Equilibrium fluctuations in the theory of surface processes on microparticles

The question of the role of equilibrium fluctuations in the adsorption theory and kinetics of surface processes occurring on the particles of the nanometer size range is discussed. Differences are put forward that need to be introduced to the fluctuation theory of surface processes on microparticles and that generalize Hill’s approach to describing the thermodynamic properties of small systems. We show the importance of allowing for the discrete character of adsorption centers on the surfaces and their heterogeneity when describing adsorption isotherms and the rates of adsorption processes.     

Use of specific surface areas in inverse gas chromatography studies at zero surface coverage

Inverse gas chromatography (IGC) is frequently used to study adsorption processes at zero surface coverage on microporous activated carbons. This allows to determine the thermodynamic adsorption parameters as equilibrium constants, V(S), standard enthalpies of adsorption, Delta HA degrees, standard free energy of adsorption, Delta GA degrees, and so on. Nevertheless, the surface areas of the adsorbents (microporous carbons in this case) are needed for this purpose. The experimental determination of the surface areas of microporous solids is not univocal and the results depend on the adsorbate employed in the measurements, usually N2 or CO2. This means that the thermodynamic parameters obtained by IGC are subjected to a degree of uncertainty depending on whether N2 or CO2 is used to determine the surface area values. The aim of this paper is to discuss which of the two surface area values is more appropriate to be used in IGC measurements at zero surface coverage. Experimental and theoretical considerations are supplied in a thorough discussion which supports that CO2 surface area value is more appropriate. Thus, it is proposed that this should be used instead of the more generally extended nitrogen specific surface area obtained by the BET equation.     

亚稳平衡吸附态与亚稳平衡系数关系的热力学研究

在相同温度、压力、pH值和离子强度等条件下,同一种金属离子在无机矿物表面上发生的吸附反应可以形成能量不同的吸附状态(如Zn可以边-边和角-角两种方式吸附在水锰矿表面上),因而反应终了时吸附质可处于不同的亚稳平衡吸附态(MEA).应用MEA理论,针对吸附反应A+H₂O=xA₁+yA₂+H₂O,推导出实际吸附反应“平衡常数”(Kreal)与亚稳平衡系数(Kme)的热力学关系式,从理论上分析了吸附反应MEA状态的变化对实际吸附反应平衡性质的影响.分析结果表明,Kme值与两种MEA状态之间的能量差呈指数关系,通过影响吸附反应的MEA状态(包括能量和组成)的反应动力学因素可影响吸附平衡常数和吸附等温式(线)等吸附热力学性质.     

平衡时溶液的表面吸附

溶液的表面吸附仍是表面热力学当中的一个具有挑战性的问题.在本文中我们定义了一个新的热力学态函数,表面吸附的平衡条件是这个态函数的微分为零.基于这个条件,我们推导了描述平衡时表面吸附的新方程.在推导过程中没有采用假想分界面.新的表面吸附方程和Gibbs表面吸附方程完全不一样.还通过分子动力学方法模拟了氯化钠溶液,模拟结果和我们的理论预测符合较好.     

Reactivity of V2O3(0001) surfaces: molecular vs dissociative adsorption of water

The adsorption of water on V2O3(0001) surfaces has been investigated by thermal desorption spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy with use of synchrotron radiation. The V2O3(0001) surfaces have been generated in epitaxial thin film form on a Rh(111) substrate with three different surface terminations according to the particular preparation conditions. The stable surface in thermodynamic equilibrium with the bulk is formed by a vanadyl (VO) (1x1) surface layer, but an oxygen-rich (radical3xradical3)R30 degrees reconstruction can be prepared under a higher chemical potential of oxygen (microO), whereas a V-terminated surface consisting of a vanadium surface layer requires a low microO, which can be achieved experimentally by the deposition of V atoms onto the (1x1) VO surface. The latter two surfaces have been used to model, in a controlled way, oxygen and vanadium containing defect centres on V2O3. On the (1x1) V=O and (radical3xradical3)R30 degrees surfaces, which expose only oxygen surface sites, the experimental results indicate consistently that the molecular adsorption of water provides the predominant adsorption channel. In contrast, on the V-terminated (1/radical3x1/radical3)R30 degrees surface the dissociation of water and the formation of surface hydroxyl species at 100 K is readily observed. Besides the dissociative adsorption a molecular adsorption channel exists also on the V-terminated V2O3(0001) surface, so that the water monolayer consists of both OH and molecular H2O species. The V surface layer on V2O3 is very reactive and is reoxidised by adsorbed water at 250 K, yielding surface vanadyl species. The results of this study indicate that V surface centres are necessary for the dissociation of water on V2O3 surfaces.     

Properties of model atomic free-standing thin films

We present a computational study of the thermodynamic, dynamic, and structural properties of free-standing thin films, investigated via molecular dynamics simulation of a glass-forming binary Lennard-Jones mixture. An energy landscape analysis is also performed to study glassy states. At equilibrium, species segregation occurs, with the smaller minority component preferentially excluded from the surface. The film's interior density and interface width depend solely on temperature and not the initialization density. The atoms at the surface of the film have a higher lateral diffusivity when compared to the interior. The average difference between the equilibrium and inherent structure energies assigned to individual particles, as a function of the distance from the center of the film, increases near the surface. A minimum of this difference occurs in the region just under the liquid-vapor interface. This suggests that the surface atoms are able to sample the underlying energy landscape more effectively than those in the interior, and we suggest a possible relationship of this observation to the recently reported formation of stable glasses by vapor phase deposition.     

Nanoparticle adsorption at liquid-vapor surfaces: influence of nanoparticle thermodynamics, wettability, and line tension

We developed a statistical mechanical theory that describes the adsorption of nanoparticles (NPs) at liquid-vapor surfaces. This theory accounts for the surface to bulk NP thermodynamic equilibrium, as well as the NP mechanical equilibrium, wettability, and line tension at liquid-vapor surfaces. The theory is tested by examining the adsorption of 5 nm diameter dodecanethiol-ligated gold NPs at the liquid-vapor surface of a homologous series of n-alkane solvents, from n-nonane to n-octadecane, where the NP wettability decreases with an increasing n-alkane chain length.     

Water adsorption and desorption pretreatment of surfaces increases the maximum amount of adsorbed water molecules by a multiple

The amount of adsorbed water on surfaces in an atmosphere with 100 % relative humidity can be increased by a multiple, if the surfaces are pretreated by cycles of adsorption and desorption of water. This was observed on surfaces of diamond, titanium dioxide and silicon dioxide at temperatures around 22 °C. With a sufficient number of such cycles a faster and stronger adsorption of water molecules was obtained, if compared with untreated surfaces. This also means an increased energy transfer from the atmosphere to the surface. Due to the pretreatment the amount of adsorbed water was more than three times increased. The observed effect is explained by small amounts of specially arranged water molecules, which remain on the surface after the desorption process and which support the adsorption of water. The observed effect can be used to moisten surfaces of small particles very efficiently from the gas phase.     

Adsorption of heterogeneously charged nanoparticles on a variably charged surface by the extended surface complexation approach: charge regulation, chemical heterogeneity, and surface complexation

Adsorption of randomly branched polyelectrolytes, "hairy" particles and internally structured macromolecules, collectively denoted as heterogeneously charged nanoparticles, on charged surfaces is important in many technological and natural processes. In this paper, we will focus on (1) the charge regulation of both the nanoparticle and the surface and (2) the surface complexation between the particle functional groups and the surface sites and will theoretically study the adsorption using the extended surface complexation approach. The model explicitly considers the electrochemical potential of a nanoparticle with an average (smeared-out) structure and charge both in bulk solution and on the surface to obtain the equilibrium adsorption. The chemical heterogeneity of the particle is described by a distribution of the protonation constant. Detailed analysis of the chemical potential of the adsorbed nanoparticle reveals that the pH and salt dependence of the adsorption can be largely explained by the balance between an energy gain resulting from the particle and surface charge regulation and the surface complexation and an energy loss from the unfavorable interparticle electrostatic repulsion close to the surface. This conclusion is also supported by the strong impacts that the chemical heterogeneity of the particle functional groups, the magnitude of the surface complexation, the number of the functional groups, and the size of the particle have on the adsorption.     

Dynamic Interfacial Behavior of Poly(oxyethylene) Lauryl Ether Based Surfactant Mixtures

The adsorption behavior of binary mixtures comprising nonionic surfactants at the air–water interface has been studied by bubble pressure tensiometry at concentrations above and below their critical micelle concentrations. Surfactants with the same hydrocarbon chains but different degree of ethoxylations were chosen as the components to understand their mixing behavior at equilibrium and dynamic conditions. At short times, the adsorption is found to be diffusion limited for individual components as well as for the mixtures, as predicted by the Ward and Tordai model. The effective diffusion coefficient of the monomers in the mixed state displays a dynamic synergism, consistent with the molecular thermodynamic model for dynamic surface tension. However, the equilibrium surface tension and micellar diffusion coefficient of the mixtures exhibit ideal behavior.     

Collisions of ideal gas molecules with a rough/fractal surface. A computational study

The frequency of collisions of ideal gas molecules (argon) with a rough surface has been studied. The rough/fractal surface was created using random deposition technique. By applying various depositions, the roughness of the surface was controlled and, as a measure of the irregularity, the fractal dimensions of the surfaces were determined. The surfaces were next immersed in argon (under pressures 2 x 10(3) to 2 x 10(5) Pa) and the numbers of collisions with these surfaces were counted. The calculations were carried out using a simplified molecular dynamics simulation technique (only hard core repulsions were assumed). As a result, it was stated that the frequency of collisions is a linear function of pressure for all fractal dimensions studied (D = 2, ..., 2.5). The frequency per unit pressure is quite complex function of the fractal dimension; however, the changes of that frequency with the fractal dimension are not strong. It was found that the frequency of collisions is controlled by the number of weakly folded sites on the surfaces and there is some mapping between the shape of adsorption energy distribution functions and this number of weakly folded sites. The results for the rough/fractal surfaces were compared with the prediction given by the Langmuir-Hertz equation (valid for smooth surface), generally the departure from the Langmuir-Hertz equation is not higher than 48% for the studied systems (i.e. for the surfaces created using the random deposition technique).     

Displacement of hexanol by the hexanoic acid overoxidation product in alcohol oxidation on a model supported palladium nanoparticle catalyst

This work characterizes the adsorption, structure, and binding mechanism of oxygenated organic species from cyclohexane solution at the liquid/solid interface of optically flat alumina-supported palladium nanoparticle surfaces prepared by atomic layer deposition (ALD). The surface-specific nonlinear optical vibrational spectroscopy, sum-frequency generation (SFG), was used as a probe for adsorption and interfacial molecular structure. 1-Hexanoic acid is an overoxidation product and possible catalyst poison for the aerobic heterogeneous oxidation of 1-hexanol at the liquid/solid interface of Pd/Al(2)O(3) catalysts. Single component and competitive adsorption experiments show that 1-hexanoic acid adsorbs to both ALD-prepared alumina surfaces and alumina surfaces with palladium nanoparticles, that were also prepared by ALD, more strongly than does 1-hexanol. Furthermore, 1-hexanoic acid adsorbs with conformational order on ALD-prepared alumina surfaces, but on surfaces with palladium particles the adsorbates exhibit relative disorder at low surface coverage and become more ordered, on average, at higher surface coverage. Although significant differences in binding constant were not observed between surfaces with and without palladium nanoparticles, the palladium particles play an apparent role in controlling adsorbate structures. The disordered adsorption of 1-hexanoic acid most likely occurs on the alumina support, and probably results from modification of binding sites on the alumina, adjacent to the particles. In addition to providing insight on the possibility of catalyst poisoning by the overoxidation product and characterizing changes in its structure that result in only small adsorption energy changes, this work represents a step toward using surface science techniques that bridge the complexity gap between fundamental studies and realistic catalyst models.     

How to distinguish energetic surface heterogeneity from electrostatic interactions in the case of hydrogen ion adsorption from solution onto oxides

It was shown that the adsorption of uncharged particles onto energetically heterogeneous surfaces with Gaussian-like distribution of adsorption energy can be described by the same adsorption isotherm as the adsorption of hydrogen ions from solution onto homogeneous oxide surfaces with surface potentials given by a quasi-Nernst formula. It exemplifies that the separation of electrostatic from energetic factors in the case of ion adsorption onto heterogeneous oxide surfaces is very difficult. Nevertheless, application of calorimetric data for proton adsorption makes it possible to estimate both factors.     

Controlling deposition and release of polyol-stabilized latex on boronic acid-derivatized cellulose

Poly(glycerol monomethacrylate)-stabilized polystyrene (PGMA-PS) latex particles undergo specific, pH-dependent adsorption onto regenerated cellulose film bearing surface phenylboronic acid groups (cellulose-PBA). Deposition occurs at pH 10 and is driven by the boronate ester formation with the polyol latex surface coating. In contrast, no deposition occurs at pH 4, and previously deposited particles can be readily desorbed at this lower pH. In control experiments, conventional anionic sulfate-stabilized polystyrene latex did not deposit onto the hydrophilic cellulose surface. The distribution of boronate groups in the cellulose was determined by exposure to Alizarin Red S dye, which forms a fluorescent complex with phenylboronic acid; confocal microscopy was used to determine a surface density of 3 nm(2) per boronic acid group on the cellulose surface. Although the boronic acid binding constant with PGMA is relatively low (5.4 L/mol), the cooperative interactions between multiple PBA surface sites and the many PGMA chains per latex particle are sufficient to induce specific latex adsorption, providing a convenient new tool for controlling nanoparticle deposition on surfaces.     

Statistical modeling of equilibrium adsorption of non-ideal mixtures on zeolites

A new model for equilibrium adsorption of a binary mixture on zeolites that takes into account the energy nonuniformity of the adsorption field in the zeolite cavities was developed on the basis of statistical thermodynamics. The nonuniformity of the adsorption field produces rearrangement of molecules in the cavity volume, decreasing the entropy, internal energy, and Helmholz free energy. A procedure for calculation of the thermodynamic functions from the data on the adsorption of pure components was proposed. The limiting cases of maximum ordering of the molecules in the cavity and their random distribution were considered. The approach proposed was exemplified by the substantially non-ideal system nitrogen--argon--zeolite NaX at 160 K. The proposed model describes the behavior of this mixture much better than that of the ideal adsorbed solution theory.     

On numerical classification of solution adsorption isotherms

To numerically classify solution adsorption isotherms, a difference or deviation measure, DS(mc) (the relative difference between two sums of the adsorption maximum's characteristics of selectivity isotherms x 100), is derived. The measure is applicable to completely miscible binary solutions on solids. This quantity evaluates the difference between an adsorption system and the ideal adsorption system (ideal adsorbed and bulk phases, homogeneous surface, and equal molar area solution components) at the point of maximum adsorption. For model systems, DS(mc)s are calculated at several levels of surface heterogeneity (Gaussian distribution of surface energy) and for different signs of phase nonideality (regular solution phases) on a homogeneous surface and on a simple two-site-type heterogeneous surface. All heterogeneous surfaces have negative DS(mc) values, but nonideal phases have DS(mc)s with signs opposite to the sign of deviation from Raoult's law. DS(mc)s from both U- and S-shape isotherms are reported for 16 experimental systems consisting of hydrocarbon mixtures and both alcohol + hydrocarbon and alcohol + water solutions or acetone + carbon tetrachloride on several silica gels and a variety of carbons.     

A qualitative approach to adsorption mechanism identification on microporous carbonaceous surfaces

The paper presents results of research on identification of localized and other adsorption mechanisms, on geometrically heterogeneous graphite-like carbonaceous surfaces. It attempts to get an insight into properties of individual adsorptive molecule movement near attractive adsorption sites, arising from adsorbent surface geometrical heterogeneities. In particular, a shape and volume of space occupied by the continuously moving molecule mass center are investigated. To this aim, kinematic equilibrium of the particle moving near a hypothetical microporous carbonaceous adsorbent wall is considered, and then compared with thermodynamic equilibrium. The proposed approach enables to examine effects of certain surface geometry on the shape and volume of space occupied by adsorbed particles, and so to outline temperature conditions for the localized adsorption mechanism predomination. Thus, it provides a cognitive basis to answer the question, what particular mechanism (localized or other—e.g. mobile) should be assumed for a class of adsorption systems in order to select the most appropriate mathematical adsorption model. Hence, it makes it possible for more reliable examination of real porous structures, based on adsorption measurements.     

Particle transfer to solid surfaces

Several theories for predicting deposition rates of flowing colloidal particles onto various collector surfaces are compared and discussed. Successful theories must include, besides hydrodynamic conditions in the vicinity of the collector and the energy of interaction between particles and the wall, a variety of other important factors affecting deposition such as particle detachment, aging of particle-collector bonds, masking, influence of the stability of the dispersion and surface heterogeneity and roughness.Several experimental procedures for measuring deposition rates are discussed and experimental data are compared with theoretical predictions. In the absence of energy barriers current theories predict deposition rates within 10–25% of their actual value. In their presence, discrepancies are usually much larger, mainly because theory neglects several of the above mentioned factors.Examples of dynamic simulation and Monte Carlo calculations suggest that these methods can incorporize the factors affecting deposition more readily than current theories.     

Simultaneous particle and vapor deposition in a laminar boundary layer

A number of industrial and technical applications involve simultaneous particle and vapor deposition from a hot gas onto cold surfaces. For example, deposition of particles and corrosive vapors reduces the lifetime of heat exchangers in power plants and in blades of gas turbines. On another hand, in the OVD (outside vapor deposition) process used in manufacturing optical fibers, a controlled deposition of silica particles and dopant vapors is necessary for obtaining products with prescribed characteristics. In the present paper, a detailed mathematical and numerical model of this process is developed for the flow region near a stagnation point of a two-dimensional body, such as a cylinder. Using an inverse scavenging factor as a small parameter, an analytical asymptotic solution is found for vapor distribution in a condensation layer adjacent to the surface. The results of numerical calculations are presented in the form of surface temperature dependencies of dimensionless deposition rates and profiles of physical parameters of the system at a wide range of surface/gas temperature ratios.