydration and drop drying of two model calcium salts: Implications for foliar nutrient absorption and deposition

AbstractBackground:Hygroscopic salts are often applied as foliar fertilizers and calcium (Ca) salts have been used as model for cuticular permeability studies, with focus on their Point of Deliquescence (POD).Aim:This study was aimed at characterizing water sorption and desorption processes at 20, 30, and 40°C temperature (T), of two model Ca salts commonly used as foliar sprays.Methods:Trials were developed with pure Ca‐chloride and Ca‐nitrate salts in addition to 2 M and 150 mM Ca solutions. Salts were analyzed by Dynamic Vapour Sorption (DVS) and climate chamber experiments.Results:For both Ca salts, water sorption and desorption processes followed hysteresis curves which varied with T. Salt POD was T‐dependent and generally decreased with increasing T. For both Ca salts, the process of efflorescence began at or below 10% RH, at a certain level of water desorption Ca‐salts becoming highly concentrated solutions (HCS).Conclusions:Owing to the low point of efflorescence (POE) of both salts, drops of Ca‐chloride and Ca‐nitrate supplied as foliar sprays will not fully dry under normal RH and T conditions in most world areas. Both Ca salts have a similarly high foliar penetration potential despite their different POD. The significance of the results for the potential absorption of foliar fertilizers and salts by the foliage is discussed under an agronomic and eco‐physiological viewpoint.

Water sorption thermodynamic behavior of whey protein isolate/ polyvinyl alcohol blends for food packaging

In food packaging systems, moisture content influences chemical and physical film properties, also determining processes such as food spoilage, and properties of food texture and crispiness level. The study of water permeation and sorption processes of new materials intended to be used as packaging is very important to determine the best application conditions and to predict the film behavior under different moisture conditions inside and/or outside the packaging. In order to determine the suitable temperature and water activity (aw) application conditions for whey protein isolate (WPI)/polyvinyl alcohol (PVOH) blends as food flexible packaging, water permeation and water sorption thermodynamic behavior of these materials were evaluated. WPI/PVOH films and blends had solubility preponderant over the diffusion on the water permeation process. Water sorption experimental data were well described by the GAB model, and curves showed a more expressive increase of water sorption at aw > 0.75, with lower equilibrium moistures (Ye) at room than at chilled temperatures. Differential enthalpy decreased and differential entropy increased by the Ye gain, and the occurrence of enthalpy-entropy compensation was confirmed with enthalpy driving the sorption process. The addition of PVOH to the WPI matrix made the water sorption process more spontaneous. Water sorption thermodynamic analysis indicates that the application of WPI/PVOH blends as packaging is best suitable for foods and external environments with aw below 0.75 and at room temperature.

The importance of outgassing conditions when using water vapour to characterize activated carbons

The singular mechanism of water adsorption in carbon pores has been roughly elucidated due to the intense research conducted in this field in the last years. Thus, it has been demonstrated that the surface chemistry of the carbon material plays a crucial role in the initialization of the water sorption process. However, the potential changes induced in the surface properties, and therefore in the water isotherm, during the outgassing of the sample prior to the water sorption measurement, have not been addressed yet. On the contrary, the same preparation protocol used for the analysis of non-associating simple fluids (N2, CO2, Ar) is commonly followed. Aiming at filling this gap, two activated carbons with different textural and surface properties have been outgassed under different conditions of time and temperature and subsequently characterized by H2O, N2 and CO2 isotherms and thermogravimetric analysis. The obtained results show that the optimal conditions of the outgassing treatment are not necessarily the same for all the adsorbates tested and will highly depend on the characteristics of the sample. In this regard, the recommended outgassing temperature for water sorption isotherms of nanoporous carbons seems to be lower than for other adsorbates and may not exceed 100 °C.

Thermodynamic properties of water desorption in Buchenavia capitata (Vahl) Eichler

ABSTRACT Thermodynamic properties provide information on energy demand and kinetic parameters in water sorption processes in agricultural products. Such information is essential to analyze and scale drying and storage equipment. The objective of this study was to determine the thermodynamic properties involved in the water desorption process in Buchenavia capitata (Vahl) Eichler seeds with moisture contents ranging from 13.31 to 7.21% dry basis (d.b.), obtained by the indirect static method, at temperatures of 10, 20, 30 and 40 °C. The latent heat of vaporization, differential entropy and Gibbs free energy increase with the reduction of the moisture content of the seeds. The enthalpy-entropy compensation theory is valid for the desorption process, which is controlled by the enthalpy. The desorption of water in the seeds is not a spontaneous process.

Moisture sorption isotherms of fresh and blanched yacon tuber flours

Information on moisture adsorption isotherms, which describe the inherent relationship between water activity (aw) and moisture content in food materials, is essential for drying, storing, packaging, and predicting shelf-life for successful application of yacon tuber flour in various fields of the food industry. The moisture sorption isotherms of fresh and blanched yacon tuber flours were obtained at 25℃, 35℃, and 45℃ and aw in the range of 0.11-0.75 by the static gravimetric method. Six different isotherm models were used to fit the experimental data and to analyze water vapor sorption. The results showed that water vapor sorption of both fresh and blanched yacon flours followed a type III shape over the humidity range. The blanched samples exhibited lower equilibrium moisture content than fresh samples. The monolayer moisture content estimated by the Guggenheim, Anderson, and De Boer (GAB) model was lower than that estimated by the Brunauer, Emmett, and Teller model. By quantifying the relative percent error and the coefficient of determination of the isotherm models relative to data, the GAB and Halsey models were identified to be the best-fitting isotherm to describe the water sorption process of both fresh and blanched yacon flours.

Isosteric heat of water adsorption and desorption in homoionic alkaline-earth montmorillonites

The aim of the present work is to study by means of thermodynamic measurements and Infrared spectroscopy, the effect of the interlayer cations on the adsorption–desorption of water in the case of a montmorillonite exchanged with alkaline-earth metals. For the first time, the net isosteric heat of water adsorption and desorption is determined from isotherms recorded at three temperatures. The net isosteric heat is a very useful parameter for getting more insights into the sorption mechanism since it provides information about the sorption energy evolution which can be complementary to that obtained from structural or gravimetric measurements. The homoionic montmorillonite samples are prepared from purification and cationic exchanged in aqueous solution of the raw material, i.e. the reference SWy-2 Wyoming material. XRD at the dry state and elemental chemical analysis confirm that the treatment does not deteriorate the clay structure and yield the expected homoionic composition. The sorption isotherms measured at various temperatures show that the nature of the interlayer, i.e. exchangeable, cation changes the adsorbed/desorbed amount of water molecules for a given water relative pressure. The total amount of water adsorbed at PP∘ = 0.5 follows the cation sequence Ca ∼ Mg>Ba while the sorption isosteric heats follow a slightly different sequence, i.e. Ca > Mg>Ba. This discrepancy between the adsorption and desorption heat is due to the higher irreversibility of water sorption process in the Ca exchanged montmorillonite. Finally, analysis of the IR spectra recorded at room temperature and under a primary vacuum reveals that the amount of adsorbed water follows the same sequence as that of the isosteric heat of adsorption and shows the coexistence of liquid-like and solid-like water confined in the interlayer space.