Model for the water-amorphous silica interface: the undissociated surface

The physical and chemical properties of the amorphous silica-water interface are of crucial importance for a fundamental understanding of electrochemical and electrokinetic phenomena, and for various applications including chromatography, sensors, metal ion extraction, and the construction of micro- and nanoscale devices. A model for the undissociated amorphous silica-water interface reported here is a step toward a practical microscopic model of this important system. We have extended the popular BKS and SPC/E models for bulk silica and water to describe the hydrated, hydroxylated amorphous silica surface. The parameters of our model were determined using ab initio quantum chemical studies on small fragments. Our model will be useful in empirical potential studies, and as a starting point for ab initio molecular dynamics calculations. At this stage, we present a model for the undissociated surface. Our calculated value for the heat of immersion, 0.3 J x m(-2), falls within the range of reported experimental values of 0.2-0.8 J x m(-2). We also study the perturbation of water properties near the silica-water interface. The disordered surface is characterized by regions that are hydrophilic and hydrophobic, depending on the statistical variations in silanol group density.     

界面层模型和表面力

本文对表面现象理论进行了研究,提出物理面界层模型;提出两相各自的界面张力、两相的表面力(表面内力、表面外力)的概念,导得其计算公式;并从分子间相互作用的理论观点讨论了表面力,在理论计算公式中考虑了诱导力的贡献。计算结果表明,提出的理论模型是正确的。     

The reduction of gold nanoparticles in the surface layer of modified silica

Silicas with deposited hydridepolysiloxane layers were used for the in situ preparation of gold nanoparticles by the reduction of metal ions from a solution of chloroauric acid. The metal-containing silicas obtained were characterized by X-ray powder diffraction, transmission electron microscopy, and UV, IR, and laser correlation spectroscopy.     

X-ray study of the electric double layer at the n-hexane/nanocolloidal silica interface

The spatial structure of the transition region between an insulator and an electrolyte solution was studied with x-ray scattering. The electron-density profile across the n-hexane/silica sol interface (solutions with 5, 7, and 12 nm colloidal particles) agrees with the theory of the electrical double layer and shows separation of positive and negative charges. The interface consists of three layers, i.e., a compact layer of Na(+), a loose monolayer of nanocolloidal particles as part of a thick diffuse layer, and a low-density layer sandwiched between them. Its structure is described by a model in which the potential gradient at the interface reflects the difference in the potentials of "image forces" between the cationic Na(+) and anionic nanoparticles and the specific adsorption of surface charge. The density of water in the large electric field (approximately 10(9)-10(10) Vm) of the transition region and the layering of silica in the diffuse layer is discussed.     

Stability of the gold/silica thin film interface: electrochemical and surface plasmon resonance studies

This article reports chemical stability studies of a gold film electrode coated with thin silicon oxide (SiOx) layers using electrochemical, surface plasmon resonance (SPR) and atomic force microscopy (AFM) techniques. Silica films with different thicknesses (d = 6.4, 9.7, 14.5, and 18.5 nm) were deposited using a plasma-enhanced chemical vapor deposition technique (PECVD). For SiOx films with d >/= 18.5 nm, the electrochemical behavior is characteristic of a highly efficient barrier for a redox probe. SiOx films with thicknesses between 9.5 and 14.5 nm were found to be less efficient barriers for electron transfer. The Au/SiOx interface with 6.4 nm of SiOx, however, showed an enhanced steady-state current compared to that of the other films. The stability of this interface in solutions of different pH was investigated. Whereas a strongly basic solution led to a continuous dissolution of the SiOx interface, acidic treatment produced a more reticulated SiOx film and improved electrochemical behavior. The electrochemical results were corroborated by SPR measurements in real time and AFM studies.     

Transient finite element analysis of electric double layer using Nernst-Planck-Poisson equations with a modified Stern layer

A finite element implementation of the transient nonlinear Nernst-Planck-Poisson (NPP) and Nernst-Planck-Poisson-modified Stern (NPPMS) models is presented. The NPPMS model uses multipoint constraints to account for finite ion size, resulting in realistic ion concentrations even at high surface potential. The Poisson-Boltzmann equation is used to provide a limited check of the transient models for low surface potential and dilute bulk solutions. The effects of the surface potential and bulk molarity on the electric potential and ion concentrations as functions of space and time are studied. The ability of the models to predict realistic energy storage capacity is investigated. The predicted energy is much more sensitive to surface potential than to bulk solution molarity.     

Trends in surface and interface analysis

Summary The rapid progress in high technology constantly poses new challenges for Analytical Chemistry and prompts the development of new techniques and procedures. The influence is particularly strong in surface and interface analysis, which is developing at a rapid pace. This paper discusses some of the frontier areas like high-resolution depth-distribution analysis of trace elements, quantitative depth distribution analysis of ultra thin-layer systems, quantitative trace element analysis in monolayers, 3-dimensional stereometric analysis, molecular analysis, in-situ atomic resolution analysis of surfaces (chemical nanoscopy). Methodological approaches are discussed as well as results obtained mainly with solid state mass spectrometry and atomic force microscopy.Abbreviations and acronyms AES Auger Electron Spectrometry - AEM Analytical Electron Microscopy - AFM Atomic Force Microscopy - BSE Back Scattered Electrons - EPMA Electron Probe Micro Analysis - LRI-SNMS Laser Resonance Ionization Sputtered Neutrals Mass Spectrometry - SE Secondary Electrons - SEM Scanning Electron Microscopy - SIMS Secondary Ion Mass Spectrometry - STM Scanning Tunneling Microscopy - TEM Transmission Electron Microscopy - TOF-MS Time-of-Flight Mass Spectrometer - TXRF Total Reflection X-Ray Spectrometry - XPS X-Ray Photoelectron Spectroscopy Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

A quantitative theory of the Stern electric double layer

A quantitative theory of the Stern electric double layer is suggested. It is based on the view that every ion possesses a geometrical and an electrokinetic radius, that the ionic atmosphere begins from the geometrical one, and that the difference between these radii is the Stern quantity delta. The equations of the mentioned radii and the quantity delta are established and the values of the different potentials characterizing an ion and its ionic atmosphere are determined.     

Transient behavior of the hydrophobic surface/water interface: from nanobubbles to organic layer

We report the formation and subsequent change of the water-depleted layer at a hydrophobic surface/water interface. With water as the solvent, surface plasmon resonance measurements indicate time dependent evolution of two separate states. The first state is the water-depleted layer, and it is characterized by a layer of nanobubbles on the surface and is short-lived in time (order of 10 min). The second state is a final equilibrium state, which occurs in approximately 30 h, where a layer is formed with organic characteristics. If, instead of water, an aqueous solution is exposed to the hydrophobic surface, the evolution from nanobubbles to an organic like layer shows dependency on the surface energy of the liquid media.     

Dynamics of the surface layer in cyanobiphenyl-aerosil nanocomposites with a high silica density

Composites were prepared from an aerosil and 4-n-alkyl-4'cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of ∼7 g aerosil in 1 cm³ of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10^-2-10⁹ Hz) in a large temperature range (220-370 K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel-Fulcher-Tammann law, which is found to be typical for glass-forming liquids. The quasi two-dimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd-even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface).     

The significance of stagnant layer conduction in electrokinetics

This paper examines the consequences of the occurrence of conduction processes in the stagnant fluid layer between the surface of a colloidal particle and the shear plane when it is undergoing an electrokinetic process. It is shown that the most widely used model system for studying electrokinetics (viz. monodisperse polystyrene sulfate latex) displays significantly different behaviour in different experiments depending on the details of the post-polymerisation clean-up procedure. Nonetheless, the results provide important insights into how electrokinetic results should be interpreted. A procedure is suggested which should allow a more realistic evaluation of the diffuse double layer potential from electrokinetic results without the need for extensive conduction studies.     

The stability of titania‐silica interface

Silica is very often the catalyst support of choice for transition metal oxides such as titania, and specially anatase. Titania is an excellent absorber and photocatalyst for many organic molecules degradation. In order to understand the chemical nature of the interaction between titania and silica, we have performed a theoretical study using density functional theory aiming to elucidate the role on the stability of the interface of the specific type of interactions, H‐bonding, covalent bonding of the pristine surfaces, and covalent bonding after silicon and titanium ions interdiffusion. The calculations were carried out for hydrogen and oxygen terminated surface, comparing the bonding types and the forces acting along the interface. The interface dynamics was studied for interfaces under applied stress in order to elucidate their stability and failure limits. The shearing forces and the mechanisms of interface failure were determined. Interfaces with interdiffused Si and Ti ions were studied to improve the interface stabilization. The results demonstrate that high‐temperature treatment leading to formation of Si O Ti bonds at the interface is responsible for the formation of strong and flexible binding interaction between both oxides. At high strains, the Si O Ti interface failure is observed due to lattice mismatch between the SiO₂ and TiO₂. The failure is a result of forces acting orthogonal to the interface shearing. In case of hydrogen terminated surface, the interface binding is a result of hydrogen bond network. Such interface is fragile at moderate shearing forces along the applied strain. The hydrogen bond network decreases the elastic properties and flexibility of the interface. The SiO₂/TiO₂ interface is further stabilized by Si/Ti ion interdiffusion. The ionic interdiffusion process also increases the interface flexibility. Thus, in order to obtain more stable anatase photocatalyst supported on silica, the synthetic routes should favor silicon and titanium ions interdiffusion along the interface.     

Silicon surface texturing by electro-deoxidation of a thin silica layer in molten salt

A new method of silicon surface texturing is reported, which is based on thin silica layer electrochemical reduction in molten salts. A thermal silica layer grown on p-type silicon was potentiostatically reduced in molten calcium chloride at 850 °C. Typical nano–micro-formations obtained at different stages of electrolysis were demonstrated by SEM. X-ray diffraction measurements confirmed conversion of the amorphous thermal silica layer into crystalline silicon. The proposed approach shows promise in photovoltaic applications, for instance, for production of antireflection coatings in silicon solar cells.     

Progress in materials analysis with SIMS: Quantitative surface and interface characterization

Summary The potential and limitations of SIMS for quantitative surface and interface characterization of technical materials are described. Quantification procedures, figures of merit and analytical strategies are discussed for surface characterization of single cristals (dopant elements in Si), homogeneous metallic thin films (multielement trace analysis), heterogeneous glass layer structures (corrosion behaviour), polycristalline metals with precipitations (C, O, Al, Si in steel) and interface analysis by depth profiling (Cr distribution in Si₃N₄/GaAs) as well as step scanning (segregation studies of P in W-NiFe).

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Fortschritte der Werkstoffanalyse mit SIMS: Quantitative Grenzflächencharakterisierung

Zusammenfassung Die Möglichkeiten und Grenzen von SIMS für die quantitative Grenzflächencharakterisierung in Werkstoffen wurden beschrieben. Quantifizierungsverfahren, analytische Gütekennziffern und Strategien wurden an folgenden Beispielen diskutiert: Oberflächenanalyse von Einkristallen (Dotierungselemente in Silizium), homogenen metallischen Dünnfilmstrukturen (Multielement-Spurenanalyse) und polykristallinen Materialien mit Ausscheidungen (C, O, Al, Si in Stahl); Grenzflächenanalyse durch Aufnahme von Tiefenprofilen (Verteilung von Cr zwischen Si₃N₄ und GaAs) und Linienprofilen (Segregation von P in W-NiFe).

     

Graft polymerization from a silica surface initiated by adsorbed peroxide macroinitiators. I. Adsorption and structure of the adsorbed layer of peroxide macroinitiators on a silica surface

The adsorption features of two peroxide macroinitiators (PMIs) with various functionalities from their semi-dilute solutions on the silica surface were thoroughly investigated in the present work. These investigations include the study of the adsorption kinetics of PMI in diverse solvents and a detailed examination of the adsorbed layer structure with the aid of ellipsometry, scanning force microscopy (SFM), and contact angle measurements. Rearrangements of PMI macromolecules at the solid surface are supposed to be the main reason for the appearance of extremes on the kinetic curves and, besides, have a more pronounceable effect on adsorption rate than their diffusion rate to the surface even at the initial stage of the process. Both island-like and densely packed structures of absorbed layers were revealed by combining contact angle measurements and SFM. Surprisingly, even in the case when saturation of the adsorbed layer is reached, PMI does not completely occupy the substrate surface which is at least particularly reachable for the wetting liquids. PMIs adsorbed at the solid surface are intended for the formation of tethered polymer "brushes" via the initiation of "grafting from" polymerization.     

Estimation of the extent of the water-rich layer associated with the silica surface in hydrophilic interaction chromatography

The possible presence of a mobile phase layer rich in water on the surface of silica columns used under conditions typical in hydrophilic interaction chromatography was investigated by the injection of a small hydrophobic solute (benzene) using acetonitrile-water mobile phases of high organic content. Benzene does not partition into this layer and is thus partially excluded from the pores of the phase up to a water content of about 30%, after which hydrophobic retention of the solute on siloxane bonds is observed. In 100% acetonitrile, the retention volume of benzene was smaller than that estimated either by pycnometry or by calculation from the basic physical parameters of the column. This result might be attributable to the larger size of the benzene molecule: the elution volume of a molecule is the pore volume minus a surface layer half the diameter of the analyte molecule. However, some influence of strongly adsorbed water that remains on the surface of the phase even after extensive purging with dry acetonitrile cannot be entirely discounted. The results suggest that about 4-13% of the pore volume of a silica phase is occupied by a water-rich layer when using acetonitrile-water containing 95-70% (v/v) acetonitrile.     

Chemistry of the silica surface: liquid-solid reactions of silica gel with trimethylaluminum

The reaction of trimethylaluminum and dry, high-surface-area (500 m2/g) silica gel in a mixed slurry was studied using multinuclear, solid-state NMR spectroscopy. The products of the initial reaction were characterized, and their progress through subsequent washing with diethyl ether and reactions with measured amounts of water was followed. The quantitative distribution of different chemical forms of carbon deposited on the silica surface by the initial reaction was measured. The products of the initial reaction are dominated by methyl species of the types Al(CH3)n (with Si-O-Al linkages), Si-O-CH3, and (Si-O)4-nSi(CH3)n; aluminum is seen to exist predominantly as a five-coordinate species. Subsequent treatment with diethyl ether fails to remove any surface species, but instead the ether becomes strongly associated with the surface and highly resistant to removal. Stepwise additions of water hydrolyze the Al-CH3 and Si-O-CH3 moieties, leading to conversion of five-coordinate aluminum to four- and six-coordinate aluminum, and affect the partial release of the surface-associated diethyl ether; Si-CH3 moieties remain. The effect of aromatic and saturated solvents on the initial reaction was examined and found to cause a small but significant change in the distribution of products. Structures of aluminum-centered species on the silica surface consistent with the spectroscopic data are proposed.     

Characterization of the silica surface

Aerosil samples, heat-treated and then silylated with various silanes at various temperatures have been characterized by adsorption of benzene at 293 K. Adsorption and desorption isotherms were plotted and the specific surface areas were determined using the BET method. Cross-plots of adsorption were made between the silylated and the unsilylated samples to reveal in a simple way the resulting changes in surface properties. Modification of aerosils using silanes was found to alter the adsorptive properties of aerosils for benzene to extents depending on the silane used as well as the concentration of residual surface silanols.