Van Der Waals Attractive Energy Research Articles

Controlling the anisotropic self-assembly of polybutadiene-grafted silica nanoparticles by tuning three-body interaction forces.

Recently, the significant improvements in polymer composites properties have been mainly attributed to the ability of filler nanoparticles (NPs) to self-assemble into highly anisotropic self-assembled structures. In this work, we investigate the self-assembly of core-shell NPs composed of a silica core grafted with polybutadiene (PB) chains, generating the so-called "hairy" NPs (HNPs), immersed in tetrahydrofuran solvent. While uncoated silica beads aggregate forming uniform compact structures, the presence of a PB shell affects the silica NPs organization to the point that by increasing the polymer density at the corona, they tend to self-assemble into linear chain-like structures. To reproduce the experimental observations, we propose a theoretical model for the two-body that considers the van der Waals attractive energy together with the polymer-induced repulsive steric contribution and includes an additional three-body interaction term. This term arises due to the anisotropic distribution of PB, which increases their concentration near the NPs contact region. The resulting steric repulsion experienced by a third NP approaching the dimer prevents its binding close to the dimer bond and favors the growth of chain-like structures. We find good agreement between the simulated and experimental self-assembled superstructures, confirming that this three-body steric repulsion plays a key role in determining the cluster morphology of these core-shell NPs. The model also shows that further increasing the grafting density leads to low-density gel-like open structures.

Sistema de Gestión de la Calidad basado en ISO 9001:2015 y su influencia en la satisfacción de los servicios de una empresa automotriz peruana

La norma ISO 9001:2015 promueve adoptar un enfoque basado en procesos, garantizando la mejora continua de las organizaciones. La investigación tuvo por objetivo mejorar la satisfacción de los servicios brindados en una empresa automotriz peruana empleando un Sistema de Gestión de la Calidad basado en ISO 9001:2015. Se aplicó un diseño de tipo pre-experimental de corte cuantitativo, con una muestra de 30 clientes, a quienes se aplicó un instrumento con respuestas tipo Likert para evaluar la calidad del servicio y satisfacción del modelo de proceso aplicado. Los resultados evidenciaron mejoras con respecto al índice de satisfacción de los clientes, asimismo se realizó exitosamente el rediseño y estandarización de los procesos de la empresa. Se concluye que el sistema influenció significativamente en la satisfacción de los servicios brindados por la empresa automotriz.

Study on Colloidal Model of Petroleum Residues through the Attraction Potential between Colloids.

The samples of DaGang atmospheric residue (DG-AR), Middle East atmospheric residue (ME-AR), TaHe atmospheric residue (TH-AR), and their thermal reaction samples were chosen for study. All the samples were fractioned into six components separately, including saturates plus light aromatics, heavy aromatics, light resins, middle resins, heavy resins, and asphaltenes. The dielectric permittivity of the solutions of these components was measured, and the dielectric permittivity values of the components can be determined by extrapolation, which increased steadily from saturates plus light aromatics to asphaltenes. Moreover, the Hamaker constants of the components were calculated from their dielectric permittivity values. The Van der Waals attractive potential energy between colloids corresponding to various models could be calculated from the fractional composition and the Hamaker constants of every component. It was assumed that the cores of colloidal particles were formed by asphaltenes and heavy resins mainly; the other fractions acted as dispersion medium. For the three serials of thermal reaction samples, the Van der Waals attraction potential energy between colloids for this kind of model was calculated. For TH-AR thermal reaction samples, the Van der Waals attraction potential energy presented the maximum as thermal reaction is going on, which was near to the end of coke induction period.

Particle Dispersion Model for Predicting the Percolation Threshold of Nano-Silver Composite

In the current study, particle dispersion models of electrically conductive adhesives are developed through establishing the effects of 80 nm of silver (Ag) particles by employing van der Waals attraction energy, which acts as an energy of interaction of particles that are embedded in an epoxy solution system. The arrangement of the particle dispersion in the epoxy colloidal system is determined by identifying the morphology in the experimental works. The characteristics of particle dispersion are analyzed based on electrical conductivity effect with respect to the volume fraction factor, which is set in the range of 2–8 vol%. In developing the particle dispersion model, the model was simulated through representative volume elements (RVEs) by implementing the standard error of the RVE size model versus electrical conductivity (S/cm2). The model size was determined by the ratio between the width of the RVE model and the particle size, which was set as 3, 4, 5, 6, 7, and 8. The accurate size that represents the precision of the estimation of the electrical conductivity result has been successfully determined by a particle dispersion model developed at a model size of δ = 6. A significant improvement in the optimization of the particle dispersion model’s synergistic effect on electrical conductivity has been obtained with new techniques in relation to the experimental data. The numerical results were almost the same as the experimental results, where the percolation threshold occurred at a filler loading of 6 vol% and reached at 1.32 × 10−1 S/cm

Solid-State NMR-Restrained Ensemble Dynamics of a Membrane Protein in Explicit Membranes

Solid-state NMR has been used to determine the structures of membrane proteins in native-like lipid bilayer environments. Most structure calculations based on solid-state NMR observables are performed using simulated annealing with restrained molecular dynamics and an energy function, where all nonbonded interactions are represented by a single, purely repulsive term with no contributions from van der Waals attractive, electrostatic, or solvation energy. To our knowledge, this is the first application of an ensemble dynamics technique performed in explicit membranes that uses experimental solid-state NMR observables to obtain the refined structure of a membrane protein together with information about its dynamics and its interactions with lipids. Using the membrane-bound form of the fd coat protein as a model membrane protein and its experimental solid-state NMR data, we performed restrained ensemble dynamics simulations with different ensemble sizes in explicit membranes. For comparison, a molecular dynamics simulation of fd coat protein was also performed without any restraints. The average orientation of each protein helix is similar to a structure determined by traditional single-conformer approaches. However, their variations are limited in the resulting ensemble of structures with one or two replicas, as they are under the strong influence of solid-state NMR restraints. Although highly consistent with all solid-state NMR observables, the ensembles of more than two replicas show larger orientational variations similar to those observed in the molecular dynamics simulation without restraints. In particular, in these explicit membrane simulations, Lys40, residing at the C-terminal side of the transmembrane helix, is observed to cause local membrane curvature. Therefore, compared to traditional single-conformer approaches in implicit environments, solid-state NMR restrained ensemble simulations in explicit membranes readily characterize not only protein dynamics but also protein-lipid interactions in detail.

Effect of Rough Nanoparticle Orientation on DLVO Energy Interaction

This study investigated the effect of the orientation of rough nanoparticles on Derjaguin and Landau, Verwey and Overbeek (DLVO) energy interaction. Rippled sphere model was used to survey van der Waals attraction energy and repulsive double-layer interaction energy between nanoparticles. The effect of particle size, asperity size, number of asperities, and concentration was studied for different orientations. Spherical coordinates were used to evaluate the effect of changes in orientation under controlled conditions. Surface element integral method was used to calculate DLVO energy interactions between rough particles at specific orientations. The DLVO energy results show that a change in orientation significantly affected the stability of the nanoparticles. The stability of dispersion varied as the contact surface between nanoparticles changed.

Developed Mathematical Equation for Predicting Turbidity Removal Using Synthetic Fiber Filter

A new mathematical model for predicting the contact efficiency of a single collector in the treatment of turbid water is presented in this study. The removal mechanisms applied were interception, diffusion, and sedimentation. In order to obtain the model for the single-collector efficiency, the effect of the density current caused by turbidity and differences in temperature was considered and the impact of multiple-size particles was also included when calculating the respective mechanisms. The sticking coefficient was also calculated, including the effect of the van der Waals attraction energy and electrical double layer forces. The result of the modeling indicated that the predicted efficiency using the developed equation showed good agreement with the experimental values over a wide range of parameters and the increases and decreases in the model performance were simulated very well. The quality of the data was analyzed and this indicated that the experimental work and the developed model showed fairly good agreement.

Rationalizing Catalyst Inks for PEMFC Electrodes Based on Colloidal Interactions

A preliminary kinetic model was developed for polymer electrolyte membrane fuel cell (PEMFC) catalyst inks in order to understand their particle stability. The Derjaguin Landau Verwey Overbeek (DLVO) model containing van der Waals attractive and electrostatic repulsive interaction energy was applied to the aqueous ink dispersions, while a modified DLVO type interaction containing a Coulombic term instead of the electrostatic term was applied to the non-aqueous dispersions. Solvents were compared based on their particle size distribution and stability ratios. Results show that the carbon black particles are stable in a higher dielectric medium whereas they tend to aggregate in a lower dielectric medium. A low ionic concentration for the aqueous medium also helped to improve the ink stability by providing a better electrostatic particle repulsion. Experiments conducted with ethyl acetate, iso-propanol and deionized water agree with the model predictions.

A theoretical approach for estimation of ultimate size of bimetallic nanocomposites synthesized in microemulsion systems

In this research a new idea for prediction of ultimate sizes of bimetallic nanocomposites synthesized in water-in-oil microemulsion system is proposed. In this method, by modifying Tabor Winterton approximation equation, an effective Hamaker constant was introduced. This effective Hamaker constant was applied in the van der Waals attractive interaction energy. The obtained effective van der Waals interaction energy was used as attractive contribution in the total interaction energy. The modified interaction energy was applied successfully to predict some bimetallic nanoparticles, at different mass fraction, synthesized in microemulsion system of dioctyl sodium sulfosuccinate (AOT)/isooctane.

The stability of BaFe12O19 nanoparticles in polar solvents

We have studied suspensions of hard-magnetic BaFe12O19 particles in water, ethanol and 1-butanol. The surfaces of these particles were previously modified with the surfactant dodecylbenzylsulphonic acid. The stabilities of the suspensions were estimated from their saturation concentrations and zeta potentials. We found that the 1-butanol suspensions were more stable than the ethanol-based suspensions and much more stable than the water-based suspensions. We analyzed the suspensions and the dispersed particles using gravimetry, conductometry and transmission electron microscopy, measured their zeta potentials, and calculated the interparticle-attraction interaction energies due to the van der Waals and magnetic dipole–dipole forces. The magnitudes of the attraction energies varied significantly with the particles’ sizes and the separation distances between the particles, and we found that the contribution of the van der Waals attraction energy can be neglected with respect to the magnetic dipole–dipole attraction. The observed differences in the stability of the suspensions were explained on the basis of the calculated electrostatic and steric repulsion energies. Electrosteric stabilization was possible in the 1-butanol and the ethanol for particles with radii and thicknesses up to 15 nm, while a too small electrostatic repulsion and the absence of steric repulsion in the water resulted in rapid agglomeration.

Coagulation of tobacco mosaic virus in alcohol–water–LiCl solutions

The coagulation and colloidal stability of tobacco mosaic virus (TMV) in alcohol–water–LiCl solutions were studied. Without the addition of LiCl salt, the coagulation was promoted by the increase of hydrophobicity of the alcohols that is proportional to their alkyl chain length and concentration. Addition of the LiCl salt reduced the electrostatic repulsion between TMV particles resulting in coagulation in methanol–water and ethanol–water solutions. In water–alcohol–LiCl mixture, the coagulation of TMV was driven by both the hydrophobic interaction of the solution and the screening effect of the salt simultaneously. To understand the particle–particle interaction during the coagulation, the interaction energy was calculated using DLVO theory. Considering the electrostatic repulsive energy, van der Waals attractive energy, and hydrophobic interaction energy, the total energy profiles were obtained. The experiment and model calculation results indicated that the increase of alcohol concentration would increase hydrophobic attraction energy so that the coagulation is promoted. These results provide the fundamental understanding on the coagulation of biomolecular macromolecules.

Control of the Structure of Self-Spreading Lipid Membrane by Changing Electrolyte Concentration

We have controlled the structure of self-spreading lipid bilayer membranes prepared on surface-oxidized silicon substrates by changing electrolyte concentration. Analysis of the fluorescence intensity, considering the optical interference effect, clarified the stacking structure of the lipid membrane. By varying the electrolyte concentration, we can vary the number of single multilamellar lobes adsorbed on the underlying self-spreading bilayer. This dependence of the stacking ability on the electrolyte concentration was investigated on the basis of changes in the bilayer-lobe interaction energies, including van der Waals, electrostatic double layer, and hydration interaction energies. Theoretical estimation suggests that the observed electrolyte concentration dependence can be explained by the combination of the van der Waals attractive interaction energy and the repulsive double-layer interaction energy.

Consequences of H-dimerization on the photophysics and photochemistry of oxacarbocyanines

The photophysical/photochemical behaviour of the monomers and the H dimers of four oxacarbocyanines (dyes D–G in the scheme) was investigated in water. In contrast with the usually observed effect of H dimerization, the dimers of dyes D–G were found to fluoresce with efficiencies comparable to or larger than those of the corresponding monomers. Analysis of the decay paths of the lowest excited singlet state showed, however, that dimerization causes a decrease of the radiative rate constants and an enhancement of intersystem crossing to the triplet manifold, as expected from application of exciton theory to a model H dimer. Twisting about one of the polymethine bonds contributes to the decay from the spectroscopic minimum of monomers, though yielding a rather small amount of a distorted cis isomer. The process is inhibited in dimers, likely due to a pronounced increase of activation energy connected with a loss of van der Waals attractive energy at the twisted geometry.

Swelling behavior of PMMA- g-PEO microgel particles by organic solvents

Aqueous dispersions of cross-linked poly(methylmethacrylate)- g-poly(ethylene oxide) [PMMA- g-PEO] microgel particles have been prepared from mixtures of methylmethacrylate [MMA] and MMA–PEO macromonomer, with ethylene glycol dimethacrylate [EGDM] as the cross-linking monomer (0.2–0.5% wt%). The hydrodynamic radius of these (unswollen) microgel particles ranged from 73 to 85 nm, and the particles were essentially monodisperse with regard to their size distribution. Their swelling behavior has been investigated in the presence of both water-miscible and water-immiscible organic solvents. In general, with the addition of a water-miscible solvent, deswelling behavior was observed. However, the microgel particles were swollen on addition of 1,4-dioxan, which is a good solvent for PMMA. With water-immiscible organic solvents, the extent of swelling depended on the solvency properties of the organic liquid for PMMA. In the presence of benzene, the somewhat large increases in particle size have been attributed to weak flocculation. This has been assumed from an estimate of the van der Waals attraction energy between the swollen microgel particles.

Calculation of the van der Waals force between a spherical particle and an infinite cylinder

Formulae for the van der Waals attraction energy and force between a spherical particle and an infinite cylinder are derived by the method of additive summation of the pair interactions described by the potential of the general form U m =− α m ρ − m . The formula of Rosenfeld and Wasan for the non-retarded vdW force between a sphere and a cylinder ( m=6) is confirmed and the compact expression for the retarded force ( m=7) is obtained. The comparison is given for the forces of the retarded vdW interaction between a sphere and a cylinder, another sphere, a row of spheres and a half-space. Also, the compact formulae for the energy of the vdW interaction of a point-like particle (atom, molecule) with a sphere and a cylinder are derived for the case of arbitrary m.

Generation of defects in model lubricant monolayers and their contribution to energy dissipation in friction

The structural, mechanical (friction) and spectroscopic properties of model lubricant films made of self-assembled and Langmuir–Blodgett monolayers on quartz, mica and gold have been investigated with atomic force microscopy, the surface forces apparatus and sum-frequency generation. In these films, the molecules tend to form densely packed structures, with the alkane chains mostly vertical and parallel to each other. The SFG results suggest that under moderate pressures of a few tens of MPa, the methyl end group of the alkane chains is rotated to accommodate a terminal gauche distortion. The molecule, however, retains its upright close-packed structure with a lattice periodicity when ordered, which can be resolved by AFM. At pressures above 0.1 GPa, changes in the form of collective molecular tilts take place that lower the height of the monolayer. Only certain angles of tilt are allowed that are explained by the interlocking of methylene units in neighboring chains. The discrete angular tilts are accompanied by increases in friction. A model based on the van der Waals attractive energy between chains is used to explain the stability of the films and to estimate the cohesive energy changes during tilt and, from that, the increases in friction force.

Influence of stiffness of carbon-nanotube probes in atomic forcemicroscopy

We report the influence of stiffness of carbon nanotubes for probesof a scanning probe microscope on images. Multiwalled carbon-nanotube probesare fabricated by manipulation under the direct view of a scanning electronmicroscope. Using this manipulation, it is also revealed that a Hamakerconstant of 60×10-20 J for the van der Waals attraction is for a sidewall ofthe nanotube and the metallic surface at a vacuum of ~10-3 Pa. The force curvemeasurements at a steep slope in air reveal the influence of the force actingnot only on the tip of the probes but also on the wall of the tip. The originof this effect is discussed in terms of the van der Waals attraction and theadhesive energy estimated from the force curve. This phenomenon is suppressedusing a nanotube probe consisting of bundled nanotubes at the base toimprove the stiffness for samples of high roughness.

Electrostatic Interactions between Two Identical Thin Disks of Arbitrary Orientation in an Electrolyte Solution

The electrostatic interactions between two identical, sufficiently thin, charged disks in an electrolyte solution are investigated. The charged entities can assume an arbitrary orientation in space, and their surfaces are either at constant charge density or at constant potential. We show that if the surface potential is low, the governing Poisson−Boltzmann equation (PBE) can be solved analytically for the former. A boundary collocation method is adopted to solve the PBE for the latter. The interaction energies of the system under consideration including the electrostatic repulsive energy and the van der Waals attractive energy are estimated. We found that for a fixed center-to-center distance between two disks these energies are at maximum if they are on the same plane and at minimum if they are in an opposite position. The difference between the maximal and the minimal electrical repulsion energies increase with κr0 for both constant surface charge and constant surface potential, κ and r0 being, respect...

The effect of surface modification on the stability characteristics of poly(N-isopropylacrylamide) latices under Brownian and flow conditions

The stability of thermoreversible microgel particles of poly(N-isopropylacrylamide) having carboxylate surface charge groups has been studied in the presence of electrolyte and non-adsorbing polymer. Methylation of the surface charge groups leads to a decrease in the electrophoretic mobility of the particles and also the interparticle electrostatic repulsive potential, resulting in the material becoming more susceptible to flocculation. The Hamaker constant of the microgel particles increases with the decrease in the hydrodynamic size of the particles following heating. This brings about an increase in the van der Waals attractive energy which results in the particles aggregating in the presence of sufficient electrolyte. Under conditions of flow through membranes, where shearing forces are operative, the flocculation observed following the heating of the dispersions results in the blockage of pores.

Van der Waals interaction at a material wedge

The problem of evaluating the van der Waals attraction energy of an atom or microparticle adsorbed near the edge of a straight material wedge of arbitrary opening angle is considered. In order to disentangle more clearly the effect of the wedge geometry from the material parameters, a simple model is used in which the particle is represented by a harmonic oscillator and the substrate by a perfect conductor. By a straightforward extension of the method of images of classical electrostatics, the dipolar coupling of the particle to the substrate is constructed from which new oscillator frequencies can be determined as a function of its position relative to the edge and of the wedge angle. The van der Waals energy is given by the shift in zero-point energy of the oscillator coupled to itself via its images. For a physisorbed atom crossing the edge of a wedge material, the detailed shape of the van der Waals energy barrier (respectively, well) offered by a convex (respectively, concave) wedge is displayed and compared with approximate results deduced from a pairwise summation of Lennard-Jones interactions. The interest of these results for the dynamics and kinetics of sorption by microporous solids is briefly discussed.