Condensation Of Water Molecules Research Articles

Refinement of Kelvin equation for condensation of water in CSH slits based on molecular simulation

The adsorption and condensation of water molecules in the pores of cementitious materials play a crucial role in the mechanical performance and durability of concrete. The Kelvin equation is generally employed to describe the water condensation in porous materials. However, it requires modification for condensation behavior in nano and micro scales of pores. In the present work, various widths of slit models for several cementitious minerals were established. The adsorption isotherms in slit pores were calculated using the hybrid of Grand Canonical Monte Carlo and Molecular Dynamics simulations, obtaining the relationship between the critical pressures for condensation and pore sizes. Then, a modified Kelvin equation was proposed, with discussion on a new reasonable method for determining the real thickness of adsorbed water film. The results demonstrate that condensation will occur in the slits within the critical width range, which promotes the surface adsorption to achieve complete adsorption. Therefore, the location of the interface between the adsorbed water film and condensed water bulk is fixed in the same mineral during condensation. The modified Kelvin equation shows higher accuracy than traditional Kelvin equations at the nano and micro scale. This work promotes a deeper understanding of the interaction mechanism of water with cementitious minerals and inspires more fundamental realization in the design of concrete durability.

Interaction between dilute water vapor and dodecane thiol ligated Au nanoparticles: Hydrated structure and pair potential of mean force.

The interaction between two ligated nanoparticles depends on whether they are isolated or immersed in a liquid solvent. However, very little is known about the influence of solvent vapor on the interaction between two ligated nanoparticles. Recent experiments yield the surprising result that the cyclic exposure of solvent free suspended monolayers of dodecane thiol ligated gold nanoparticles (AuNPs) to water vapor and dry nitrogen generates reversible cyclic decreases and increases in Young's modulus of the monolayer, implying corresponding cyclic changes in the AuNP-AuNP interaction. We examine how water vapor interacts with an isolated dodecane thiol dressed AuNP and how water vapor affects the interaction between a pair of nanoparticles, using all-atom molecular-dynamics simulations. We find that there is condensation of water molecules onto the ligand shell of an AuNP in the form of clusters of 100-2000 molecules that partially cover the shell, with most of the water in a few large clusters. A water cluster bridges the AuNPs, with a sensibly constant number of water molecules for AuNP-AuNP separations from the edge-to-edge contact up to center-to-center separations of 100 Å. The wet AuNP-AuNP interaction has a slightly deeper and wider asymmetric well than does the dry interaction, a change that is qualitatively consistent with that implied by the observed water vapor induced change in Young's modulus of a monolayer of these AuNPs. We find that macroscopic analyses of water drop-deformable surface interactions and dynamics provide both guidance to understanding and qualitatively correct predictions of the phenomena observed in our simulations.

Silver Doped Polyaniline-Graphene Based Barium Ferrite Composite as Humidity Sensor and Photocatalyst

In this work, the response of humidity sensing and dielectric properties of PANI/Ag/graphene/BaFe12O19 (PAGB) composite prepared by chemical interfacial oxidative polymerization method using the ammonium peroxydisulfate as an oxidant is presented. The synthesized PAGB composite was characterized by using FT-IR, XRD, SEM and EDAX techniques. The measured dielectric constant and dielectric tangent loss values were found to decrease with increase in logarithmic frequency. The electrical resistance of the composite was also found to decrease from 2500 MΩ to 7 MΩ, on exposure to a wide range of relative humidity (RH) varying between 10% and 97% RH. This observed decrease is due to capillary condensation of water molecules which cause the changes in electrical conductivity of composite. The synthesized PAGB-50% composite displayed a high sensitivity at low humidity ranging from 20% to 60% RH. The composite has been proved to be a better candidate in the design of humidity sensors. In addition, PAGB composite exhibited efficiency towards the degradation of organic anionic dye, acid orange-8, with high potential of reusability.

Nature-inspired PDMS cumulonimbus micro-energy-harvesting cloud

Here in this work, nanoenergy-generating films mimicked from cumulonimbus cloud were prepared using deionised water (DI) in polydimethylsiloxane (PDMS). Various characterisation methods have been adopted to verify the content of water as well as the resemblance of the morphological structure of the water-entrapped polymer films with clouds which include scanning electron microscopy and optical reflectance imaging techniques. The water content entrapment in the polymer is verified using the ATR-IR spectroscopy by finding absorbance peaks at 3266 and 1634 cm−1. Polyethylene terephthalate (PET) sheets with a copper coating (Cu-PET) were used as contact electrodes, and the device was fabricated by the simple sandwiching process. Dielectric studies of the device under an applied electric field were used to illustrate the presence of interfacial hopping as well as orientational polarisation in the device. The device fabricated shows an enhancement of 218% and 562% with an increase in water content from 4:1 to 1:1 and a decrease of 26% and 36.5% with a further increase in water for the generated voltage and current measured, respectively. The optimum concentration of polymer to water is selected as 1:1 and the decrease in current and voltage may be due to the condensation of water molecules leading to fewer contact sites between the water and polymer molecules. The devices showed an increment of voltage and current of 171.1 V and 29.16 nA, respectively, with an increase in the force of 1–5 N by using an automated input system. The device was tested under various mechanical inputs, and the generated energy used as sensors in the knock sensor, home doorbell system and seabed vibration detector and as a secondary powering source for charging mobile phones and lighting LED.

Influence of Water on the Interface between Hydrophilic Ionic Liquid and Single Crystal Bismuth Electrodes

Ionic liquids (ILs) as solvent-free electrolytes are new favorites for interfacial chemistry as well as for many other potential applications in electrochemical systems, such as supercapacitors, solar cells, fuel cells and secondary ion batteries. One of the main advantages of ILs is the good electrochemical stability. Differently from the hygroscopic, bulky ILs, many ILs are completely miscible with water, conventionally defined as hydrophilic. As an inevitable impurity, water has a strong effect on the surface of ILs within the electrochemical stability region where condensation of water molecules is dependent on applied potential [1-2]. The electrochemical faradic processes at the interface affect both charge transfer and energy storage processes. Those prompt us to focus on the interaction of ILs and water at electrified electrode interface, and the formation kinetics of the electrical double layer (EDL). Adsorption of particles, such as molecules or ions, contributes to the variety of EDL structure at ionic liquid│electrode interface. Halide ions, namely chloride, bromide and iodide, have been studied as part of a group of strongly adsorbing anions to form a dense layer at Bi(hkl) electrodes [3-4]. Electrosorption of water on electrode surface depends not only on the water-ion interaction, but also on the water-eletrode interaction [2]. To further explore the interaction mechanism of water molecules at electrified electrode interface, classical electrochemical methods (cyclic voltammetry and electrochemical impedance spectroscopy) as well as in situ scanning tunneling microscopy (STM) method have been applied to study the effect of water on the interfacial structure of ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate (EMImOTf)│Bi(hkl) interface. Single crystal bismuth electrodes as working electrodes have variable electronic properties and surface structure. Electrical equivalent circuit analysis derived from impedance data has been applied to characterize the interfacial processes [3]. The fitted data double layer capacitance (Cdl ) has a good agreement with the series capacitance (Cs ) at lower frequency, suggesting relative sluggishness of dried IL system. Compared with dry EMImOTf, systems with up to 0.1wt% water additive have a pronounced peak in Cdl vs. potential curves, observed within less negative potential region, showing that water has a much stronger interaction with IL anions than with the cations. OTf⁻ anions combined with aliphatic imidazolium cations are generally thought specifically adsorb at metallic electrodes due to the presence of sulfonyl groups. The different metallic properties of bismuth single crystal planes also show a difference in capacitive behavior. Other parameter, such as charge transfer resistance (Rct ) reveals how water adsorption process varies with applied electrode potential. The in situ STM results show that there are no surface reconstruction processes at Bi(111) surface within the studied polarization range. It should be noted, however, that for other single crystal planes surface reconstruction process are observed by varying the applied potential value, being affected by both anion adsorption and water content. Acknowledgements This work was supported by the Estonian Ministry of Education and Research (projects no. IUT 20-13, PUT1033, PUT1107 and PSG 249) and Estonian Centres of Excellence project TK141.

Transport Mechanisms in Capillary Condensation of Water at a Single-Asperity Nanoscopic Contact

Transport mechanisms involved in capillary condensation of water menisci in nanoscopic gaps between hydrophilic surfaces are investigated theoretically and experimentally by atomic force microscopy (AFM) measurements of capillary force. The measurements showed an instantaneous formation of a water meniscus by coalescence of the water layers adsorbed on the AFM tip and sample surfaces, followed by a time evolution of meniscus toward a stationary state corresponding to thermodynamic equilibrium. This dynamics of the water meniscus is indicated by time evolution of the meniscus force, which increases with the contact time toward its equilibrium value. Two water transport mechanisms competing in this meniscus dynamics are considered: (1) Knudsen diffusion and condensation of water molecules in the nanoscopic gap and (2) adsorption of water molecules on the surface region around the contact and flow of the surface water toward the meniscus. For the case of very hydrophilic surfaces, the dominant role of surface water transportation on the meniscus dynamics is supported by the results of the AFM measurements of capillary force of water menisci formed at sliding tip-sample contacts. These measurements revealed that fast movement of the contact impedes on the formation of menisci at thermodynamic equilibrium because the flow of the surface water is too slow to reach the moving meniscus.

Humidity sensing properties of porous iron oxide/silica nanocomposite prepared via a formamide modified sol–gel process

Humidity sensing properties were investigated for a thermally stable, high surface area and porous composite material composed of iron oxide/silica (Fe:Si as 1:10 atomic ratio). The investigated composite material was formed by calcination for 3 h at 600 °C of the xerogel precursor, obtained via a formamide-modified sol–gel process. The produced composite material was characterized by a high surface area, S BET = 557 m 2 g −1, microporosity, S αs = 533 m 2 g −1, average pore size amounts to 2.1 nm, and average particle diameter amounts to 4.3 nm. Humidity sensing properties were measured by direct current (DC) electrical measurements and impedance spectroscopy (IS). The DC bulk conductivity and the relaxation frequency determined from the analysis of the IS spectra, showed similar dependences on RH, whereas the dielectric constant ( ɛ b) was close to that of a continuous water phase and was independent on RH for RH > 45%. The present results are discussed in terms of the established models proposed for humidity sensing by metal oxides due to monolayer, multilayer and condensation of water molecules on oxide surfaces. The efficient sensing properties observed for the investigated nanocomposite material were explained in terms of its nanoporous texture.

Entropy Changes in Aqueous Solutions of Non-polar Substances and in Bio-complex Formation

The entropy changes, ΔSapp, (i) for dissolution in water of non-polar substances and (ii) for protein-ligand complexation show linear dependences on the logarithm of the absolute temperature. For every compound, the slope m(S)=ΔCp for the line ΔSapp=f(ln T) depends on the size of the molecule and is exactly equal to the slope m(H)=ΔCp found in the diagram ΔHapp=f(T). This means that the slopes are rigorously proportional (with a ratio m(S)/nw=Cp,w) where nw is the number of involved water molecules as determined from the enthalpy change ΔHapp=f(T). It is also worth noting that the value of nw is positive (as well as m(S) and m(H)) in the dissolution of non-polar substances, whereas it is negative (as well as m(S) and m(H)) in bio-complex formation and in micelle formation. The number nw (nw>0) involved in the dissolution of non-polar substances depends on the size of the cavity (excluded volume) formed in the structure of water. These water molecules that have been excluded from the structure of the solvent absorb thermal energy that compensates for the negative enthalpy change, whereas the formation of the cavity implies there should be a large negative entropy contribution. The low solubility of non-polar substances in water depends on the highly negative entropy effect due both to cavity formation and to loss of configurational entropy by the gas trapped in a cage of water molecules. In processes involving association, as in micelle formation and in protein complexation, the cavities surrounding the separate units coalesce and the resultant cavity is smaller than the sum of the previous ones. The nw water molecules (nw<0) needed to fill the excess cavity return to the structure of the bulk solvent and release thermal energy, which compensates for the endothermic enthalpy. The affinity in the association processes is bound, for the most part, to the entropy produced by occupation of part of the cavity by condensation of water molecules. The association processes are therefore entropy driven.

K-1704 ソノルミネッセンシング・バブルのDSMC法による解析(J08-1 熱流体現象の原子・分子論的アプローチ(1))(J08 熱流体現象の原子・分子論的アプローチ)

To analyze a single sonoluminescing bubble captured in the standing wave, a computational method, which consists of the Direct Simulation Monte Carlo method for the mixture of argon and water molecules inside the bubble and the Yasui's equation for the boundary condition, is proposed, considering the evaporation and condensation of water molecules across the bubble wall as well as the thermal diffusion. The calculation results show agreements of the characteristic histories on the bubble radius and temperature inside the bubble with the experimental one, and the species segregation of mixture gas inside the bubble, which may be associated with the pressure gradient in the interior of bubble during rapid bubble collapse.

2809 DSMC 法による単一気泡の挙動の解析

In this study, a computational method, which consists of the Direct Simulation Monte Carlo method for the inside of the bubble and the well-known Keller's equation for the boundary of the bubble, is proposed for an analysis of a single spherical sonoluminescing bubble in water at molecular level. The present method includes the effect of evaporation and condensation of water molecules at the bubble wall which has never been considered in the previous method. Also air molecules inside the bubble are replaced with the mixture of uncondensable argon molecules and condensable water ones. The results show the improved characteristic histories of the bubble radius, temperature and number of molecules inside the bubble which agree qualitatively with the experimental ones.

Heat of adsorption of water on SrF2 : relation to two-dimensional condensation of water adsorbed on SrF2

The heat of adsorption and the adsorption isotherm of water on the SrF2 surface have been measured at 301 K by direct calorimetry to evaluate the energetic properties of the SrF2 surface and of water adsorbed on it. The differential heats of adsorption, qdiff, reveal an appreciable maximum in the coverage range 0.5–0.8, where a distinct step appears in the adsorption isotherm. These phenomena can be interpreted in terms of a two-dimensional condensation of water molecules resulting from their lateral interactions on the homogeneous solid surface. The behaviour of water molecules on the solid surface was characterized by entropies estimated from the adsorption isotherm and calorimetric data. It was thus revealed that water molecules are in the localized state on the SrF2 surface and their lateral interaction energy is ca. 7.3 kJ mol–1. On the other hand, the energy of interaction of water with the SrF2 surface is ca. 58 kJ mol–1, which is indicative of a strong interaction in this system.Water that cannot be desorbed by evacuation at 301 K is present on the SrF2 sample, and such water once removed by heat treatment at higher temperatures is restored by subsequent exposure of the sample to saturated water vapour. This rehydration energy (ca. 95 kJ mol–1) seems to be relatively small compared with that of usual metal oxides (ca. 150 kJ mol–1). From heat of adsorption and IR data, it has become apparent that the rate of rehydration of the SrF2 surface can be retarded by heat treatment at higher temperatures.

STM IMAGES OF NaCI MICROCRYSTAL IN THE AIR

The STM images of NaCl microcrystal at room temperature have been studied in the air. The results show that the ionic conductivity plays a crucial role in the image formation. The resolution of the microcrystals is restricted due to surface condensation of water molecules on the sample, and the phenomenon of "dragging effect" appears on the NaCl images. The effects of bias voltage and humidity to the tunneling process are also discussed.

A theory of the formation of ice crystals

As is known from laboratory experiments and investigations in the free atmosphere, ice crystal formation is initiated either at vapor saturation with respect to water or by crystallization of a droplet. Nuclei with special properties are needed. An explanation of their effectiveness can be given by application of the laws of oriented overgrowth (Epitaxy) between two crystals. Any host crystal having a plane with a geometrical pattern similar to, for instance, the base plane of the ice crystal, can orient an adsorbed layer of H2O molecules already in the ice crystal pattern. The closer the fit between the two crystal planes, the greater the area of oriented H2O molecules will be. By piling up other layers of H2O molecules a very small ice crystal, called “ice seed”, is created which can grow in supercooled water. This crystal is so small that according to theThomson-Gibbs Law a high supersaturation would be needed to create it directly from vapor. Calculations of the effective temperature of mica, AgI and quartz as freezing nuclei are in fairly good agreement with the experiment. This theory permits an explanation of the observed thresholds of ice formation which occur at about −20°, −30°, −40° and −60°C. Occasionally capillary condensation of water molecules on a freezing nucleus will occur when the vapor is saturated with respect to ice but not with respect to water; this has frequently been erroneously ascribed to ice formation by a real sublimation nucleus.