Capillary Condensation Of Water Research Articles

Capillary Condensation of Water in Graphene Nanocapillaries.

Recent experiments have revealed that the macroscopic Kelvin equation remains surprisingly accurate even for nanoscale capillaries. This phenomenon was so far explained by the oscillatory behavior of the solid-liquid interfacial free energy. We here demonstrate thermodynamic and capillarity inconsistencies with this explanation. After revising the Kelvin equation, we ascribe its validity at nanoscale confinement to the effect of disjoining pressure. To substantiate our hypothesis, we employed molecular dynamics simulations to evaluate interfacial heat transfer and wetting properties. Our assessments unveil a breakdown in a previously established proportionality between the work of adhesion and the Kapitza conductance at capillary heights below 1.3 nm, where the dominance of the work of adhesion shifts primarily from energy to entropy. Alternatively, the peak density of the initial water layer can effectively probe the work of adhesion. Unlike under bulk conditions, high confinement renders the work of adhesion entropically unfavorable.

Role of humidity and surface roughness on direct wafer bonding

Bodies made from elastically stiff material usually bind very weakly unless the surfaces are flat and extremely smooth. In direct wafer bonding flat surfaces bind by capillary bridges and by the van der Waals interaction, which act between all solid objects. Here we study the dependency of the work of adhesion on the humidity and surface roughness in hydrophilic direct wafer bonding. We show that the long-wavelength roughness (usually denoted waviness) has a negligible influence on the strength of wafer bonding (the work of adhesion) from the menisci that form from capillary condensation of water vapor.Graphic abstractAFM surface topography for Si(100) wafer

Simulation studies of water adsorption on MIL-101(Cr) revealing the role of inhomogeneous potential field composed of open metal sites and organic linkers

Simulation studies reveal that the capillary condensation of water on MIL-101(Cr) occurs due to the formation of a liquid film that covers the hydrophobic pore surface, driven by the presence of open metal sites.

Effects of agglomerate structure and operating humidity on the catalyst layer performance of PEM fuel cells

Understanding the transport mechanism in catalyst layer (CL) agglomerates is crucial to enhance Pt utilization and overall fuel cell performance. In this study, high-resolution porous agglomerates are stochastically reconstructed, and capillary condensation of water in agglomerates is directly simulated using a lattice Boltzmann pseudopotential model. On this basis, a pore-scale electrochemical model is developed to investigate the effects of agglomerate structure and operating humidity on local transport resistance and agglomerate effectiveness at different Pt loadings. The results reveal that liquid water formed above 70%RH improves the electrochemical surface area (ECSA) of agglomerates with low ionomer content. An optimum I/C of 0.4 in agglomerates is determined by balancing the activated ECSA and oxygen transport resistance. Through the comparison of local transport resistance with limiting current data, an additional oxygen dissolution resistance of 60–350 s m−1 at the pore/ionomer or ionomer/Pt interface is quantified. The lowest local transport resistance of the agglomerate is achieved at about 70%RH, while condensed water at higher humidities hampers oxygen diffusion within the agglomerate, leading to reduced Pt utilization. Agglomerate size significantly affects the local transport resistance only when excessive ionomer or liquid water severely blocks primary pores. Finally, a novel multiscale coupling strategy integrating the porous agglomerate sub-model with a continuous-scale CL model is proposed, offering innovative insights into comprehending the relationship between CL structure and the performance of proton exchange membrane fuel cells.

Cyclical water vapor sorption-induced structural alterations of mine roof shale

Understanding the nanoscale structural changes of shale in an unsaturated gas-water-shale system can shed light on the underlying mechanisms on water retention-induced shale deterioration. The simplified hypothesis here is that pore-scale shale structure can be passively altered by pore deformation/failure under cyclical water vapor treatments and can be described by a partially saturated effective stress law. Cyclical water vapor sorption tests were conducted on dried and water-saturated shale powders prepared by a hand-crushing (larger particle size) method and a cryogenic ball-milling (smaller particle size) method. Techniques including XRD, FESEM-EDS, and nanoindentation were used to provide evidence that the nano-porous shale is highly heterogeneous in nano-to micro-scale. The sorption curves of both samples with initial water-saturation treatments are higher than the raw samples, indicating enhanced sorption capacities in the treated samples. The degree of hysteresis among eight cycles (five cycles on raw sample and three cycles for treated sample) for both samples exhibit significant differences at low relative humidities, namely lower than ∼0.6. Very slight differences were observed among eight cycles when the relative humidity was higher than 0.6. An obvious decrease in the effective diffusion coefficients is observed in the treated samples compared to the raw samples, which is attributed to the physical alterations induced by the initial drying of water-saturated sample. In situ SANS data for shale at elevated relative humidity conditions suggest that the decrease in the scattering contrast between rock solid matrix and pores can be attributed to water capillary condensation. This occurs mainly in pores with radii smaller than ∼8.3 nm but larger than ∼1.25 nm and at a relative humidity of ∼59%. The capillary condensation continuously takes place in larger pores as the relative humidity increases to ∼87%. The results can ultimately provide the data for analyzing the interactions within an unsaturated air-water-shale system.

Condensation of capillary water and decreased surface energy cause increased soil water repellency in sandy soil

AbstractPredicting soil water availability to crops in water‐repellent sandy soil is complicated as soil water repellency (SWR) responds non‐linearly to soil water content. Others have hypothesised that the development of a monolayer of water molecules results in SWR increasing before SWR declines with a further increase in soil water content. In a previous study, we found that SWR increases when above 0.28–0.86% threshold soil water content. Thus, our objective was to determine the underlying mechanisms responsible for why SWR increases when above a certain threshold soil water content in a sandy soil. A water adsorption isotherm was constructed by exposing a water‐repellent sandy soil to increasing relative humidity (dynamic vapour sorption technique) to evaluate if the development of a monolayer of water molecules was responsible for the increased SWR response. The increased SWR when above 0.66% threshold soil water content was found to coincide with the capillary condensation of water in the soil. The inverse gas chromatography technique was used for the first time in soil particles' surface energy analysis to investigate why SWR increases when above the threshold soil water content by determining the total, dispersive (non‐polar), and specific surface (polar) energy of the soil at two relative humidities (0% and 90%). Wettable sandy soil (98% soil organic carbon removed) was included as a control to further assess if soil organic carbon in the water‐repellent soil influences the surface energy of the soil. The mean of total, dispersive, and specific surface energy decreased for both wettable and water‐repellent sandy soils when exposed to 90% relative humidity, suggesting that there was limited effect of soil carbon on the increased SWR when above the threshold soil water content since most organic carbon was removed from the wettable soil. We also investigated if there is any difference in the surface energy heterogeneity when exposed to 90% relative humidity to gain insight into surface chemistry heterogeneity of the soil particles' surfaces. Exposing soils to 90% relative humidity decreased the heterogeneity of the total and dispersive surface energy of both wettable and water‐repellent sandy soil indicating a more uniform surface chemistry than when exposed to 0% relative humidity.Highlights Examined why soil water repellency (SWR) in a sandy soil increases at low soil water content. Explored underlying mechanisms via water adsorption isotherm and surface energy of a sandy soil. Increased SWR coincided with capillary water condensation and is likely due to counterion effects. Quantitative data presented new mechanisms on why SWR increases with increasing soil water content.

Effect of ionomer swelling and capillary condensation of water on porous characteristics in cathode catalyst layers of polymer electrolyte membrane fuel cells under humidified conditions

Reducing oxygen diffusion resistance in the cathode catalyst layer of polymer electrolyte fuel cells under humidified conditions is essential for reducing the cost of expensive platinum catalysts. We designed an ionomer swelling model that reproduces isotherm adsorption and oxygen diffusion coefficient measurements, to elucidate the effect of pore blockage due to ionomer swelling and capillary condensation on the decrease in oxygen diffusivity. Coupled simulations of ionomer swelling, capillary condensation, and oxygen diffusion are performed using this model, and the water and swelled ionomer distributions at each relative humidity are elucidated. When the humidity is low, ionomer swelling is dominant for the oxygen diffusivity reduction, whereas when the humidity is high, both ionomer swelling and capillary condensation affect them.

Vapor/gas separation through carbon molecular sieve membranes: Experimental and theoretical investigation

The separation of H2O vapor from (hydrogen-rich) gaseous streams is a topic of increasing interest in the context of CO2 valorisation, where the in situ water removal increases product yield and catalyst stability. In this work, composite alumina carbon molecular sieve membranes (Al-CMSM) were prepared from phenolic resin solutions loaded with hydrophilic boehmite (γ-AlO(OH)) nanosheets (0.4–1.4 wt. % in solution) which partially transform to γ-Al2O3 nanosheets upon thermal decomposition of the resin, improving the hydrophilicity and thus the adsorption-diffusion contribution of the H2O permeation. The γ-Al2O3 nanosheets showed no influence on the pore size distribution of the membranes in the range of micropores, but they increased the membrane hydrophilicity. In addition, the use of boehmite in the resin solution causes an increase in the viscosity and thus an increase in the carbon layers thickness deposited on the porous α-Al2O3 support (from 1 to 3.3 μm). Furthermore, the alumina sheets introduce defects in the carbon matrix, increasing the tortuosity of the active layer, as concluded via phenomenological modelling and parametric fitting of the experimental results. As a consequence, the water permeability exhibits a maximum (1.3ꞏ10−6 molꞏs−1 Pa−1 m−1 at 150 °C) with boehmite/alumina content of ca. 0.8 wt. %, as the combined effects of increasing hydrophilicity (which favour H2O permeability) and increasing thickness and tortuosity (which hamper permeability) upon increasing boehmite loading. Similarly, the H2O/gas perm-selectivity is optimum at 1.2 wt. % boehmite loading. We further investigated the H2O permeation mechanism by modelling the mono- and multi-layer adsorption and capillary condensation of water in microporous media, which result as the main transport mechanisms in the explored conditions.

In Situ Atomic-Scale Imaging of Interfacial Water under 3D Nanoscale Confinement.

Capillary condensation of water from vapor is an everyday phenomenon which has a wide range of scientific and technological implications. Many aspects of capillary condensation are not well understood such as the structure of interfacial water, the existence of distinct properties of confined water, or the validity of the Kelvin equation at nanoscale. We note the absence of high-spatial resolution images inside a meniscus. Here, we develop an AFM-based method to provide in situ atomic-scale resolution maps of the solid–water interface of a nanomeniscus (80–250 nm3). The separation between the first two hydration layers on graphite is 0.30 nm, while on mica it is 0.28 nm. Those values are very close to the ones expected for the same surfaces immersed in bulk water. Thus, the hydration layer structure on a crystalline surface is independent of the water volume.

Capillary condensation under atomic-scale confinement.

Capillary condensation of water is ubiquitous in nature and technology. It routinely occurs in granular and porous media, can strongly alter such properties as adhesion, lubrication, friction and corrosion, and is important in many processes used by microelectronics, pharmaceutical, food and other industries1-4. The century-old Kelvin equation5 is frequently used to describe condensation phenomena and has been shown to hold well for liquid menisci with diameters as small as several nanometres1-4,6-14. For even smaller capillaries that are involved in condensation under ambient humidity and so of particular practical interest, the Kelvin equation is expected to break down because the required confinement becomes comparable to the size of water molecules1-22. Here we use van der Waals assembly of two-dimensional crystals to create atomic-scale capillaries and study condensation within them. Our smallest capillaries are less than four ångströms in height and can accommodate just a monolayer of water. Surprisingly, even at this scale, we find that the macroscopic Kelvin equation using the characteristics of bulk water describes the condensation transition accurately in strongly hydrophilic (mica) capillaries and remains qualitatively valid for weakly hydrophilic (graphite) ones. We show that this agreement is fortuitous and can be attributed to elastic deformation of capillary walls23-25, which suppresses the giant oscillatory behaviour expected from the commensurability between the atomic-scalecapillaries and water molecules20,21. Our work provides a basis for an improved understanding of capillary effects at the smallest scale possible, which is important in many realistic situations.

Separation of particles of different surface energies through control of humidity

Many of the methods to classify and concentrate minerals and the subsequent extraction of metals takes place in water-based environments (aqueous solutions). Sustainable processing through the reduction of water consumption will become a key factor to make mining operations viable in the long term. In humid environments, capillary condensation of water can occur between the particle and substrate. The objective herein is to identify separation windows in which control of relative air humidity (RH) yields different substrate adhesion for hydrophilic and hydrophobic particles of different values of interfacial energy. Plasma cleaned glass beads, and trichloro(octadecyl)silane (TCOD) treated beads were poured on a plasma cleaned glass disk and an impact caused the detachment of particles. Impact tests performed under a range of RH showed that separation of plasma cleaned and TCOD treated particles can be achieved in 80% of the tests at humidity levels between 45% and 55%. The recovery of plasma cleaned particles was five times greater than TCOD treated particles at humidity levels between 50% and 55%.

Kinetic analysis on water adsorption of thermally upgraded lignite

An in-situ water adsorption analyzer was designed and set up for obtaining water dynamic adsorption process of thermally upgraded lignite under relative humidity (RH) of 55%–85% at 20 °C and 30 °C. Four upgraded samples with different pore structures and oxygen functional groups were prepared at 120, 160, 240 and 320 °C. Grey correlation degree analysis method was used to study the correlation between water adsorption capacity and its influencing factors. The results showed that upgrading temperature was the dominant factor influencing water adsorption of upgraded coals compared to RH and environmental temperature. The dynamic adsorption process of water on coal could be well described by Intra-particle diffusion model, and it could be divided into surface adsorption stage, pore diffusion stage, and dynamic equilibrium stage. The rate-controlling factors of surface adsorption stage were oxygen functional groups and specific surface area, while those of pore diffusion stage were pore volume and pore size. During adsorption under RH 55%, water was firstly adsorbed on coal surface through oxygen functional groups with a relatively fast rate, which was orderly followed by formation of water clusters and capillary condensation of water in pores with a relatively slow rate.

Relation between oxygen gas diffusivity and porous characteristics under capillary condensation of water in cathode catalyst layers of polymer electrolyte membrane fuel cells

We present a coupled simulation method for evaluating oxygen diffusivity in cathode catalyst layers of polymer electrolyte membrane fuel cells under capillary condensation of water partially filling the pore network. The voxel data of a catalyst layer formed with platinum-supported carbon and Nafion ionomer are prepared from sequential cross-sectional images taken by a cryo-focused ion beam scanning electron microscope. Capillary condensation of water, which effectively alters the pore network, is simulated using the lattice density functional theory. Then, the effective porosity εeff, chord length distribution, and characteristic length of the partially wet catalyst layer are evaluated. The effective diffusion coefficients of oxygen in the Knudsen regime are evaluated using the mean-square displacement method. The ratio εeff/τ, with τ being the tortuosity factor, roughly exhibits intermediate behavior between the Bruggeman correlations of sphere- and cylinder-filling for εeff ≳ 0.2, while εeff/τ drastically deviates from the Bruggeman correlation for εeff ≲ 0.2. The effective oxygen diffusion coefficients at 350 K are predicted as a function of relative humidity and total pressure based on the kinetic theory of gases. The proposed method gives deeper insights into oxygen diffusivity under capillary condensation, contributing to more efficient material design.

Influence of air humidity on particle filtration performance of a pulse-jet bag filter

A pulse-jet bag filter, representative of those implemented in waste incineration plants for flue gas treatment, was studied at laboratory scale to evaluate the influence of air humidity on the filtration performance. Several cycles of clogging/unclogging were performed with non-hygroscopic submicronic particles with a nanosized fraction with on-line cleaning at operating conditions as similar as possible to those found in the flue gas treatment of waste incineration plants (150 °C – 73 g/m3 of absolute humidity), and in dry conditions (150 °C).The results revealed a significant influence of air humidity on the bag filter performance: faster increase of bag filter pressure drop during clogging in presence of humidity due to water capillary condensation, lower efficiency of particulate cake unclogging and better collection efficiency of particles between 110 and 300 nm in diameter.

Simulation of capillary condensation of water and prediction of gas oxygen diffusivity in the mesoporous cathode catalyst layer of polymer electrolyte fuel cell

Simulation of capillary condensation of water and prediction of gas oxygen diffusivity in the mesoporous cathode catalyst layer of polymer electrolyte fuel cell

Metal Oxide Heteronanostructures Prepared by Electrospinning for the Humidity Detection: Fundamentals and Perspectives

Humidity is a critical environmental parameter for several production processes and its control/monitoring is of great importance in maintaining the quality of goods and products. In this context, metallic oxide ceramic nanostructures are materials of great technological interest in the fabrication of moisture sensors because they have good chemical/structural stability and high surface area/volume ratio. The electrical response of these sensors relates to the chemisorbed and physisorbed layers of water molecules on the surface of the ceramic particles and to the capillary condensation of water in the microscopic pores between the particles. Based on these aspects, this work presents the fundamentals, electrical/electronic properties, influence of dopants, novel preparation procedure by electrospinning and perspectives of application of TiO2:WO3 metal oxide heteronanostructures as humidity sensors.

Merging Electrochemistry and Water Capillary Condensation to Understand the Corrosion Mechanism of Steel in Carbonated Concrete

Reinforced concrete is one of the most used man-made materials worldwide, one of its main causes of degradation worldwide is carbonation induced corrosion, accounting every year for billions of dollars for main­tenance, repair and rebuilding. The alkali produced by the Portland cement hydration (mainly Ca(OH)2) allows steel passivation. CO2 from the atmos­phere can enter the concrete pores and react with the Ca(OH)2, leading to a decrease of pH that can propagate to the reinforcement level. In presence of oxygen the steel starts to corrode, the corrosion products precipitate in the surrounding of the reinforcement, generating stresses that can lead to cracking and spalling of the concrete cover. The topic of carbonation induced corrosion is becoming of key importance in the reduction of environmental footprint of Portland cement production, responsible for about 8% of the man-made CO2 and thus for global warming and climatic change. The replacement of Portland cement by using supplementary cementitious materials (SCM) is one of the most viable, short-term solutions. This, however, might impair the durability of reinforced concrete; reducing the Portland cement content leads to a reduced production of calcium hydroxide and therefore to a much faster carbonation process. When the carbonation front reaches the reinforcement before the end of service life, the corrosion propagation becomes an important part of the life of the structure. To be able to collect data of corrosion propagation rates in a reasonable time a new experimental set up has been designed, which consists of small (8x8 cm) and thin (0.6 cm) mortar samples instrumented with a reference electrode (Ag/AgCl), 5 steel wire electrodes and a stainless steel grid counter electrode. The thin sample allows rapid carbonation and equilibration of environmental humidity. Electrical resistivity, oxygen cathodic current, open circuit potential and corrosion rate of the steel can be measured. The results have shown that the corrosion rate increases markedly with increasingly higher relative humidity (up to 99% RH), maximum values around 2 µA/cm2 have been found in wet/dry cycles. Increasing corrosion rate was associated with a decrease of the corrosion potential, the ohmic resistance of the electrochemical cell was found to correlate with the corrosion rate. The oxygen reduction limiting current was always about five to ten times higher than the corrosion rate. The influence of cement type and water/binder ratio was of minor importance. The mechanism of corrosion of steel in carbonated concrete is under debate since 1980 - and after almost 40 years there is still no agreement. The traditionally proposed controlling mechanisms such as resistive, cathodic or anodic control have been found not suitable for corrosion of steel in carbonated mortar, which has always been considered as a uniform process. Instead, being the cement paste a porous system, the capillary water condensation had to be taken into account. With the Kelvin equation and the porosity curves, the volumetric water content could be calculated for every different sample and relative humidity condition. A correlation between the water content and the corrosion rate was found. Merging electrochemistry and water capillary condensation theory allowed explaining the mechanism of corrosion of carbon steel embedded in a porous system. The porosity and degree of pore saturation define the electrochemically active area with respect to the total steel surface. This active area varies by orders of magnitude depending on the exposure condition: from immersed conditions to 99% RH the active area goes down to ca. 40 %, at 80% RH only 1 %. The volumetric water content also determines the available diffusion volume of produced Fe2+ ions, which modifies the anodic reaction reversible potential. A larger diffusion volume leads to a lower Fe2+ concentration and therefore lower reversible potential and higher corrosion current according to the Evans diagram. The two contributions can be recognized and the huge variation of the corrosion rate with the exposure condition can thus be explained. In conclusion the corrosion mechanism of carbon steel in carbonated concrete could be determined and the corrosion rate can be uniformly expressed, for every different humidity state, water to binder ratio and binder type, as a function of pore solution pH, water content and porosity. Figure 1

Converting Water Adsorption and Capillary Condensation in Usable Forces with Simple Porous Inorganic Thin Films.

This work reports an innovative humidity-driven actuation concept based on conversion of chemical energy of adsorption/desorption using simple nanoporous sol-gel silica thin films as humidity-responsive materials. Bilayer-shaped actuators, consisting of a humidity-sensitive active nanostructured silica film deposited on a polymeric substrate (Kapton), were demonstrated as an original mean to convert water molecule adsorption and capillary condensation in usable mechanical work. Reversible solvation stress changes in silica micropores by water adsorption and energy produced by the rigid silica film contraction, induced by water capillary condensation in mesopores, were finely controlled and used as energy sources. The influence of the film nanostructure (microporosity, mesoporosity) and thickness and the polymeric substrate thickness on actuation force, on movement speed and on displacement amplitude are clearly evidenced and discussed. We show that the global mechanical response of such silica-based actuators can easily be adjusted to fabricate tailor-made actuation systems triggered by humidity variation. This study provides insight into hard ceramic stimulus-responsive materials that seem to be a promising alternative to traditional soft organic materials for surface-chemistry-driven actuation systems.

Alumina-coated ordered mesoporous silica as an efficient and stable water adsorbent for adsorption heat pump

With regard to the large amount of available but unused low-temperature heat (<373 K), adsorption heat pumps (AHP) can provide an energy-efficient cooling technology. Considering general operation conditions of AHP, the adsorbents should show high water uptake in the P/Po range of 0.1–0.3 at 303 K. With a uniform and tunable pore structure, MCM-41-type ordered mesoporous silica can be a promising candidate as an adsorbent. However, typical MCM-41 materials with a pore diameter of 3–5 nm show water capillary condensation at P/Po > 0.3, which is too high for AHP. This indicates that the pore diameter should be further reduced below the typical mesopore diameter range of these materials. Another serious limitation of these materials as a water adsorbent is their low hydrothermal stability. In this work, in order to reduce the mesopore diameter and to increase the hydrothermal stability of MCM-41 simultaneously, the silica mesopore surface was uniformly coated with an alumina layer. Due to the reduced mesopore size (2.05 nm), the alumina-coated MCM-41 showed S-shaped water isotherm with capillary condensation in the ideal P/Po range of 0.1–0.3. The adsorbent also showed remarkably enhanced hydrothermal stability. Consequently, the adsorbent showed a cycling water capacity of 0.27 gwater gads−1 (1.5 times larger than FAM-Z01 which is a commercialized adsorbent for AHP) without a loss of capacity up to 10 cycles.

Water Capillary Condensation Effect on the Photocatalytic Activity of Porous TiO2 in Air

Spectroscopic ellipsometry was utilized to follow in situ the photodegradation of organic species at TiO2 interfaces during UV irradiation in real operating conditions. Mesoporous crystalline TiO2 thin films were used as photoactive catalyzers, while lauric acid was used as the model pollutant. After infiltration of lauric acid in the TiO2 film, the local photocatalytic activity was deduced by monitoring the variation of refractive index, which is directly related to the total organic content (TOC) elimination, during irradiation at 364 nm within different atmosphere compositions in O2 and H2O vapor. Results revealed that the presence of both H2O (hydroxyl groups) and O2 is critical due to their role as electron and hole scavengers when adsorbed at the surface of TiO2. The photodegradation activity speed is not constant along the test as a result of the variation in −OH and H2O content into the pore. Interestingly, while the activity was not significantly influenced between 10% and 20% of O2 and between 2...