Interfacial Steps Research Articles

EXTRACTION KINETICS OF ZINC BY TRILAURYLAMM0NIUM CHLORIDE AT DIFFERENT CHLORIDE CONCENTRATIONS

ABSTRACT The rate of the extraction of zinc by trilaurylammonium chloride (TLAHC1) dissolved in toluene was investigated at different aqueous chloride concentrations, using lithium chloride as bulk electrolyte, ranging from 0.5 mol.dm -3 to 2.0 mol dm -3. The experiments were performed using a modified Lewis cell at 25 °C. The rate of metal extraction was found to increase by increasing the aqueous chloride concentration. The data treatment indicated that ZnCl2 and ZnCl4 2- are both kinetically active species. The addition reaction with ZnCl2 is favoured at low chloride concentration while the anion exchange of ZnCl4 2- prevails at high chloride concentration. The addition reaction was explained in terms or interfacial reaction steps. The anion exchange reaction, due to its fast nature, seems to be controlled by diffusion.

Actual comparison of experimental and simulated lattice images of the GaAs/AlAs interface

MOVPE grown (001) GaAs/AlAs superlattices were investigated with high resolution transmission electron microscopy. Images along [110] and [100] orientations were made and compared with calculated images. Good agreement is shown between experimental and calculated images for a consistent set of images with varying defocus and sample thickness values. Interfacial steps were observed in a superlattice, grown on a substrate misoriented 4° to [0 1 0]. It is clearly shown that the steps are inclined with respect to the [100] imaging direction. It is argued that to distinguish GaAs and AlAs layers in lattice images, differences in the fine pattern are of more use than differences in the mean intensity. The [110] imaging orientation has several advantages over the much advocated [100] orientation.

Effect of hydrophobicity of extractant on extraction kinetics of copper with N-8-quinolylsulfonamide.

Using four kinds of alkylbenzene-substituted N-8-quinolylsulfonamides of different alkyl-chain length and their copper chelate complexes, kinetic studies concerning the extraction of copper with extractant and the stripping of copper with sulfuric acid were carried out in a membrane extractor using a hollow fiber. It was found that the interfacial reaction between the adsorbed copper complex (CuR+ad) and the adsorbed extractant (HRad) is a rate-controlling step among the interfacial reaction steps of the extraction and stripping processes, regardless of the alkyl-chain length of the extractants, and that the reaction rate constant of the ratecontrolling step is proportional to the equilibrium constant of this reaction for each of the four kinds of extractants. The validity of a diffusion model with the interfacial reaction was confirmed by agreement of the observed extents of copper extracted and stripped with those calculated over a wide range of experimental conditions.

Review of the principal contrast effects observed at interphase boundaries using transmission electron microscopy

SUMMARYThis review is concerned with the interpretation of contrast features observed at interphase interfaces using conventional transmission electron microscopy. The principal features which arise are interfacial dislocations and steps, four types of fringes, namely thickness, Δw, displacement and moiré fringes, and lattice images. The contrast mechanisms leading to these features are discussed in the framework of the dynamical theory of electron diffraction. In particular, the influence on image contrast of the cyrstal composition and structure, the interfacial configuration and the crystallographic relationship of the two phases in the specimen is considered. In general, electron diffraction in bi‐crystalline specimens is complicated, and reliable interpretation can only be carried out when the diffraction conditions have been carefully established. The diffraction conditions most widely used are discussed, and their relative merits compared.

Liquid solubilization dynamics: Oleic acid in bile salt

The solubilization dynamics of oleic acid by aqueous sodium taurodeoxycholate solutions at 37°C does not conform to previous resistance in series models involving diffusion and interfacial steps. This is especially true as the ionic strength of the aqueous solution is increased by adding NaCl or by increasing the taurodeoxycholate concentration above 2%. Experiments conducted with a liquid/liquid stirred cell show that as the ionic strength is increased, the flux of oleic acid into the aqueous solution exceeds a reasonable value for the diffusion limit, assuming that oleic acid diffuses only in mixed micelles, and that the flux becomes independent of flow at these high ionic strengths. Two alternative models for the solubilization process are proposed. One involves a rapid exchange of oleic acid between mixed micelles and oleic acid free micelles in parallel with mixed micelle diffusion; and the second involves formation of a small emulsion droplet at the interface, which contains significantly more oleic acid than a mixed micelle, and which rapidly dissolves into micellar solution as the droplet leaves the interface.

Electron microscope studies of the structure and propagation of the Pd2Si/(111)Si interface

Abstract The structure of the epitaxial Pd2Si/(111)Si interface has been investigated by transmission electron microscopy. Observations normal to the interface using the weak-beam technique demonstrate the presence of misfit dislocations and support a model where atomically smooth regions of the interface are coherent and where misfit dislocations are associated with interfacial steps. High-resolution lattice images of (110) cross-sectional specimens are consistent with this model, showing characteristic image features which may be explained using theoretical multislice image simulations. It is shown that there are additional image features attributable to palladium-rich surface overlayers present on the cross-sections. The determination of the atomic structure of the interface is discussed.

Atomically sharp cracks in brittle solids: an electron microscopy study

The issue of bond rupture versus microplasticity as an essential mechanism of crack propagation in brittle solids is addressed. A detailed survey of existing theoretical and experimental evidence relating to this issue highlights the need for direct observations of events within the crack-tip “process zone”, at a level approaching 10 nm. Transmission electron microscopy is accordingly used to study arrested cracks about sharp-contact (Vickers indentation and particle impact) sites in Si, Ge, SiC and Al2O3. The nature of the deformation which accommodates the irreversible contact impression is first investigated, in the light of Marsh's proposal of an “equivalence” between indentation and crack-tip zone processes. Interfacial and tip regions of the surrounding cracks are then examined for any trace of a plasticity-controlled fracture process. Dislocation-like images are indeed evident at the crack planes, but these are shown to be totally inconsistent with any conventional slip mechanism. The close connection between the dislocation patterns and moire fringe systems along the cracks points to “lattice mismatch” contrast in association with a partial closure and healing operation at the interface. Analysis of all other details in the crack patterns, e.g. the presence of a crack-front contrast band indicative of a residual strain field and the disposition of interfacial fracture steps relative to the dislocation/moire system, reinforces this interpretation. It is concluded that the concept of an atomically sharp crack provides a sound basis for the theory of fracture of brittle solids.

Dynamic Interfacial Electrochemical Phenomena at Living Cell Membranes: Application to the Toad Urinary Bladder Membrane System

The concept of electrochemical information transfer in vivo has been studied using the isolated toad urinary bladder membrane system to evaluate a recently developed approach to membrane impedance which emphasized the coupling of interfacial processes to standard transport phenomena. Laplace plane analysis was utilized to examine frequency response from 1 Hz to 1 MHz. The results indicate that, under the linear conditions of this experiment, Na+ion is the predominant current carrier and that the rate‐determining step is its penetration into the membrane phase from the aqueous phase. Thus, membrane transport appears to be a very rapid step, whereas the interfacial step of phase transfer regulates transport in this system. The implication of this result in terms of potential dependent interfacial steps in the control of cellular function is discussed.