Effective Lateral Interactions Research Articles

The Continuum Quasichemical Approximation in Vapor–Liquid Systems

The continuum quasichemical approximation is used to improve the accuracy of describing molecular distributions in a vapor–liquid system. It considers displacements of the molecular center of mass from the center of a cell within the lattice gas model. It also allows for (as in its discrete variant) direct correlations of interacting molecules. The probability density of a molecule being inside the cell is used as a continuous function of its coordinate. An algorithm for solving a system of integral equations is developed with respect to the pair distribution function. The effect the continuum description of the particle distribution has on the concentration dependences of the main thermodynamic functions is investigated. The approach is shown to explain the concentration dependence of the parameter of effective lateral interaction.

Membrane-mediated interaction of amphipathic peptides can be described by a one-dimensional approach.

Amphipathic alpha-helical peptides, among other peripheral components of plasma membranes, are promising antimicrobial agents. Partial incorporation of a peptide into a lipid monolayer causes elastic deformations. Deformations induced by two peptides distant from each other are independent; when peptides get closer, interference between the deformations causes effective lateral interaction. We quantified the energy of membrane deformations for arbitrary configuration of two amphipathic peptides on the membrane surface. The global minimum of the deformation energy proved to be achieved when two parallel peptides are in registry at the distance of about 6 nm between the axes of peptides. The energy calculated in the unidimensional approach provides a good approximation for the dependence of the energy of peptides being in the registered configuration upon the distance between them, valid for a broad range of peptide lengths. The effective interactional length of peptides for the unidimensional approach is close to their actual length. If two parallel peptides are shifted along their axes with respect to each other, the interaction energy is also well approximated by the unidimensional potential, within the projection of one peptide onto the other. In the case when the axes of alpha helices cross at a substantial angle, the main contribution to peptide interactions comes from their edges: the effective length of peptides for the unidimensional approach is almost equal to the characteristic length of decay of deformations. Based on the results we obtained it can be concluded that interaction of membrane inclusions is quite adequately described by the potential calculated in the unidimensional approach.

Potentials with small‐angle neutron scattering technique for understanding structure–property relation of 3D‐printed materials

Carbon fiber (CF)‐embedded acrylonitrile butadiene styrene polymer composites printed on the large‐scale printer at Oak Ridge National Laboratory were investigated by small‐angle neutron scattering to correlate the microstructure of the composites with their mechanical strength. The microstructure of the polymer domains and the alignment of CF were characterized across the interfaces between layers of the hot‐melt extruded material and were compared with CF‐free ABS. The small‐angle neutron scattering data show that the CF‐containing material displays strong anisotropic scatterings suggesting molecular alignment along the printing direction that is not present in the CF‐free ABS. Scattering data analysis across the interfacial layer revealed enhanced molecular alignment along the printing direction near the boundaries and inhomogeneous size distribution of polymer domains upon the addition of CF. We attribute the compromised strength across interfacial layers from CF‐containing material to this inhomogeneous size distribution which prevents effective lateral interaction between layers. POLYM. ENG. SCI., 59:E65–E70, 2019. © 2018 Society of Plastics Engineers

Early stages of growth of Pb, Sn and Ge on Ru(0001): A comparative density functional theory study

As has recently been shown experimentally, adatoms from group 14 exhibit distinct growth characteristics on Ru(0001) at submonolayer coverages. In particular, Pb and Sn atoms agglomerate in dense islands of the c(2 × 4) or c(2 × 8) symmetry with local coverage of 1/2 monolayer (ML), respectively, already for much lower deposition doses, whereas Ge atoms self-assemble in the dilute 21×21 structure with adatom density of 1/7 ML. To trace the observed differences, we employed ab-initio density-functional-theory simulations and followed the early stages of growth of Pb, Sn, and Ge on Ru(0001) in a systematic manner by examining the potential energy surfaces for successive adsorption of up to several atoms of each element within a large surface unit cell. This way the effective lateral interactions between adatoms were recognized in the low-coverage regime, and their most favorable dimer and trimer configurations were identified. Interpreting them as initial building blocks of the adsorbate structure enabled us to clarify the tendency of various considered elements towards formation of a dense or dilute ordered overlayer on Ru(0001) at low nominal coverages.

Langmuir–Hinshelwood Mechanism Including Lateral Interactions and Species Diffusion for CO Electro-Oxidation on Metallic Surfaces

The electrochemical oxidation of CO on metallic surfaces following a lattice-gas model, including effective lateral interactions between one of the adsorbates and species diffusion, is studied. The reaction occurs through a Langmuir–Hinshelwood (LH) mechanism in which adsorbed CO reacts with adsorbed hydroxyl species. The mean field approximation and dynamic Monte Carlo simulations have been compared. The effect of the molecular distribution and surface mobility of reacting species on the potential dependence of the CO oxidation rate is analyzed. The inclusion of lateral interactions into the reaction mechanism reconciles different experimental observations, such as island formation and fast CO diffusion. Results highlight the importance of effective interactions in the reaction kinetics and suggest that they should be taken into account when interpreting experimental data. Simulations are useful for an improved qualitative understanding of the kinetics of CO oxidation and other electrochemical LH reactions.

Mean Field Approximation of Langmuir–Hinshelwood CO–Surface Reactions Considering Lateral Interactions

The mean field approximation to model the CO electrocatalytic oxidation on metal surfaces, including attractive and repulsive effective lateral interactions between the species, is employed. Adsorbed CO reacts with adsorbed OH from water dissociation through a Langmuir–Hinshelwood mechanism. Simulated results suggest alternative explanations to experimental observations during CO stripping oxidation on different electrodes by potential step chronoamperometry or voltammetry. Even though the model ignores higher order interactions, such as three-body or long-ranged interactions, the results extend the conceptual framework for transient kinetics and stripping voltammograms for CO oxidation on metallic surfaces or any other electrochemical Langmuir–Hinshelwood reaction. In spite of the high level of simplicity, the model is a promising tool for the understanding of the electrocatalytic activity and results were qualitatively compared with experimental results.

Blank voltammetry of hexagonal surfaces of Pt-group metal electrodes: Comparison to density functional theory calculations and ultra-high vacuum experiments on water dissociation

This paper discusses the relationship between the blank voltammetry of various hexagonally close-packed transition-metal electrodes, and DFT calculations of the binding energy of H, OH and O as well as UHV experiments on the dissociation of water on the same surfaces. The binding energies of H, OH and O can be used to predict the “phase diagram” of the electrode surface including the potentials of the transition between different surface states. The width of the voltammetric peaks corresponding to these transitions can be used to estimate the effective lateral interactions between the adsorbates involved, where effectively attractive interactions may often be explained by a replacement reaction. The detailed comparison shows that only for Pt(1 1 1) and Pd(1 1 1) the existence of a “double-layer region” is fully consistent with the available experimental and computational data. The Ru(0 0 0 1) surface does not have a double-layer region, but is covered with residues of water dissociation at every potential. The situation is unclear for Rh(1 1 1) and Ir(1 1 1) electrodes, for which a double-layer region has been claimed in the literature, but in both cases the claims are (at least partially) inconsistent with theoretical predictions.

Nanosecond pulsed laser ablation of silicon in liquids

Laser fluence and laser shot number are important parameters for pulse laser based micromachining of silicon in liquids. This paper presents laser-induced ablation of silicon in liquids of the dimethyl sulfoxide (DMSO) and the water at different applied laser fluence levels and laser shot numbers. The experimental results are conducted using 15 ns pulsed laser irradiation at 532 nm. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology shows that the explosive melt expulsion could be a dominant process for the laser ablation of silicon in liquids using nanosecond pulsed laser irradiation at 532 nm. Silicon surface’s ablated diameter growth was measured at different applied laser fluences and shot numbers in both liquid interfaces. A theoretical analysis suggested investigating silicon surface etching in liquid by intense multiple nanosecond laser pulses. It has been assumed that the nanosecond pulsed laser-induced silicon surface modification is due to the process of explosive melt expulsion under the action of the confined plasma-induced pressure or shock wave trapped between the silicon target and the overlying liquid. This analysis allows us to determine the effective lateral interaction zone of ablated solid target related to nanosecond pulsed laser illumination. The theoretical analysis is found in excellent agreement with the experimental measurements of silicon ablated diameter growth in the DMSO and the water interfaces. Multiple-shot laser ablation threshold of silicon is determined. Pulsed energy accumulation model is used to obtain the single-shot ablation threshold of silicon. The smaller ablation threshold value is found in the DMSO, and the incubation effect is also found to be absent.

An amplitude equation approach to contextual effects in visual cortex.

A mathematical theory of interacting hypercolumns in primary visual cortex (V1) is presented that incorporates details concerning the anisotropic nature of long-range lateral connections. Each hypercolumn is modeled as a ring of interacting excitatory and inhibitory neural populations with orientation preferences over the range 0 to 180 degrees. Analytical methods from bifurcation theory are used to derive nonlinear equations for the amplitude and phase of the population tuning curves in which the effective lateral interactions are linear in the amplitudes. These amplitude equations describe how mutual interactions between hypercolumns via lateral connections modify the response of each hypercolumn to modulated inputs from the lateral geniculate nucleus; such interactions form the basis of contextual effects. The coupled ring model is shown to reproduce a number of orientation-dependent and contrast-dependent features observed in center-surround experiments. A major prediction of the model is that the anisotropy in lateral connections results in a nonuniform modulatory effect of the surround that is correlated with the orientation of the center.

Spin polarization and the energetics of chlorine adsorption on the GaAs(110) surface

We have investigated the energetics of chlorine adsorption on the GaAs(110) surface using first principles plane-wave pseudopotential calculations. Both spin-polarized and spin-compensated calculations were performed for a set of eight different configurations of adsorbed chlorine atoms. We find that spin polarization plays an important role in the energetics of adsorption, particularly when we compare chlorine adatom binding energies for configurations with different numbers of “half-filled” dangling bonds. Our calculations show that scanning tunneling microscopy data of chlorine adatom configurations, with observations of islands at lower temperatures and chains at higher temperatures, can be understood in terms of the energetics. Previous calculations, which did not include spin-polarization effects, were not able to provide a definitive conclusion on these observations. From the energetics of the various configurations studied, we can extract a set of effective lateral interaction parameters between chlorine atoms adsorbed on GaAs(110). However, analysis of this set of parameters shows that the range of the effective lateral interactions probably extends to larger distances than the unit cells used in our calculations.

Equilibrium and non-equilibrium applications of lattice-gas models in electrochemistry

We discuss applications of statistical-mechanical lattice-gas models to study static and dynamic aspects of electrochemical adsorption. The strategy developed to describe 3pecific systems includes microscopic model formulation, calculation of zero-temperature phase diagrams, numerical simulation of thermodynamic and structural quantities at non-zero temperatures, and estimation of effective lateral interactions. We briefly review earlier work, including studies by Monte Carlo simulation of the adsorption of urea on Pt(100). As an illustrative example, we discuss in some detail recent applications to the underpotential deposition of Cu with sulfate on Au(111). Experimental and numerical results are included for equilibrium coverages and voltammetric currents at slow potential sweep rates, and new results from dynamic Monte Carlo simulations are presented. In particular, we present simulated results for current transients far from equilibrium, following sudden changes in the electrode potential.

Adsorption of urea on the Pt(100) electrode: experiments and lattice-gas modeling

We report a complete experimental and theoretical description of urea adsorption on the Pt(100) electrode. We have used voltammetry, radiochemistry, LEED and quantitative Auger electron spectroscopy, as well as Monte Carlo simulations of a lattice-gas model developed for this study. We demonstrate that the simulation results agree well with those of our experiments. These include an ordered c(2 × 4) adsorbed urea phase and sharply peaked voltammograms. We interpret our observations as associated with a phase transition between the urea phase for potentials on the positive side of the voltammetric peak and a (1 × 1) monolayer of hydrogen phase on the negative side. The main products of this research are the lattice-gas model that describes the microscopic adsorption geometry and the effective lateral interaction energies of the urea surface species involved in the phase transition behavior.

Lateral interactions and enhanced adsorption

We extend earlier work on the effects of lateral adsorbate-adsorbate interactions in systems with two different adsorbate species to consider in detail enhanced adsorption phenomena. We give a detailed explanation of the enhancement mechanism for a lattice-gas model in thermodynamic equilibrium, and provide explicit quantitative criteria which must be satisfied by the effective lateral interactions in systems exhibiting strong, intermediate, or weak enhancement behavior. It is the examination and understanding of the topological details of the ground-state and phase diagrams of the model that allow the formulation of these criteria. The theoretically obtained criteria are supported by precise numerical calculations (transfer-matrix with strip width six) of adsorption isotherms for a three-state lattice-gas model with nearest-neighbor interactions on a triangular lattice. The applicability of this theoretical framework is illustrated by an analysis of experimental adsorption isotherms for the electrochemical adsorption of naphthalene on copper and n-decylamine on nickel, previously obtained by Bockris et al. As suggested by Damaskin et al. we attribute the potential dependence of the organic coverage to the influence of coadsorbed hydrogen. We find that nonlinear least-squares fits of numerical lattice-gas isotherms to the experimental data produce good agreement between the experimental and numerical adsorption isotherms, as well as effective lattice-gas interaction energies consistent with independent estimates from the literature.

Ar and Kr adsorption on Ag(111)

Precise structural and thermodynamic studies of Kr and of Ar adsorbed on Ag(111) are made using low energy electron diffraction. The phase diagram, lattice constants of the unconstrained monolayer and of the monolayer in equilibrium with the bilayer, latent heats of adsorption and isosteric heats are measured. The results are similar to those of an earlier study of Xe adsorbed on Ag(111). The results are compared to model calculations using effective lateral interactions which are similar to those for Xe/Ag(111). Comparison of the results for Xe, Kr, and Ar on Ag(111) is made using corresponding states scalings. A comparison is also made with properties of the non-registry phases of Xe, Kr, and Ar on basal plane graphite.