Properties Of Water Films Research Articles

Effects of key parameters on water film properties and temperature field distribution inside transpiring wall reactor

Transpiring wall reactor (TWR) can efficiently weaken corrosion and salt deposition problems during supercritical water oxidation. Its corrosion and salt deposition resistance are mainly affected by water film properties and temperature field distribution. Therefore, the effects of crucial parameters on water film properties and temperature field distribution are investigated via numerical simulation in this work. The results indicate that the upper supercritical oxidation zone is mainly affected by the second layer of transpiration water instead of the third layer. The second, third layer optimal injection locations are at 0.036 m, 0.060 m in this study, respectively. Excessively thick transpiring wall decreases water film coverage rate thereby damaging TWR, conversely makes supercritical zone shrink and thus weakening degradation ability. Annular gap primarily affects the temperature distribution near the transpiring wall. Under the conditions of the work, transpiring wall thickness of 0.002 m and annular gap of 0.004 m are recommended.

Effects of structural parameters on water film properties of transpiring wall reactor

AbstractCorrosion and salt deposition problems severely restrict the industrialization of supercritical water oxidation. Transpiring wall reactor can effectively weaken these two problems by a protective water film. In this work, methanol was selected as organic matter, and the influences of vital structural parameters on water film properties and organic matter removal were studied via numerical simulation. The results indicate that higher than 99% of methanol conversion could be obtained and hardly affected by transpiration water layer, transpiring wall porosity and inner diameter. Increasing layer and porosity reduced reactor center temperature, but inner diameter's influence was lower relatively. Water film temperature reduced but coverage rate raised as layer, porosity, and inner diameter increased. Notably, the whole reactor was in supercritical state and coverage rate was only approximately 85% in the case of one layer. Increasing reactor length affected slightly the volume of the upper supercritical zone but enlarged the subcritical zone.

Effect of operating parameters on water film properties of transpiring wall reactor

Transpiring wall reactor can effectively minimize reactor corrosion and salt deposition problems, which seriously hinder the commercialization of supercritical water oxidation. In this work, methanol was selected as organic matter, and the influences of key operating parameters on water film properties and organic matter removal were investigated mainly via numerical simulation. The results show that methanol conversion could be higher than 99% and hardly affected by various feed and transpiration water parameters. The highest value of reactor center temperature significantly increased by about 300 K as feed preheating temperature changed from 673 K to 823 K. Decreasing feed flow rate, feed concentration, feed preheating temperature and transpiration water temperature, and increasing transpiration intensity reduced water film temperature but raised coverage rate, which are beneficial to forming excellent water film with good corrosion and salt deposition resistance. These findings can provide significant reference for the optimization of reactor operating parameters.

Mechanism of heat transfer in heterogeneous droplets of water under intense radiant heating

Thermocouple measurements of temperature have been performed at three main points of heterogeneous water droplet–high-temperature gases system: on the surface and in the depth of a solid inclusion, as well as on the free surface of the water droplet. Investigations have been carried out for water droplets of an initial volume of 5–15 μl with single inclusions of cubic graphite particles of a typical size of 1 mm. The gas temperature varied from 700 K to 1200 K, which corresponds to the main practical applications: thermal purification of water from solid and liquid impurities, fire extinguishing, treatment of heat-loaded surfaces of power equipment, etc. A hypothesis about the dominant role of radiant heat transfer in vaporization within heterogeneous water droplets has been grounded. It has been shown that in a short period (a few seconds), the surface temperature of an opaque solid inclusion within a droplet can reach the boiling point of water. A significant change in the optical properties of water with increasing temperature has been revealed, i.e., water became partially transparent to the infrared radiation. Presence of an opaque heterogeneous inclusion enhances this effect due to intensification of the heating of the water film. The heat and mass transfer characteristics obtained in the experiments were used for designing a model that takes into account the radiative properties of water film and adequately reproduces the results of thermocouplemeasurements. Based on the findings of the investigations, a conclusion has been formulated that models of high-temperature evaporation of water droplets should be developed with due account of changes in the optical properties of water and formation of a vapor buffer layer around inclusions.

Diversity at the Water-Metal Interface: Metal, Water Thickness, and Confinement Effects.

The structure and properties of water films in contact with metal surfaces are crucial to understand the chemical and electrochemical processes involved in energy-related technologies. The nature of thin water films on Pd, Pt, and Ru has been investigated by first-principles molecular dynamics to assess how the chemistry at the water–metal surface is responsible for the diversity in the behavior of the water layers closer to the metal. The characteristics of liquid water: the radial distribution functions, coordination, and fragment speciation appear only for unconfined water layers of a minimum of 1.4 nm thick. In addition, the water layer is denser in the region closest to the metal for Pd and Pt, where seven- and five-membered ring motifs appear. These patterns are identical to those identified by scanning tunneling microscopy for isolated water bilayers. On Ru densification at the interface is not observed, water dissociates, and protons and hydroxyl groups are locked at the surface. Therefore, the acid–base properties in the area close to the metal are not perturbed, in agreement with experiments, and the bulk water resembles an electric double layer. Confinement affects water making it closer to ice for both structural and dynamic properties, thus being responsible for the higher viscosity experimentally found at the nanoscale. All these contributions modify the solvation of reactants and products at the water–metal interface and will affect the catalytic and electrocatalytic properties of the surface.

SOME REMARKS ON ATTACHMENT OF A GAS BUBBLE TO ANOTHER PHASE BOTH IMMERSED IN WATER

In this paper the importance of definition of hydrophobicity and aquaoleophilicity in terms of contact angle as well as the properties of water films in flotation and oil agglomeration were briefly presented. It was shown that the hysteresis of contact angle for a considered system depends on the way of measurement and geometry of the system due to the presence of other that excess pressure and capillary forces and buffering properties of the capillary force. It was suggested that, the measured advancing and receding contact angles should be, when possible, recalculated into the Young (rest, equilibrium) contact angle. It was discussed that quartz is not a good model of hydrophilic surface because its contact angle with a gas phase in water is not zero and that a spontaneous attachment between highly hydrophobic materials such as hydrocarbons and Teflon in dynamic system, such as flotation, does not occur.

Structure and properties of water film adsorbed on mica surfaces.

The structure profiles and physical properties of the adsorbed water film on a mica surface under conditions with different degrees of relative humidity are investigated by a surface force apparatus. The first layer of the adsorbed water film shows ice-like properties, including a lattice constant similar with ice crystal, a high bearing capacity that can support normal pressure as high as 4 MPa, a creep behavior under the action of even a small normal load, and a character of hydrogen bond. Adjacent to the first layer of the adsorbed water film, the water molecules in the outer layer are liquid-like that can flow freely under the action of external loads. Experimental results demonstrate that the adsorbed water layer makes the mica surface change from hydrophilic to weak hydrophobic. The weak hydrophobic surface may induce the latter adsorbed water molecules to form water islands on a mica sheet.

Spectroscopic Monitoring of the Acidity of Water Films on Ru(0001): Orientation‐Specific Acidity of Adsorbed Water

We examined the acid–base properties of water films adsorbed onto a Ru(0001) substrate by using surface spectroscopic methods in vacuum environments. Ammonia adsorption experiments combined with low-energy sputtering (LES), reactive ion scattering (RIS), reflection–absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) measurements showed that the adsorbed water is acidic enough to transfer protons to ammonia. Only the water molecules in an intact water monolayer and water clusters larger than the hexamer exhibit such acidity, whereas small clusters, a thick ice film or a partially dissociated water monolayer that contains OH, H2O and H species are not acidic. The observations indicate the orientation-specific acidity of adsorbed water. The acidity stems from water molecules with H-down adsorption geometry present in the monolayer. However, the dissociation of water into H and OH on the surface does not promote but rather suppresses the proton transfer to ammonia.

Influence of alkanethiol self-assembled monolayers with various tail groups on structural and dynamic properties of water films

Molecular dynamics simulations are performed to investigate the structural and dynamic properties of a water layer lying on a clean Au(111) surface and on alkanethiol self-assembled monolayers (SAMs) with three different tail groups: methyl, carboxyl, and hydroxyl. The effects of these functional groups on the local structure of the water are quantified by analyzing the reduced density profiles of the oxygen and hydrogen atoms, the average number of hydrogen bonds, and the distribution of the O-H bond angle, respectively. Meanwhile, the dynamic properties of the water layer are evaluated by analyzing the diffusion coefficients of the water molecules in the xy-plane and z-direction. The simulation results indicate that in both the hydrophobic and the hydrophilic alkanethiol SAMs, the formation of a two-layer water structure is suppressed. And the water molecules can approach the SAMs composed of hydroxyl tails most closely and SAMs composed of methyl tails furthest. Due to the existence of hydrogen bonds between water molecules and hydrophilic alkanethiol SAMs, the distribution of water molecules is more uniform than that in the hydrophobic interface. Meanwhile, the water-water hydrogen bond network weakens. Furthermore, the mobility of the water molecules in the hydrophilic interface is reduced more significantly than in the hydrophobic interface. The results developed in this study yield detailed insights into the microscopic interfacial phenomena.

Effect of surface physicochemical properties on the lubricating properties of water film

Effect of surface physicochemical properties on the water film confined within a nanogap was investigated. The film thickness and friction force were measured by the Relative Optical Interference Intensity (ROII) method and a UMT-2MT tribotester. It was found that the confined water film formed the thicker lubricate film than the prediction of elastic–isoviscous lubrication theory. Experimental results indicate that the higher the solid/water interfacial energy is, the thicker lubricate film the highly viscous “interphase” water layer forms and the lower the friction force is.

Rheological Properties of Water Films Nanoconfined in Parallel Au Plates by Molecular Dynamics Simulations

Rheological properties of water films nanoconfined in two parallel Au plates are investigated with the aid of molecular dynamics simulations. The density distribution, velocity profile, and diffusion coefficients of the water film in a Couette flow are studied. Shear viscosity and its dependence on the shear rate of the water film are also examined in the present research. It is found that the density of the water molecules near the plates is much higher than that in the other regions. This indicates that many water molecules are adsorbed by the plates and adsorbed layers are formed in the vicinity of the plates. The diffusion of the whole film increases dramatically as the shear rate becomes greater than 1010 s-1. The shear viscosity decreases as the shear rate increases, especially for the water film with a small thickness, which indicates the shear-thinning behavior for viscosity of the nanoconfined film. Moreover, an increase in shear viscosity with a decrease in the film thickness can also be found in the present study.

Anomalous properties of absorbed water films in layered silicates

The electrical properties of water films in residual electric fields on the surface of crystals and in slitlike capillaries of layered silicates are investigated. The results obtained indicate a substantial change in the water structure, degradation of the dipole-orientation polarization of molecules, and a decrease in the crystallization temperature.

Hydration of polar and nonpolar molecules at the surface of amorphous solid water

On the basis of time-of-flight secondary ion mass spectrometry, properties of amorphous solid water above the glass transition temperature $(136\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and the hydration of polar $(\mathrm{HCOOH},{\mathrm{C}}_{3}{\mathrm{H}}_{7}\mathrm{OH})$ and nonpolar $({\mathrm{C}}_{6}{\mathrm{H}}_{14},{\mathrm{C}}_{6}{\mathrm{F}}_{14})$ molecules on the ${\mathrm{D}}_{2}\mathrm{O}$-ice surface have been investigated. No evidence was obtained for the irreversible transition of the amorphous solid water into the crystalline phase: the self-diffusion of water molecules occurs above $140\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ irrespective of the preparation temperatures of the water-ice film ranging from $15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to $165\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, whereas the morphology of the film changes drastically at $165\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ due to the evolution of liquidlike water. It is also demonstrated that the change in conformation of the hydrated $\mathrm{HCOOH}$ molecule, as well as the occurrence of hydrophilic/hydrophobic hydration of the ${\mathrm{C}}_{3}{\mathrm{H}}_{7}\mathrm{OH}$ molecule, can be analyzed successfully from the temperature evolutions of the secondary-ion intensities. These polar molecules basically stay on the surface and tend to quench the morphological change of the water film due to the reduction of surface tension. The nonpolar ${\mathrm{C}}_{6}{\mathrm{H}}_{14}$ and ${\mathrm{C}}_{6}{\mathrm{F}}_{14}$ molecules readily dissolve in the ${\mathrm{D}}_{2}\mathrm{O}$ layer below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and dehydration of the incorporated molecules occurs at $165\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ concomitantly with the evolution of the liquidlike water. It is thus concluded that the hydrophobic hydration of nonpolar molecules is intimately related to the properties of water films.

Computer Simulation Study of a Water Film in a Narrow Unwetted Slit

Thin films of TIP4P water, confined between two ideal hard walls, have been simulated by the grand canonical Monte Carlo and molecular dynamics methods. Several water film properties, such as function of the slit width (equal to 2, 2.5, 3, and 6 molecular diameters, respectively), have been studied. On the basis of two isotherms at 300 and 600 K, the chemical potential of two-phase coexistence was found, thereby allowing an estimate of the Laplace and disjoining pressures in the capillaries, indicating a strong hydrophobic interaction involving the two walls. The values of the average isothermal compressibility of films are somewhat higher than those for a corresponding bulk liquid. Local characteristics, such as the density and the orientational distribution functions, the components of the pressure tensor, the electric field and electric potential, as well as the self-diffusion coefficients along the slit were calculated for all the studied systems.

Studies of the adsorbed water layers on solid surfaces by means of the thermal analysis special technique

The paper presents the thermogravimetric studies of water film properties on the solid surfaces using the programmed desorption under the quasi-isothermal conditions. The samples wetted in a different way before the measurements were heated under the suitable quasi-isothermal conditions of temperature increase in the derivatograph using a platinum labyrinth crucible in order to evaporate water films from the surface. During the measurements, the mass loss Q-TG and the differential Q-DTG curves were registered as a function of temperature and/or time which are characterized by the inflections showing individual stages of water desorption from the samples. On the basis of experimental data, the surface wetting mechanism and kinetics of the thermodesorption of water films, discontinuous properties of adsorbed water layers on porous surfaces, adsorption capacity of samples and nature of active sites (e.g. hydroxyl groups) have been presented. It was stated that this method could be successfully applied in the studies of adsorbed water films and surface wetting phenomena.

Thermoanalytical studies of water films on porous silicas at subambient and elevated temperatures

Physicochemical properties of water films formed on the surfaces of porous silicas were studied by high-resolution thermogravimetry (TGA) and differential scanning calorimetry (DSC) over a temperature range from −70°C to 250°C. Silica samples were exposed to water under different conditions in order to prepare surface films of variable thickness. It was shown that TGA and DSC measurements are useful for investigating the physicochemical properties of surface water films and the sorption properties of silica supports.

Studies of silica gel surface wetting phenomena by means of controlled-rate thermal analysis

The paper presents the controlled-rate thermal analysis studies of water film properties on silica gel samples using thermogravimetry and differential scanning calorimetry techniques. On the basis of experimental data the adsorption capacity of silica gel sample, concentration of hydroxyl groups, kinetic of desorption process of water films, surface wetting mechanism and behavior of water films in subambient and elevated temperatures have been investigated. It was stated that this method could be successfully applied in the studies of adsorbed water films and surface wetting phenomena.

Water film properties on mineral surfaces in flotation processes

This paper presents the properties of hydration layers on mineral surfaces and their importance in the enrichment by flotation processes. Water layers possessing different properties from those in bulk water may be spontaneously formed due to polar and dispersive interactions of the molecules on both hydrophobic and hydrophilic surfaces. The layers of the highest binding energy (per molecule) constitute an energy barrier against the approach of an air bubble, and hence they create a relatively low flotability of even hydrophobic minerals. It is stated that for good Notability the specific properties of vicinal water should be changed to achieve the properties of bulk water. This may be done by the addition of suitable components (collectors) which eliminate polar interactions and reduce the dispersive interactions to 25–35 mJm −2. The structure and thermodynamic properties of hydration layers formed on hydrophobized mineral surfaces become similar to those in bulk water, and they may be spontaneously disrupted. The work of destruction of such layers becomes comparable to the work of water cohesion. This results in a significant Notability of minerals.

Contactless determination of the properties of water films on roads

The properties of electromagnetic waves reflected by wet road surfaces have been investigated by computer modelling in the range of 0.5-10 GHz. The properties of the reflected wave depend on the thickness of the film and the concentration of salt in the water film. Reflection behaviour between 2 and 10 GHz is influenced mostly by the thickness of the film, whereas the reflection at frequencies below 1 GHz depends strongly on the salt concentration. Thus a dual frequency (e.g. 2.6 GHz and 0.5 GHz) reflectometer can be employed to detect simultaneously the thickness of a water film up to approximately 2 mm and its salt concentration up to 100 g l-1.

The properties of oxide and water films formed during the atmospheric exposure of iron and low alloy steels

The primary oxide and water films on iron and low alloy steels have been characterized using ellipsometry. Ellipsometry data obtained during stepwise water film formation are consistent with a model of two layers on iron where the water layer grows and the primary oxide layer remains unchanged in thickness. The thickness of water films on primary oxide layers falls within 0–4 nm and depends significantly on atmospheric conditions. The growth properties of water films are influenced by SO2. Stepwise drying studies give results which indicate modification of the primary oxide-water film interfaces.