Fickian diffusion constrained on spherical surfaces: voltammetry.

A model is developed for the voltammetric response due to surface charge injection at a single point on the surface of a sphere on whose surface the electroactive material is confined. Accordingly, charge diffusion is constrained to the spherical surface and thus mimicks the voltammetric response of immobilised microparticles derivatised with electroactive material. The full cyclic voltammetric response is investigated, and the peak currents, the peak-to-peak separation and the symmetry of the voltammetric wave are shown to be indicative of the heterogeneous kinetics and the geometry of the adsorbed microparticle. The results show strong deviations from the responses expected for planar diffusion.     

Voltammetric sizing of a sphere.

The size of a glass sphere positioned in the center of a microdisk electrode is determined by using a simple electrochemical procedure and is confirmed, additionally, by a microscopical measurement of the sphere at the time of the electrochemical measurement. The cyclic voltammetric response of the naked electrode and of the electrode with the sphere positioned in its center is recorded over a wide range of scan rates (0.002-1.5 V s(-1)). The size of the sphere is then determined by comparison of the experimental voltammogram with simulations for each individual scan rate.     

Static investigation of adsorption and hetero‐diffusion of copper,silver, and gold adatoms on the (111) surface

In this work, we have used the static molecular simulations combined with an interatomic potential derived from the embedded‐atom method to study the adsorption and hetero‐diffusion on the (111) surface of Cu, Ag, and Au adatoms by using LAMMPS code. The investigation is performed for six heterogeneous systems such as Ag/Au(111), Ag/Cu(111), Au/Ag(111), Au/Cu(111), Cu/Ag(111), and Cu/Au(111). First, we have investigated the relaxation trends and the bond lengths of the atoms in the systems. The calculation results show that, the top layer spacing between the first and second layers of the Au(111), Ag(111), and Cu(111) substrates is contracted. This contraction is found to be more important in the Au(111) substrate. On the other hand, the strong reduction of the binding length is found in Au/Cu(111) for the different adsorption sites. In addition, the binding, adsorption, and static activation energies for all studied systems were examined. The results indicated that the binding and adsorption energies reached their maximum values in the Au/Cu(111) and Au/Ag(111) systems, respectively. Moreover, the static activation barriers for hopping diffusion on the (111) surfaces are found to be low compared with those found in the (100) and (110) surfaces. Therefore, our calculations showed that the difference in energy between the hcp and fcc sites on the (111) surfaces is very small. Copyright © 2017 John Wiley & Sons, Ltd.     

The effect of covalent surface immobilization on the bactericidal efficacy of a quaternary ammonium compound

This study investigates the effect of surface immobilization on the bactericidal function of a quaternary ammonium compound. Quaternary ammonium silane (QAS) coated planar surfaces did not produce any measurable mortality of Staphylococcus aureus, while 1 µm QAS‐coated microparticles did produce S. aureus mortality. The experiments using QAS‐coated microparticles indicate that the ability of QAS molecules to disrupt the cell wall is not hindered by covalent immobilization of QAS to a surface. These results provide evidence that S. aureus cells on a QAS‐coated planar surface are not exposed to a sufficient number of QAS molecules to produce significant mortality. This result has important implications for the development of self‐decontaminating coatings. Covalent immobilization is used to prevent leaching of the bactericidal compound. However, covalent immobilization may result in a significant tradeoff in bactericidal performance. Published in 2007 by John Wiley & Sons, Ltd.     

Change of the roginski “control band” in the case of surface diffusion of adsorbed particles

The effect of surface particle migration between different surface patches on the distribution of the catalytic activity of patches of a linearly inhomogenous surface has been studied. It has been shown that the diffusion can result in an equilibrium distribution of particles A_ads on the surface patches and, therefore, shift of the “control band” and significant change of the reaction rate.     

Molecular modeling study on surface,thermal, mechanical and gas diffusion properties of chitosan

The motivation of this work is to provide reliable and accurate modeling studies of the physical (surface, thermal, mechanical and gas diffusion) properties of chitosan (CS) polymer. Our computational efforts have been devoted to make a comparison of the structural bulk properties of CS with similar type of polymers such as chitin and cellulose through cohesive energy density, solubility parameter, hydrogen bonding, and free volume distribution calculations. Atomistic modeling on CS polymer using molecular mechanics (MM) and molecular dynamics (MD) simulations has been carried out in three dimensionally periodic and effective two dimensionally periodic condensed phases. From the equilibrated structures, surface energies were computed. The equilibrium structure of the films shows an interior region of mass density close to the value in the bulk state. Various components of energetic interactions have been examined in detail to acquire a better insight into the interactions between bulk structure and the film surface. MD simulation (NPT ensemble) has also been used to obtain polymer specific volume as a function of temperature. It is demonstrated that these V–T curves can be used to locate the volumetric glass transition temperature (T_g) reliably. The mechanical properties of CS have been obtained using the strain deformation method. Diffusion coefficients of O₂, N₂, and CO₂ gas molecules at 300 K in CS have been estimated. The calculated properties of CS are comparable with the experimental values reported in the literature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1260–1270, 2007     

Silylation of an OH-terminated self-assembled monolayer surface through low-energy collisions of ions: a novel route to synthesis and patterning of surfaces

Using a multi-sector ion-surface scattering mass spectrometer, reagent ions of the general form SiR(3) (+) were mass and energy selected and then made to collide with a hydroxy-terminated self-assembled monolayer (HO-SAM) surface at energies of approximately 15 eV. These ion-surface interactions result in covalent transformation of the terminal hydroxy groups at the surface into the corresponding silyl ethers due to Si--O bond formation. The modified surface was characterized in situ by chemical sputtering, a low-energy ion-surface scattering experiment. These data indicate that the ion-surface reactions have high yields (i.e. surface reactants converted to products). Surface reactions with Si(OCH(3))(3) (+), followed by chemical sputtering using CF(3) (+), yielded the reagent ion, Si(OCH(3))(3) (+), and several of its fragments. Other sputtered ions, namely SiH(OCH(3))(2)OH(2) (+) and SiH(2)(OCH(3))OH(2) (+), contain the newly formed Si--O bond and provide direct evidence for the covalent modification reaction. Chemical sputtering of modified surfaces, performed using CF(3) (+), was evaluated over a range of collision energies. The results showed that the energy transferred to the sputtered ions, as measured by their extent of fragmentation in the scattered ion mass spectra, was essentially independent of the collision energy of the projectile, thus pointing to the occurrence of reactive sputtering.A set of silyl cations, including SiBr(3) (+), Si(C(2)H(3))(3) (+) and Si(CH(3))(2)F(+), were similarly used to modify the HO-SAM surface at low collision energies. A reaction mechanism consisting of direct electrophilic attack by the cationic projectiles is supported by evidence of increased reactivity for these reagent ions with increases in the calculated positive charge at the electron-deficient silicon atom of each of these cations. In a sequential set of reactions, 12 eV deuterated trimethylsilyl cations, Si(CD(3))(3) (+), were used first as the reagent ions to modify covalently a HO-SAM surface. Subsequently, 70 eV SiCl(3) (+) ions were used to modify the surface further. In addition to yielding sputtered ions of the modified surface, SiCl(3) (+) reacted with both modified and unmodified groups on the surface, giving rise not only to such scattered product ions as SiCl(2)OH(+) and SiCl(2)H(+), but also to SiCl(2)CD(3) (+) and SiCl(2)D(+). This result demonstrates that selective, multi-step reactions can be performed at a surface through low-energy ionic collisions. Such processes are potentially useful for the construction of novel surfaces from a monolayer substrate and for chemical patterning of surfaces with functional groups.     

A study of surface diffusion of ternary (Cu-Ag-Zr) adatoms clusters for applications in thin film formation

This study presents the diffusion of heterogeneous ternary (Cu-Ag-Zr) adatoms clusters on Ag(111) using molecular dynamics techniques which could be important for the surface phenomena's and helpful for the ternary cluster's growth and formation of ternary alloy-based thin films. The mechanism of nanoscale surface diffusion is investigated for 1Cu-1Ag-1Zr, 2Cu-2Ag-2Zr, 3Cu-3Ag-3Zr, and 4Cu-4Ag-4Zr clusters at temperatures 300, 500, and 700 K. The diffusion mechanism displays that the diffusion of trimer cluster exhibits hopping, sliding, and shearing at 300 K, whereas for hexamer, nonamer, and decamer, the diffusion rate is low; however, breathing, anchoring, and concentrated motion dominates. At 500 K, trimer and hexamer show the process of atomic exchange; however, the atomic exchange is not observed in the case of nonamer and decamer diffusion. The atomic exchange mechanism of Cu and Zr adatoms dominates at 700 K for all size clusters, except Ag adatoms, where Zr adatoms show a relatively more tendency. Separation and rejoining of the one and two adatoms (likely Zr adatom) are also witnessed at high temperature. The pop-up of Ag adatoms also occurs in very short intervals over the remaining adatoms of clusters. Interestingly, during trimer diffusion, the adsorption of the Zr- or Cu-adatom among the trimer cluster into the substitutional site is found. At 700 K, vacancy generation, filling of vacancies, and migration of vacancy, in the neighborhood of the adatoms cluster, also observed. Moreover, the rate of diffusion decreases with the size increase of the clusters and increases with the increase in temperature.     

Imaging with chemical analysis: adsorbed structures formed during surface chemical reactions

Imaging surfaces and interfaces with structural and chemical specificity has been essential for understanding a variety of phenomena occurring in adsorbed layers during surface chemical reactions. A recent achievement of chemical imaging with spectroscopic analysis is the experimental proof of theoretically predicted spontaneous formation of regular patterns of metal adatoms during surface chemical reactions. An attractive feature of this finding is that the reaction rate and adlayer coverage can be employed to precisely control the morphology of the structures. The mechanisms of these self-organisation phenomena, driven by the interplay between energetic principles and kinetics, opens a conceptually novel route to creating a wide range of surface-supported functional structures at the micro- and nanometre length scales.     

Mathematical modelling and simulation of adsorption processes at spherical microparticles.

A model for the adsorption process at spherical microparticles under transient diffusion conditions has been developed and solved using numerical simulation. This model allowed us to demonstrate that the system is controlled by two main dimensionless parameters: the adsorption rate constant ka' and the saturation parameter beta. Analytical models for the adsorption process at spherical microparticles under steady-state mass transport conditions have been derived. These models use previously developed empirical relationships for the calculation of the mass transfer coefficient (kc). The properties of the system were studied for both the case where mass transport is described by diffusion only and the case where it is the result of a coupled diffusion/convection process. These mathematical tools were then used to analyse the results obtained for the uptake of CuII by glassy carbon powder modified with the monomer L-cysteine methyl ester and to extract a minimum value for the adsorption rate constant which was found to be of the order of 10(-4) cm s(-1).     

聚乙烯亚胺改性聚丙烯腈中空膜固定脂肪酶

聚丙烯腈是富腈基的高分子聚合物,易修饰改性,广泛应用于膜分离应用.我们以聚丙烯腈中空膜为载体,采用化学法交联聚乙烯亚胺并固定脂肪酶,固定过程中引入海藻酸钠,用CaCl_2进行后处理,得到固定化脂肪酶PAN-PEI-SA/E-CaCl_2载酶量为31.70(mg enzyme)/(g support),酶活为50.20 U/(g support),15次重复使用可保留58.77%的酶活,与游离酶相比耐酸性和耐温性有所提高,相同条件下与Nov 435相比,酶活更高,这表明最终得到的固定化脂肪酶有良好的工业应用前景.     

Effect of surface orientation on the segregation kinetics of Sb from a Cu single crystal

In a previous theoretical study it has been suggested that the bulk vacancy formation energy near a surface depends on the orientation of the surface. It has been suggested also that this dependency of the vacancy formation energy would influence the bulk diffusion coefficient near the surface. The experimental results presented in this paper support this hypothesis. The experimental results were obtained by measuring the bulk‐to‐surface segregation of Sb for a Cu(111) single crystal with 0.088 at.% Sb and for a Cu(110) single crystal with 0.082 at.% Sb. The experimental results were fitted with the vacancy‐modified Darken model and it was clear that the bulk diffusion coefficient beneath the (110) surface is higher than the bulk diffusion coefficient beneath the (111) surface. Copyright © 2003 John Wiley & Sons, Ltd.     

Chemical‐surface modification of polymers using atmospheric pressure nonequilibrium plasmas and comparisons with vacuum plasmas

We demonstrate that stable microwave‐coupled atmospheric pressure nonequilibrium plasmas (APNEPs) can be formed under a wide variety of gas and flow‐rate conditions. Furthermore, these plasmas can be effectively used to remove surface contamination and chemically modify polymer surfaces. These chemical changes, generally oxidation and crosslinking, enhance the surface properties of the materials such as surface energy. Comparisons between vacuum plasma and atmospheric plasma treatment strongly indicate that much of the vacuum‐plasma literature is pertinent to APNEP, thereby providing assistance with understanding the nature of APNEP‐induced reactions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 95–109, 2002     

Effect of surface mobility on the particle sliding along a bubble or a solid sphere

The sliding velocity of glass beads on a spherical surface, made either of an air bubble or of a glass sphere held stationary, is measured to investigate the effect of surface mobility on the particle sliding velocity. The sliding process is recorded with a digital camera and analyzed frame by frame. The sliding glass bead was found to accelerate with increasing angular position on the collector's surface. It reaches a maximum velocity at an angular position of about 100 degrees and then, under certain conditions, the glass bead leaves the surface of the collector. The sliding velocity of the glass bead depends strongly on the surface mobility of a bubble, decreasing with decreasing surface mobility. By a mobile surface we mean one which cannot set up resistive forces to an applied stress on the surface. The sliding velocity on a rigid surface, such as a glass sphere, is much lower than that on a mobile bubble surface. The sliding velocity can be described through a modified Stokes equation. A numerical factor in the modified Stokes equation is determined by fitting the experimental data and is found to increase with decreasing surface mobility. Hydrophobic glass beads sliding on a hydrophobic glass sphere were found to stick at the point of impact without sliding if the initial angular position of the impact is less than some specific angle, which is defined as the critical sticking angle. The sticking of the glass beads can be attributed to the capillary contracting force created by the formation of a cavity due to spontaneous receding of the nonwetting liquid from the contact zone. The relationship between the critical sticking angle and the particle size is established based on the Yushchenko [J. Colloid Interface Sci. 96 (1983) 307] analysis.     

Reactivity of alkoxysilyl compounds: chemical surface modification of nano‐porous alumina membrane using alkoxysilylazobenzenes

In order to clarify the chemical adsorption properties of alkoxysilyl compounds to metal‐oxide surface quantitatively and to obtain the information on the reaction conditions for the efficient surface modification, the chemical surface modifications of nano‐porous alumina membranes (NPAMs) by typical alkoxysilyl compounds of 4‐(triethoxysilyl)azobenzene, 4‐(diethoxyphenylsilyl)azobenzene, 4‐(ethoxydiphenylsilyl)azobenzene and 4‐(methoxydimethylsilyl)azobenzene were examined. The chemical surface modifications were performed by immersing NPAMs into the solutions of the alkoxysilylazobenzenes. Especially for 4‐(triethoxysilyl)azobenzene, the modification was investigated precisely by changing the solvent, temperature, concentration and water content of the solutions to reveal the effects of the reaction conditions on the adsorption property of alkoxysilyl compounds to metal oxides. The NPAMs having chemically modified surface were prepared successfully by the immersing method, and the alkoxysilylazobenzenes were confirmed to be bound on the NPAM surface through Si? O? Al bonds, which were indicated to be formed mainly by the direct condensation reaction between the alkoxysilyl groups of the azobenzenes and the hydroxy groups on the NPAM surface. The amounts of the azobenzenes adsorbed on the NPAM surface were estimated quantitatively by a visible absorption spectroscopy, and the results showed that the solutions with non‐polar solvents, higher temperatures and higher concentrations are suitable for the efficient surface modification. Copyright © 2010 John Wiley & Sons, Ltd.     

Evaluation of the free‐volume theory of diffusion

General characteristics of the free‐volume theory of diffusion are discussed, and a recent data‐theory comparison involving free‐volume theory is critically evaluated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 501–507, 2003     

Lattice microbes: High‐performance stochastic simulation method for the reaction‐diffusion master equation

Spatial stochastic simulation is a valuable technique for studying reactions in biological systems. With the availability of high‐performance computing (HPC), the method is poised to allow integration of data from structural, single‐molecule and biochemical studies into coherent computational models of cells. Here, we introduce the Lattice Microbes software package for simulating such cell models on HPC systems. The software performs either well‐stirred or spatially resolved stochastic simulations with approximated cytoplasmic crowding in a fast and efficient manner. Our new algorithm efficiently samples the reaction‐diffusion master equation using NVIDIA graphics processing units and is shown to be two orders of magnitude faster than exact sampling for large systems while maintaining an accuracy of ～ 0.1%. Display of cell models and animation of reaction trajectories involving millions of molecules is facilitated using a plug‐in to the popular VMD visualization platform. The Lattice Microbes software is open source and available for download at http://www.scs.illinois.edu/schulten/lm © 2012 Wiley Periodicals, Inc.     

The effect of vacuum-ultraviolet laser wavelengths on the surface treatment of polyolefinic polymers

The laser-induced modification of the surface morphology of poly(propylene) fibers and films was investigated. It is known that strongly absorbing fibers such as poly(ethylene terephthalate) and polyamide obtain characteristic surface morphologies after treatment with 193 or 248 nm laser irradiation. Polyolefinic material cannot be modified directly (i.e., without doping) by irradiation with these wavelengths. Therefore experiments were carried out using a 157 nm F₂-laser. After irradiation in a vacuum chamber at fluences in excess of 50 mJ/cm² poly(propylene) also reveals the well-known surface morphology. Compared with aromatic polymers a rather high number of pulses is needed to generate the effect. © 1993 John Wiley & Sons, Inc.