Coarse-grained molecular dynamics simulations of capillary evaporation of water confined in hydrophilic mesopores

Non-equilibrium molecular dynamics (MD) simulations were performed to investigate the capillary evaporation of water confined in hydrophilic mesopores. The electrostatics-based (ELBA) coarse-grained water model was employed to calculate the duration of the time-consuming capillary evaporation process. To evaluate the effect of hydrophilicity of mesopores on the capillary evaporation of water, three types of thin films with a cylindrical mesopore were modelled by tuning the interactions between water and wall atoms. Initially, the cylindrical mesopore was filled with water, and evaporation of the water into vacuum was simulated. The calculation results showed that when capillary evaporation occurred, the desorption rate of water was almost constant in a highly hydrophilic mesopore where a stable water layer was formed on the pore surface, whereas the rate decreased with time in a weakly hydrophilic mesopore where the water layer did not remain stable. As time progressed, the water column shortened and then broke up. The number of water molecules in the mesopores decreased exponentially with time. The difference in the hydrophilicity of the mesopores resulted in different relaxation curves of water desorption from the mesopores.     

Confinement effect on thermodynamic and structural properties of water in hydrophilic mesoporous silica

The adsorption of water on porous silica surfaces at 300 K, has been qualitatively reproduced by Grand Canonical Monte Carlo simulations (GCMC) without any adjustment of adsorbate/substrate potential parameter. The simulated adsorption isotherm and isosteric differential enthalpy of adsorption compare well to experimental data for Vycor, showing the ability of the model in describing hydrophilic properties of silica surfaces. The analysis of fluid structure in the mesoporous glass gives detailed insights into confinement and disorder effects on water adsorbed on the hydrophilic surface of a porous glass. It is shown that hydrophilic properties are not simply related to surface hydroxyl density but are also related to local structure of the silica surface.     

Heats of interaction of hydrogen with gold and platinum powders and its effect on the subsequent adsorptions of oxygen and noble gases

Exposure of pure gold powders to hydrogen flow at 125 °C and atmospheric pressure causes heat evolution accompanied by hydrogen adsorption. The exposure takes place in a flow-through microcalorimeter, in which the metal powders are purged by nitrogen flow used as an inert carrier gas. The adsorbed hydrogen is slowly desorbed by nitrogen flow. The heats of hydrogen adsorption and its uptake by the gold powder are greatly increased by its sequential treatments with micromole quantities of oxygen and noble gases, such as helium and argon. This increase does not take place if the gold treatment is confined only to oxygen, or only to pure noble gases. The radically increased hydrogen adsorption by gold is caused by a combination of its treatments with oxygen and the noble gases. Similar results were obtained with pure platinum powder exposed to hydrogen at room temperatures. Gold powder containing adsorbed hydrogen reacts very strongly with molecular oxygen/argon mixtures, generating heats of adsorption several times higher than the heat of formation of water. The heat evolution is very rapid and is not accompanied by the formation of water. These intense interactions are not observed after complete desorption of hydrogen from the gold surfaces.     

The adsorption of argon on ZnO at 77 K

We have studied the adsorption of argon onto ZnO surfaces at 77 K by means of quasi-equilibrium adsorption volumetry coupled with high resolution microcalorimetry and Grand Canonical Monte-Carlo (GCMC) simulations. The adsorbate/surface adsorption potential function (PN type) used in the simulations, was determined on the basis of ab initio calculations (corrected for dispersion interactions). The first aspect of this work was to test the ability of a standard solid-state Hartree—Fock technique coupled with a perturbative semi-empirical approach in deriving a reliable adsorption potential function. The dispersion part of the adsorbate/surface interatomic potential was derived by using perturbation theory-based equations while the repulsive and induction interactions were derived from periodic Hartree—Fock (CRYSTAL92) calculations. GCMC simulations based on this adsorption potential allow one to calculate adsorption isotherms and isosteric heat versus loading curves as well as singlet distribution functions at 77 K for each type of ZnO (neutral and polar) faces. The combined analysis of the simulation data for all surfaces gives a good insight of the adsorption mechanism of argon onto ZnO surfaces at 77 K in agreement with experiment. As far as neutral surfaces are concerned, it is shown that adsorption first takes place within the ‘troughs’ which cover ZnO neutral surfaces. At low chemical potentials, these semi-channels are preferential adsorption sites in which we could detect a nearly one-dimensional adsorbate freezing in a commensurate phase at 77 K. The polar O faces are the most favourable surfaces for adsorption at higher chemical potentials.     

Water confined in mesoporous silica glasses: Influence of temperature on adsorption/desorption hysteresis loop and fluid structure

In natural as well as industrial processes, water is frequently confined in silica porous materials with pore sizes in the nanometer scale. Understanding the confinement effects on the fluid properties is a fundamental issue, helpful to optimize the industrial processes. The molecular simulation is a powerful tool to study complex polar fluid like water at the atomic scale. The water adsorption/desorption properties in a mesoporous silica glass are investigated by means of Grand Canonical Monte Carlo simulations (GCMC). The SPC and PN TrAZ potential are used to describe water-water and water-silica interactions. The numerical sample of mesoporous silica glass (pore size: 3.6nm) was obtained by off-lattice reconstruction, known to reproduce in a realistic way the geometrical complexity of high specific surface Vycor (pore size distribution, pore interconnections, etc). The intermolecular potential is shown to reproduce the experimental data at 300K (adsorption isotherm and isosteric heat of adsorption). The water structure is analyzed and confinement effects are emphasized. The temperature influence is studied: the hysteresis loop is shown to shrink with an increase in temperature.     

Influence of framework silica-to-alumina ratio on the water adsorption and desorption characteristics of MHI-CaX/CaY zeolite

The influence of the framework SiO₂/Al₂O₃ ratio from 2.0 to 10.0 of commercial faujasite-type CaX/CaY zeolite produced by Mitsubishi Heavy Industries Ltd. (MHI) on the water adsorption and desorption characteristics was investigated. Not only the change in electronegativity of the zeolite but the change in pore-size distribution of the zeolite affects the water adsorption and desorption characteristics of the zeolite. We found great differences in isotherms of water between CaY7.0 (SiO₂/Al₂O₃=7.0) and CaY10.0. The differences are mainly caused by the considerable change in pore-size distribution. A step-wise variation was observed in the desorption isotherm of water from CaY10.0 at approximately P/P₀=0.4. This is due to the pore distribution of CaY10.0 being relatively poor in smaller micropores in zeolite structure, since a similar phenomenon is observed in the case of argon adsorption on CaY10.0. In the experiments using a fixed bed, an apparent dependency of HTO dehydration ratio on the flow rate of the purge gas is measured with the CaY10.0 zeolite, while the water desorption from other tested CaX/CaY zeolite is independent of the flow rate of helium purge gas. This indicates that the transfer step of water diffusion through a laminar film appeared as one of the rate-controlling steps in the water desorption from CaY10.0.     

Capillary condensation in a disordered mesoporous medium: a grand canonical Monte Carlo study

The adsorption of rare gases in a disordered mesoporous silica glass has been studied by means of grand canonical Monte Carlo (GCMC) simulation. A series of porous samples has been obtained by using an off-lattice 3D reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered porous materials such as Vycor. The off-lattice functional of 115m²g^?1 Vycor is applied to a simulation box containing silicon and oxygen atoms of cubic cristoballite with a homothetic reduction in order to obtain porous samples with mean pore size around 35 Å and specific surface around 220 m² g^?1. A realistic surface chemistry is then obtained by saturating all the dangling oxygen bonds with hydrogen. Some topological properties of the different 3D reconstructions of the Vycor-like material are analysed using chord length distributions and small angle scattering data. The GCMC Ar, Kr and Xe adsorption/desorption isotherms are calculated at different temperatures. At sufficiently low temperatures, they exhibit a capillary condensation transition with an adsorption branch having a finite slope accompanied by a hysteresis loop upon desorption. It has been shown on a set of simulated argon isotherms, that evolution with temperature of the GCMC results is similar to experiment. At the temperature at which the hysteresis loop disappears, it was found that the compressibility of the dense liquid-like phase at the maximum of the so-called hysteretic coexistence curves increases significantly. In the low pressure and temperature domain, different adsorption scenarios can be interpreted on the basis of a Zisman-type of criterion for wetting. The BET surface area is shown to be strongly related to this criterion. At higher pressure, it was found that the pore size distribution obtained by using the standard BJH analysis applied to both simulated adsorption and desorption data qualitatively reproduces the main features of the chord length distribution.     

Methods for calculating the desorption rate of an isolated molecule from a surface: Water on MgO(0 0 1)

We present two types of Molecular Dynamics (MD) simulation for calculating the desorption rate of molecules from a surface. In the first, the molecules move freely between two surfaces, and the desorption rate is obtained either by counting the number of desorption events in a given time, or by looking at the average density of the molecules as a function of distance from the surface and then applying transition state theory (TST). In the second, the potential of mean force (PMF) for a molecule is determined as a function of distance from the surface and the desorption rate is obtained by means of TST. The methods are applied to water on the MgO(0 0 1) surface at low coverage. Classical potentials are used so that long simulations can be performed, to minimise statistical errors. The two sets of MD simulations agree well at high temperatures. The PMF method reproduces the 0 K adsorption energy of the molecule to within 5 meV, and finds that the well depth of the PMF is not linear with temperature. This implies the prefactor frequency f in the Polanyi-Wigner equation is a function of temperature, increasing at lower temperatures due to the reduction of the available configuration space associated with an adsorbed molecule compared with a free molecule.     

Confinement effect on thermodynamic and structural properties of water in hydrophilic mesoporous silica

The European Physical Journal E - The adsorption of water on porous silica surfaces at 300 K, has been qualitatively reproduced by Grand Canonical Monte Carlo simulations (GCMC) without any...     

外加阴离子吸附剂对复合污染黄土淋洗过程养分淋失的减缓机制(Ⅱ)

化学淋洗法是常用的污染场地修复方法,其经济性、简便性、安全性等指标较其他方法有较大优势。化学淋洗法存在的弊端之一是淋洗过程土壤伴存养分的同步淋失,若能在此瓶颈问题上有所突破,将会大大促进淋洗法场地修复的应用进程。本文研究的目标在于揭示合成秸秆基阴离子吸附剂对养分淋失的减缓机制,采用吸附平衡模式(等温线和动力学)与光谱分析技术(SEM和FT-IR)相结合,力图从微观层面验证宏观研究结果。Langmuir和准二级动力学方程能更好地描述硝酸根和磷酸根的吸附行为,由Langmuir方程计算得出q_m分别为7.507 5和4.194 6 mg·g-1,吸附反应为单层吸附和优惠吸附过程,主要控速步骤为化学吸附。阴离子吸附剂对硝酸根和磷酸根的吸附活化能(298 K)E_a分别为42.25和39.38 kJ·mol-1,吸附反应自发、吸热。纯净水和KOH能够实现阴离子吸附剂的再生,超声辅助和延长解吸时间对于解吸效果略有促进作用。吸附后阴离子吸附剂表面被点状分布的细微颗粒物覆盖,解吸后仅有少量枝状物零星分布。吸附前后FTIR图谱的主要波峰红移或蓝移,证实阴离子吸附剂表面及内部官能团通过静电作用与硝酸根和磷酸根结合。阴离子吸附剂对硝酸根和磷酸根的吸持固定是物理吸附和化学吸附共同作用的结果。     

Kinetic Monte Carlo simulations of electrodeposition: Crossover from continuous to instantaneous homogeneous nucleation within Avrami’s law

The influence of lateral adsorbate diffusion on the dynamics of the first-order phase transition in a two-dimensional Ising lattice gas with attractive nearest-neighbor interactions is investigated by means of kinetic Monte Carlo simulations. For example, electrochemical underpotential deposition proceeds by this mechanism. One major difference from adsorption in vacuum surface science is that under control of the electrode potential and in the absence of mass-transport limitations, local adsorption equilibrium is approximately established. We analyze our results using the theory of Kolmogorov, Johnson and Mehl, and Avrami (KJMA), which we extend to an exponentially decaying nucleation rate. Such a decay may occur due to a suppression of nucleation around existing clusters in the presence of lateral adsorbate diffusion. Correlation functions prove the existence of such exclusion zones. By comparison with microscopic results for the nucleation rate I and the interface velocity of the growing clusters v, we can show that the KJMA theory yields the correct order of magnitude for Iv². This is true even though the spatial correlations mediated by diffusion are neglected. The decaying nucleation rate causes a gradual crossover from continuous to instantaneous nucleation, which is complete when the decay of the nucleation rate is very fast on the time scale of the phase transformation. Hence, instantaneous nucleation can be homogeneous, producing negative minima in the two-point correlation functions. We also present in this paper an n-fold way Monte Carlo algorithm for a square lattice gas with adsorption/desorption and lateral diffusion.     

Field desorption of common gases from iridium and tungsten

The technique of field desorption has been used to study adsorption and desorption of gases from field ion tips. The formal procedure of the experiments is quite similar to thermal flash desorption. Values for the desorption field and their change with coverage have been obtained. For H₂/W a field adsorbed state could be detected. Time-of-flight analysis was applied to determine the desorbing species. The gas supply for field ion tips could be measured; it increases exponentially with the field.     

Heterogeneity of activated carbons in adsorption of phenols from aqueous solutions—Comparison of experimental isotherm data and simulation predictions

Surface heterogeneity of activated carbons is usually characterized by adsorption energy distribution (AED) functions which can be estimated from the experimental adsorption isotherms by inverting integral equation. The experimental data of phenol adsorption from aqueous solution on activated carbons prepared from polyacrylonitrile (PAN) and polyethylene terephthalate (PET) have been taken from literature. AED functions for phenol adsorption, generated by application of regularization method have been verified. The Grand Canonical Monte Carlo (GCMC) simulation technique has been used as verification tool. The definitive stage of verification was comparison of experimental adsorption data and those obtained by utilization GCMC simulations. Necessary information for performing of simulations has been provided by parameters of AED functions calculated by regularization method.     

Phase coexistence and dynamic properties of water in nanopores

The dynamical properties of a confined fluid depend strongly on the (spatially varying) density. Its knowledge is therefore an important prerequisite for molecular-dynamics (MD) simulations and the analysis of experimental data. In a mixed Gibbs ensemble Monte Carlo (GEMC)/MD simulation approach we first apply the GEMC method to find possible phase states of water in hydrophilic and hydrophobic nanopores. The obtained phase diagrams evidence that a two-phase state is the most probable state of a fluid in incompletely filled pores in a wide range of temperature and level of pore filling. Pronounced variations of the average and local densities are observed. Subsequently, we apply constant-volume MD simulations to obtain water diffusion coefficients and to study their spatial variation along the pore radius. In general, water diffusivity slightly decreases in a hydrophilic pore and noticeably increases in a hydrophobic pore (up to about 40% with respect to the bulk value). In the range of gradual density variations the local diffusivity essentially follows the inverse density and the water binding energy. The diffusivity in the quasi-two-dimensional water layers near the hydrophilic wall decreases by 10 to 20% with respect to the bulk value. The average diffusivity of water in incompletely filled pore is discussed on the basis of the water diffusivities in the coexisting phases.Received: 1 January 2003, Published online: 14 October 2003PACS: 61.20.Ja Computer simulation of liquid structure - 64.70.Fx Liquid-vapor transitions     

Liquid-phase adsorption and desorption of phenol onto activated carbons with ultrasound

The effect of 48-kHz ultrasound on the adsorption and desorption of phenol from aqueous solutions onto coconut shell-based granular activated carbons was studied at 25 degrees C. Experiments were performed at different carbon particle sizes (1.15, 2.5, 4.0 mm), initial phenol concentrations (1.06-10.6 mol/m3), and ultrasonic powers (46-133 W). Regardless of the absence and presence of ultrasound, the adsorption isotherms were well obeyed by the Langmuir equation. When ultrasound was applied in the whole adsorption process, the adsorption capacity decreased but the Langmuir constant increased with increasing ultrasonic power. According to the analysis of kinetic data by the Elovich equation, it was shown that the initial rate of adsorption was enhanced after sonication and the number of sites available for adsorption was reduced. The effect of ultrasonic intensity on the initial rate and final amount of desorption of phenol from the loaded carbons using 0.1 mol/dm3 of NaOH were also evaluated and compared.     

Water vapor desorption and adsorbent regeneration for air conditioning unit using pulsed corona plasma

The dehumidifying action of a dehumidifier or air conditioner was employed to achieve a comfortable and desirable indoor comfort. Water vapor adsorbed on an adsorbent substance needs to be regenerated when the water vapor exceeds the adsorption capacity. The conventional process for adsorbent regeneration or moisture desorption uses the heat by means of the electric heater. In the present study, the water vapor desorption from the adsorbent material was investigated using a pulsed corona plasma for possible replacement of the electric heater. The water vapor desorption for a given power (defined as desorption efficiency) using the pulsed corona plasma was found to be superior over conventional thermal desorption. The gradient of desorption (desorption rate) was found to be significantly higher for plasma desorption, so that faster desorption can be achieved without excessive gas heating. In addition, the plasma desorption was not affected by the initial moisture content in an indoor environment, which leads to more economical, controllable, and flexible air conditioning system.     

Studies of adsorption kinetics by means of the stochastic numerical simulation

This paper surveys the methods used in the theoretical studies of kinetics with a special emphasis on the works dealing with the stochastic methods and their application in studies of adsorption kinetics. One of the stochastic methods — Monte Carlo numerical simulation of the stochastic time evolution — is mainly discussed. Numerous studies show that this method, introduced by Gillespie [J. Comput. Phys. 22 (1976) 403], is very useful to investigate the adsorption kinetics. The systematic studies of adsorption kinetics of single gases and gas mixtures on solid surfaces are presented. The kinetic adsorption isotherms, involving the lateral interactions between molecules in the surface phase, energetic heterogeneity of the adsorbent surface and surface diffusion, are numerically simulated by means of the numerical program, which is presented in the appendix. These simulations show influence of the adsorbent heterogeneity, lateral interactions and surface diffusion on the adsorption kinetics.     

The isothermal kinetics of hydrogen adsorption onto iron films observed with the chemisorption-induced resistance change

A study has been made of the isothermal kinetics for hydrogen chemisorption onto evaporated thin iron films. The measurements were made over a temperature range from about 293 to 460 K and for hydrogen pressures up to about $\text{10}{}^{\mathrm{?2}}\text{N}\text{m}{}^2\text{(8 × 10}{}^{\mathrm{?5}}\text{Torr}\text{)}$. The chemisorption kinetics were measured using the chemisorption-induced resistance change to monitor the coverage of the chemisorbed hydrogen as it responded to changes in the gas phase pressure. Data from the measurements (adsorption kinetics, desorption kinetics, pressure-jump relaxation kinetics, and equilibrium isotherms) were analyzed in terms of a simple chemical reaction kinetic model which assumes that dissociative chemisorption occurs via an adsorbed molecular state. The absolute rate constants of the model obtained in this analysis were found to be independent of coverage for estimated coverages less than 0.8 monolayers. Based on the model, the desorption process in this temperature range is rate limited by desorption from the adsorbed molecular state to the gas phase.     

Sorption and permeation of gaseous molecules in amorphous and crystalline PPX C membranes: molecular dynamics and grand canonical Monte Carlo simulation studies

Amorphous and crystalline poly (chloro-p-xylylene) (PPX C) membranes are constructed by using a novel computational technique, that is, a combined method of NVT+NPT-molecular dynamics (MD) and gradually reducing the size (GRS) methods. The related free volumes are defined as homology clusters. Then the sorption and the permeation of gases in PPX C polymers are studied using grand canonical Monte Carlo (GCMC) and NVT-MD methods. The results show that the crystalline PPX C membranes provide smaller free volumes for absorbing or transferring gases relative to the amorphous PPX C area. The gas sorption in PPX C membranes mainly belongs to the physical one, and H bonds can appear obviously in the amorphous area. By cluster analyzing on the mean square displacement of gases, we find that gases walk along the x axis in the crystalline area and walk randomly in the amorphous area. The calculated permeability coefficients are close to the experimental data.