Interfacial Step Research Articles

Study of ultrathin polyamide-6,6 films on clean copper and platinum

The aim of this work is to present an interaction reactor which allows us to build very thin organic coatings on metallic substrates. This reactor is coupled to an analytic device which further enables an investigation of the interfacial region by very-surface-sensitive analytical means. The use of such an integrated installation makes it possible to follow the growth of the interfacial region step by step and to focus on the nature of the chemical structure that is identified in the interfacial region between a well-defined polymer phase and an atomically clean metallic substrate. As an illustration we compare the behavior of PA-6,6 on copper and platinum.

Effect of lipid composition on lipoprotein lipase activity measured by a continuous fluorescence assay: effect of cholesterol supports an interfacial surface penetration model.

The breakdown of normal substrates by lipases requires an interfacial binding step prior to hydrolysis. Interfacial binding and subsequent hydrolysis will be affected by the lipid components and hence physical properties of the substrate surface. In order to investigate in detail the effect of lipid structure on the activity of lipoprotein lipase (LPL), triolein-containing emulsion particles of defined composition have been used as substrates. In addition, lipase activity has been measured using a continuous fluorescence displacement assay that monitors the release of long-chain fatty acids as an alternative to normal radiochemical assays. Using this fluorescence assay, rates of hydrolysis of triolein were the same as when using a standard radiochemical assay under identical conditions. Activation by apolipoprotein CII was very similar by both methods; however, the extent of activation (2-3-fold) was less than has been reported previously using different assay conditions. In order to investigate the effect of cholesterol on LPL activity, emulsion particles were prepared in which the cholesterol/egg-phosphatidylcholine ratio was increased up to a 1:1 molar ratio. A pronounced stimulatory effect of cholesterol was observed under these assay conditions, with up to a 5-fold increase in rate compared with emulsion particles without cholesterol. Since high molar ratios of cholesterol are reported to exclude triacylglycerol from the phospholipid surface [Spooner and Small (1987) Biochemistry 26, 5820-5825], these results are not consistent with a mechanism involving LPL hydrolysis of surface triacylglycerol. Instead, they support an interfacial penetration model, allowing the enzyme's active site direct access to triacylglycerol in the lipoprotein core. Perturbation of the surface phospholipid monolayer of the emulsion particle as a result of hydrolysis by Naja naja phospholipase A2 resulted in a 10-fold activation of LPL, providing further support for an interfacial penetration model. The stimulatory effect of apolipoprotein CII was not modulated by modification of the interface with cholesterol.

Step-Induced Magnetic Defects in Fe/Cr Systems

ABSTRACTWe present the results of calculations of the magnetic order in Fe/Cr systems with a mono-atomic step at the interface. The study of such a systems is of particular interest since it is believed that topological defects play an essential role in the exchange coupling mechanism in Fe/Cr multilayered systems. We show that assuming only collinear magnetism, an interfacial step creates a very extended magnetic defect in the Cr layer. On the other hand, when the local magnetic moments are allowed to rotate, the magnetic defect is very localized near the step and the presence of the step induces a non-collinear coupling between the Fe layers in Fe/Cr superlattices.

High-resolution transmission electron microscopy of AlAs-GaAs semiconductor superlattices

A new high-resolution transmission electron microscopy (HREM) technique is proposed for observing interfacial structures in semiconductor superlattices in the cross-section, and is used to characterize AlAs-GaAs interfacial step structures. Analysis of diffracted beam amplitudes shows that the {1 1 1| beam amplitudes of GaAs are minimized at an extinction distance of 14.4 nm, whereas those of AlAs show significantly higher values. This remarkable difference in the {1 1 1| beam amplitudes leads to a marked contrast between HREM images of GaAs and AlAs around this specimen thickness, allowing edge-on observation of the interfacial atomic steps running along the direction. Artifacts produced by ion milling prevent atomic-scale observations of the interfaces so an artifact-free TEM specimen preparation technique is also presented. The HREM method provides information on the step intervals and straightness of step edges at AlAs-GaAs interfaces fabricated on just-cut and vicinal GaAs (0 0 1) substrates. The effect of the growth interruption method during molecular beam epitaxy on interface smoothing is also demonstrated.

Role of interface structure and interfacial defects in oxide scale growth

Recent studies of the structure and dynamics of solid-solid interfaces have provided some understanding about the role of the scale-metal interface in the growth of reaction product scales on pure metals. The action of interfacial defects (misfit dislocations, misorientation dislocations and disconnections) in the creation and annihilation of the point defects suporting the diffusional growth of scales is considered. Anion point defects (vacancies/interstitials) supporting scale growth by anion diffusion are annihilated/created by the climb of misorientation dislocations or disconnections in the scale at the interface. For scale growth by cation diffusion, cation point defects (vacancies/interstitials) can be annihilated/created by the climb of interfacial misfit or misorientation dislocations in the metal. Because of their necessarily high density, in most cases, the dominant climb of misfit dislocations would be favored. The blocking of interfacial reaction steps can be a means to retard the scaling kinetics and to alter the fundamental scaling mode. For instance, the interfacial segregation of large reactive element ions can pin the interface dislocations, an action which poisons the usual interfacial reaction step. Such considerations are consistent with the well-known phenomena ascribed to the reactive element effect (REE).

Characterization of GaAs/AlAs interfacial atomic step structures on a (411)A-oriented substrate by transmission electron microscope

We observed atomic structures of (411)A GaAs/AlAs hetero-interfaces using a transmission electron microscope, and studied the numerical computer simulations of the lattice images for the first time. The observed specimens were GaAs/AlAs multi-layer structures fabricated by molecular beam epitaxy, where the AlAs layers were very thin (0.75–2.1 monolayer). From the hetero-interfacial structures observed in cross-sectional view, it was found that the (411)A atomic step structures were based on Shimomura's model, which has already been proposed in a previous paper. We will discuss the concrete interfacial structure.

Characterization of GaAs/AlAs interfacial atomic step structures on a (411)A-oriented substrate by transmission electron microscope

Characterization of GaAs/AlAs interfacial atomic step structures on a (411)A-oriented substrate by transmission electron microscope

High resolution interferometric technique for in-situ studies of crystal growth morphologies and kinetics

We have developed an interferometric technique for the high resolution monitoring of interfacial morphologies. Changes in feature height of 200 Å over several days can be resolved across crystal facets several 100 μm wide. This is achieved through signal processing based on the phase information contained in the interferometric intensity field and by applying corrections determined from an interferometric reference signal. Extensive tests on {110} faces of lysozyme crystals revealed growth rate fluctuations as a result of step bunching and other interfacial position-dependent step kinetics.

The role of interface dislocations and ledges as sources/sinks for point defects in scaling reactions

The specific roles of interface misfit and misorientation dislocations, and of disconnections, in creating or annihilating the point defects supporting diffusion during scale growth are considered. Anion point defects (vacancies/interstitials) supporting scale growth by anion diffusion are annihilated/created by the climb of misorientation dislocations or disconnections in the scale at the interface. For scale growth by cation diffusion, cation point defects (vacancies/interstitials) can be annihilated/created by the climb of interfacial misfit or misorientation dislocations in the metal. Because of their necessarily high density, in most cases, the dominant climb of misfit dislocation would be favored. Consistent with experimental observations of the “reactive element effect”, large reactive element cations segregated to the metal/scale interface provide a pinning force on interface dislocations, especially on the misfit dislocations in the metal. An approximate elastic binding energy calculation suggests that a fraction of a monolayer of segregated reactive ions is adequate to pin the misfit dislocations and thereby retard the oxidation kinetics, or change the dominant diffusion mechanism. When the interfacial reaction step blocks the kinetics, a dominant fraction of the Gibbs energy change is localized across the blocked interface with a smaller concentration gradient to drive diffusion in the scale.

Mechanism of inhibition of human nonpancreatic secreted phospholipase A2 by the anti-inflammatory agent BMS-181162.

Many important mediators of inflammation result from the liberation of free arachidonic acid from phospholipid pools which is thought to result from the action of phospholipase A2 (PLA2). It is believed, therefore, that the inhibition of PLA2 would be an important treatment in many inflammatory disease states. The anti-inflammatory agent BMS-181162 (4-(3'-carboxyphenyl)-3,7-dimethyl-9-(2",6",6"-trimethyl-1"-cyclohexenyl )-2Z,4E , 6E,8E-nonatetraenoic acid) selectively inhibits PLA2 and has been shown to block arachidonic acid release in whole cells. The mechanism of inhibition of human non-pancreatic-secreted PLA2 by BMS-181162 is investigated in this paper. A scooting mode assay in which the enzyme is irreversibly bound to vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol containing 5 mol % of 1-palmitoyl-2-[1-14C]arachidonoyl-sn-glycero-3-phosphocholine, was used to characterize the inhibition. With this assay system, BMS-181162 inhibited the enzyme in a dose-dependent manner. Compounds which inhibit in the scooting mode have been shown to be competitive inhibitors in the interface (Gelb, M. H., Berg, O., and Jain, M. K. (1991) Curr. Op. Struct. Biol. 1, 836-843). This was verified by demonstrating that the inhibition was not due to the desorption of the enzyme from the lipid-water interface. Additionally, the compound did not measurably affect the rate of association onto the vesicles. Therefore, the inhibition was not the result of a modulation of the bilayer morphology nor an interaction with the interfacial binding site on the enzyme. The degree of inhibition was dependent on the reaction volume which indicates that the inhibitor is only partially partitioned into the bilayer. After compensating for this partitioning, the dose-dependent inhibition could be defined by kinetic equations describing competitive inhibition at the interface. The equilibrium dissociation constant for the inhibitor bound to the enzyme at the interface (KI*) was determined to be 0.013 mol fraction, thus demonstrating that BMS-181162 represents a novel structural class of tight-binding competitive inhibitors of human nonpancreatic secreted PLA2. Using Escherichia coli membranes as substrate, to which the enzyme binds to the interface reversibly, the inhibition showed a nonclassical kinetic pattern which is also consistent with a partial partitioning of the inhibitor into the bilayer. This was verified by a direct measurement of the amount of inhibitor remaining in solution. The implications for in vivo efficacy which result from this mechanism are discussed.

Enzyme Kinetics of Lipolysis Revisited: The Role of Lipase Interfacial Binding

A model for the enzyme kinetics of lipolysis was developed where the rate limiting step of the reaction is the interfacial binding step. Binding involves the association of the enzyme with a cluster of substrate molecules and a conformational change in the enzyme, resulting in an interfacially penetrated lipase bound to a cluster of substrate molecules. The resulting derived rate equation is identical to the Hill equation. Fits of the model to experimental velocity vs. substrate concentration data from the literature allowed for the determination of enzyme-substrate interface dissociation constants and reaction order with respect to substrate concentration.

Study of the interfacial polycondensation of isocyanate in the preparation of benzalkonium chloride loaded microcapsules

The preparation of benzalkonium chloride loaded microcapsules was performed by interfacial polycondensation of isocyanates. The present study was made in order to clarify parameters affecting microcapsule wall formation during the course of polymerization. The results presented here show that many interrelated parameters are involved during the microcapsule formation. Each individual component introduced in the preparation was shown to have an effect either on the morphology of the microcapsules or on the mechanical resistance. Benzalkonium chloride seemed to interact mainly in the interfacial polymer precipitation step through a salt effect, or influence the polycondensation reaction rate acting as a catalyst. A contribution of the hydroxylic functions of the surfactant in the polycondensation reaction of the isocyanate was also highlighted. Finally, the organic phase composition was found to be able to modulate the reactivity of hydroxylic functions of the surfactant, leading to very slow reactions in pure xylene. These effects were related to the characteristics of the microcapsules obtained according to different compositions of the formulation system.

High-Resolution Transmission Electron Microscopy of AlAs/GaAs Interfacial Structure in the Projection

We have established an imaging condition of <110> high-resolution transmission electron microscopy (HREM) for edge-on observations of AlAs/GaAs interfacial step structures. It is shown that <110> HREM images of GaAs and AlAs show a remarkable contrast around a specimen thickness equivalent to the GaAs extinction distance due to a large difference in {111} amplitudes between GaAs and AlAs. A specimen preparation technique in which chemical etching was used to remove ion milling artifacts was also developed to accomplish this observation. Using this HREM method, we have made observations of AlAs/GaAs interfaces grown by molecular beam epitaxy, giving information about the straightness of the interfacial step edges and the distribution of step intervals. These results show a marked difference in the interfacial step structure between steps parallel to [11̄0] and steps parallel to [110].

High-resolution transmission electron microscopy of vicinal AlAs/GaAs interfacial structure

AlAs-on-GaAs vicinal interfaces grown by molecular beam epitaxy were investigated by high-resolution transmission electron microscopy in the 〈110〉 cross section, and the interfacial step structures were clearly observed. For the first time, distinctly different step interval distributions are found for interfaces tilted 2° from (001) towards (111)A and towards (111)B. Step bunching is observed for A steps, while a regular array of steps is observed for B steps. The edges of A steps are also straighter than those of B steps.

Strain relaxation induced by interfacial steps of GaAs/In 0.2Ga 0.8As superlattices

A theoretical and experimental study of GaAs/InGaAs strained-layer superlattices with ideal and stepped interfaces is presented. The stable atomic positions in the interface region with a single-layer step are determined from the elastic strain energy. A local strain field is set up by the rearrangement of the atoms near the step edge. Some fringe lines running approximately perpendicular to the epilayers have been observed in the reflection electron microscopy images of GaAs/InGaAs samples grown by molecular beam epitaxy. The observations can be understood by a simple interfacial step strain-field model. Our results confirm that dislocations are not the only mechanism for surface strain relief, another case is roughness.

Phospholipase A2 engineering. Structural and functional roles of highly conserved active site residues tyrosine-52 and tyrosine-73.

Site-directed mutagenesis was used to probe the structural and functional roles of two highly conserved residues, Tyr-52 and Tyr-73, in interfacial catalysis by bovine pancreatic phospholipase A2 (PLA2, overproduced in Escherichia coli). According to crystal structures, the side chains of these two active site residues form H-bonds with the carboxylate of the catalytic residue Asp-99. Replacement of either or both Tyr residues by Phe resulted in only very small changes in catalytic rates, which suggests that the hydrogen bonds are not essential for catalysis by PLA2. Substitution of either Tyr residue by nonaromatic amino acids resulted in substantial decreases in the apparent kcat toward 1,2-dioctanoyl-sn-glycero-3-phosphocholine (DC8PC) micelles and the v(o) (turnover number at maximal substrate concentration, i.e., mole fraction = 1) toward 1,2-dimyristoyl-sn-glycero-3-phosphomethanol (DC14PM) vesicles in scooting mode kinetics [Berg, O. G., Yu, B.-Z., Rogers, J., & Jain, M. K. (1991) Biochemistry 30, 7283-7297]. The Y52V mutant was further analyzed in detail by scooting mode kinetics: the E to E* equilibrium was examined by fluorescence; the dissociation constants of E*S, E*P, and E*I (KS*, KP*, and KI*, respectively) in the presence of Ca2+ were measured by protection of histidine-48 modification and by difference UV spectroscopy; the Michaelis constant KM* was calculated from initial rates of hydrolysis in the absence and presence of competitive inhibitors; and the turnover number under saturating conditions (kcat, which is a theoretical value since the enzyme may not be saturated at the interface) was calculated from the vo and KM* values. The results indicated little perturbation in the interfacial binding step (E to E*) but ca. 10-fold increases in KS*, KP*, KI*, and KM* and a less than 10-fold decrease in kcat. Such changes in the function of Y52V are not due to global conformational changes since the proton NMR properties of Y52V closely resemble those of wild-type PLA2; instead, it is likely to be caused by perturbed enzyme-substrate interactions at the active site. Tyr-73 appears to play an important structural role. The conformational stability of all Tyr-73 mutants decreased by 4-5 kcal/mol relative to that of the wild-type PLA2. The proton NMR properties of Y73A suggested significant conformational changes and substantially increased conformational flexibility. These detailed structural and functional analyses represent a major advancement in the structure-function study of an enzyme involved in interfacial catalysis.

Effects of gas bubbles on the current and potential profiles within porous flow-through electrodes

This paper presents a mathematical model to calculate the distributions of currenti(x), potentialE(x), gas void fraction e(x) and pore electrolyte resistivity ϱ(x) within porous flow-through electrodes producing hydrogen. It takes into consideration the following effects: (i) the kinetics of the interfacial charge transfer step, (ii) the effect of the non-uniformly generated gas bubbles on the resistivity of the gas-electrolyte dispersion within the pores of the electrode ϱ(x) and (iii) the convective transport of the electrolyte through the pores. These effects appear in the form of three dimensional groups i.e.K=i o αL where io is the exchange current density, α is the specific surface area of the electrode andL its thickness.υ=ρ 0 L whereρ 0 is the pore electrolyte resistivity and τ=ϖ/λQ where ϖ is a constant, λ=tortuosity/porosity of the porous electrode andQ is the superficial electrolyte volume flow rate within it. Two more dimensionless groups appear: i.e. the parameter of the ohmic effect Δ=KΦ/b and the kinetic-transport parameterI=Kτ. The model equations were solved fori(x),E(x), e(x) and ϱ(x) for various values of the above groups.

Denitrogenization Mechanism from Molten Steel by Flux Treatment.

Experiments of nitrogen desorption from molten steel by flux treatment have been proceeding and the reaction rates have been investigated. The flux of the CaO-Al2O3-CaF2-BaF2-SiO2-MgO system is effective for nitrogen desorption from molten steel, The nitrogen transfer rate from the steel to the gas phase could be increased by using the present flux, when the free surface of molten steel is exposed to argon atmosphere. The reason for the increase is that surface activity elements such as oxygen and sulphur in the molten steel were decreased and the interfacial resistance between the metal and gas was reduced. Under the present conditions, the nitrogen desorption from molten steel to the gas phase was more rapid than the nitrogen absorption of slag from the molten steel. The rate determinations step of nitrogen desorption from molten steel is changed from the interfacial chemical reaction step to the mass transfer of the metal phase, depending on the reduction of oxygen and sulphur in metal.

Two-phase kinetics of the solubilization in reverse micelles

The rates of the solubilization of methylene blue, water, and different electrolytes were measured in reversed AOT micelles in a liquid/liquid system using a two-phase stirred cell. Variation in the forced convection in the cell allowed the diffusional and chemical steps of the process to be differentiated. Applying the usual concept of mass transfer resistances it is possible to prove unequivocally that the rates of solubilization of methylene blue and water are controlled by the convective transport in the aqueous and organic phase, respectively. The transfer of metal cations, however, is a very slow process that is independent of the stirring rate and dependent on the interfacial area, which indicates that an interfacial step is rate controlling. With these results, particularly related to the kinetic behavior of mixed electrolytes, a mechanism for the interfacial solubilization is proposed, and some conclusions concerning the kind of aggregation are drawn. The values of the interfacial tension and tension gradients caused by convection suggest that appearance of a locally negative interfacial tension could be the reason for the encapsulation process.

Mass transfer of acetylacetone, ethylene glycol mono- n-butyl ether and ethylene glycol monophenyl ether across water-carbon tetrachloride and water-chloroform interfaces

The interfacial mass transfer kinetics of acetylacetone (acac), ethylene glycol mono- n-butyl ether (EGBE) and ethylene glycol monophenyl ether (EGPE) across water-carbon tetrachloride (Ccl 4) and water-chloroform (CHCl 3) interfaces were studied by stirring the two phases at various speeds, maintaining a quiescent interface with a constant area. In the analysis of data, two rate-determining steps consisting of a diffusion toward the interface and a mass exchange between the interface and bulk phases are considered. It is shown that the transfer of EGBE and EGPE in both CHCl 3 and Ccl 4 systems is controlled by the diffusion step even at high stirring speeds, whereas the transfer of acac in Ccl 4 is controlled by the interfacial exchange step at high stirring speeds. An irreversible transfer has been also observed in the EGBE and EGPE systems at low stirring speeds.