Varying Surface Charge Density Research Articles

Second harmonic generation null angle polarization analysis for determining interfacial potential at charged interfaces.

Methods of quantifying the electrostatics of charged interfaces are important in a range of research areas. The surface-selective nonlinear optical technique second harmonic generation (SHG) is a sensitive probe of interfacial electrostatics. Recent work has shown that detection of the SHG phase in addition to its amplitude enables direct quantification of the interfacial potential. However, the experimental challenge of directly detecting the phase interferometrically with sufficient precision and stability has led to the proposal and development of alternative techniques to recover the same information, notably through wavelength scanning or angle scanning, each of which has their own associated experimental challenges. Here, we propose a new polarization-based approach to recover the required phase information, building upon the previously established nonlinear optical null ellipsometry (NONE) technique. Although NONE directly returns only relative phase information between different tensor elements of the second-order susceptibility, it is shown that a symmetry relation that connects the tensor elements of the potential-dependent third-order susceptibility can be used to generate the absolute phase reference required to calculate the interfacial potential. The sensitivity of the technique to potential at varying surface charge densities and ionic strengths is explored by means of simulated data of the silica:water interface. The error associated with the use of the linearized Poisson-Boltzmann approximation is discussed and compared to the error associated with the precision of the measured NONE null angles. Overall, the results suggest that NONE is a promising approach for performing phase-resolved SHG based quantification of interfacial potentials that experimentally requires only the addition of standard polarization optics to the basic single-wavelength, fixed-angle SHG apparatus.

Exploring the structural conformations of amyloid-beta oligomers on self-assembled monolayers and in water under confinement, a molecular dynamics study.

Alzheimer's disease (AD) is a devastating neurological disorder, affecting millions of people worldwide. The aggregation of amyloid-beta (Aβ) peptides and the intermediate protein tau into amyloid fibrils is believed to be a critical event in the pathogenesis of AD. Amyloid fibrils represent the endpoints of aggregation, and it is widely believed that their precursors—the soluble oligomers—are the more important neurotoxins. However, there is limited structural information available on Aβ oligomers. Although atomic force microscopy (AFM) is a widely-used method employed to study the conformation of Aβ oligomers, the resolution of AFM is limited and unable to discern atomistic structural features. Here, we construct a self-assembled monolayer (SAM), mimicking the surface properties of mica (used in AFM), with varying surface charge densities. Then, we apply the enhanced sampling algorithm REST2 (replica exchange with solute tempering) from the NAMD package, to efficiently sample the protein conformations on the SAM surface. Our simulations have resulted in cross β-sheet formation between Aβ monomers, similar to that observed experimentally. In particular, we find that residues 17-21 and 35-40 are involved in β-sheet formation, the former of which has been observed in the Aβ fibril structure. We also observe the prominence of the intramolecular Asp23-Lys28 salt bridge, which has been found to play a significant role in the formation of Aβ fibrils. Further, we notice that an asymmetric charge distribution on the SAM surface tends to result in greater β-sheet formation. Our simulations of Aβ monomers in TIP4P water, confined using a harmonic wall, similarly result in cross β-sheet formation among the monomers. Confinement has been found to increase the conformational stability of proteins, suggesting the possibility of Aβ formation in the crowded environment of the cell.

Surface roughness noise analysis and comprehensive noise effects on depth-dependent coherence time of NV centers in diamond

Noise is a detrimental issue for nitrogen-vacancy (NV) centers in diamond, causing line broadening and decreasing the coherence time (T2). Following our previous electric and magnetic field noise work, we investigate noise caused by the diamond surface roughness, which is a source for charge density fluctuations and incoherent photon scattering. We find that the varying surface charge density noise source is prevalent throughout the entire NV dynamical decoupling frequency range, while the photon scattering noise is almost negligible. Next, we combine the results from various noise sources to perform comprehensive analyses on T2 and how it varies with NV depth. At a given NV depth of 5 nm below a hydrogen- or fluorine-terminated surface, we find that these magnetic nuclei reduce the NV coherence time the most, followed by the surface electric field noise sources. The photon scattering and bulk magnetic field noise effects on T2 are weak compared to the varying charge density, electric dipole, and surface impurity noise. However, with oxygen surface termination, the surface electric field noise becomes comparable to the surface magnetic field noise. Our calculated values of T2,Hahn (few microseconds to ten microseconds) are in good agreement with the experimental values reported elsewhere. Finally, we calculate an anticipated signal-to-noise ratio (SNR) for NV AC magnetometry of external nuclear spins. In our simplified assessment, where some depth-dependent parameters (e.g. NV conversion efficiency) are held constant, we find that shallower NV layers should yield the best SNR, which is consistent with experimental findings.

Response modeling of single SnO2 nanowire gas sensors

The response of single SnO2 nanowire gas sensors with different diameters between 20 and 140 nm are evaluated by calculating the nanowire conductivity as a function of the surface charge density. The procedure involves the numerical solution of the Poisson-Boltzmann equation for the electrostatic potential in cylindrical geometry in order to model the depletion region and band bending at the SnO2 nanowire surface. In the model we take into account varying surface charge densities σ and bulk electron concentrations n0 to calculate the electrical conductivity. Considering the fact that the surface charge density depends on the nanowire surface interactions with ambient gas, the model allows us to simulate the sensor response when the nanowire is employed as gas sensing component. We report a saturation in depletion length λ versus surface charge density σ which is the principal reason for limiting the sensor responses. The results also show that the conductivity is decreasing by increasing surface charge density, the smaller the nanowire diameter the steeper the decrease. As a result the nanowire response is proportional to 1/d where d is the nanowire diameter. Furthermore, we argue about the validity of the modeling results and their relevance to experimental findings on SnO2 nanowire based gas sensors reported in literature.

Direct observation of fouling phenomena during cross-flow filtration: Influence of particle surface charge

Membrane fouling is inherent in all types of membrane filtration. In microfiltration, particles are often deposited on the membrane surface, forming a filter cake whose structure and behaviour play crucial roles in the filterability of suspensions. Filter cake formation is often measured or calculated indirectly from pressure flux data. In this study, filter cakes were characterized by direct optical observation of the surface of a single hollow-fibre membrane during cross-flow filtration. Monodisperse polymer model particles with varying surface charge densities were suspended in water and filtered. Results indicate a significant difference in the behaviour of filter cakes formed by differently charged particles. Increasing the particle surface charge generally increases both the specific resistance and compressibility of the filter cake, whereas lowering the charge increases the filter cake growth rate and steady-state thickness. Attempts to remove fouling were made using relaxation, backwash, and cross-flow shear. Relaxation alone did not remove the filter cake and recompression happened quickly. Effectively removing the filter cake required a combination of backwash and high cross-flow shear. However, a thin layer of particles remained on the membrane after cleaning, and it was demonstrated that this layer accounts for a significant amount of the overall filter cake resistance.

Structure and Li+ ion transport in a mixed carbonate/LiPF6 electrolyte near graphite electrode surfaces: a molecular dynamics study.

Electrolyte and electrode materials used in lithium-ion batteries have been studied separately to a great extent, however the structural and dynamical properties of the electrolyte-electrode interface still remain largely unexplored despite its critical role in governing battery performance. Using molecular dynamics simulations, we examine the structural reorganization of solvent molecules (cyclic ethylene carbonate : linear dimethyl carbonate 1 : 1 molar ratio doped with 1 M LiPF6) in the vicinity of graphite electrodes with varying surface charge densities (σ). The interfacial structure is found to be sensitive to the molecular geometry and polarity of each solvent molecule as well as the surface structure and charge distribution of the negative electrode. We also evaluated the potential difference across the electrolyte-electrode interface, which exhibits a nearly linear variation with respect to σ up until the onset of Li+ ion accumulation onto the graphite edges from the electrolyte. In addition, well-tempered metadynamics simulations are employed to predict the free-energy barriers to Li+ ion transport through the relatively dense interfacial layer, along with analysis of the Li+ solvation sheath structure. Quantitative analysis of the molecular arrangements at the electrolyte-electrode interface will help better understand and describe electrolyte decomposition, especially in the early stages of solid-electrolyte-interphase (SEI) formation. Moreover, the computational framework presented in this work offers a means to explore the effects of solvent composition, electrode surface modification, and operating temperature on the interfacial structure and properties, which may further assist in efforts to engineer the electrolyte-electrode interface leading to a SEI layer that optimizes battery performance.

Effect of Dehydration on the Interfacial Water Structure at a Charged Polymer Surface: Negligible χ(3) Contribution to Sum Frequency Generation Signal

Interfacial water structure at charged surfaces plays a key role in many physical, chemical, biological, environmental, and industrial processes. Understanding the release of interfacial water from the charged solid surfaces during dehydration process may provide insights into the mechanism of protein folding and the nature of weak molecular interactions. In this work, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by quartz crystal microbalance (QCM) measurements, has been applied to study the interfacial water structure at polyelectrolyte covered surfaces. Poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) chains are grafted on solid surfaces to investigate the change of interfacial water structure with varying surface charge density induced by tuning the solution pH. At pH ≤ 7.1, SFG-VS intensity is linear to the loss of mass of interfacial water caused by the dehydration of PDMAEMA chains, and no reorientation of the strongly bonded water molecules is observed in the light of χ(ppp)/χ(ssp) ratio. χ((3)) contribution to SFG signal is deduced based on the combination of SFG and QCM results. It is the first direct experimental evidence to reveal that the χ((3)) has a negligible contribution to SFG signal of the interfacial water at a charged polymer surface.

Characterization of surface charge and mechanical properties of chitosan/alginate based biomaterials

This study aims to examine mechanical properties and surface charge characteristics of chitosan/alginate-based films for biomedical applications. By varying the concentrations of chitosan and alginate, we have developed films with varying surface charge densities and mechanical characteristics. The surface charge densities of these films were determined by applying an analytical model on force curves derived from an atomic force microscope (AFM). The average surface charge densities of films containing 60% chitosan and 80% chitosan were found to be − 0.46 mC/m 2 and − 0.32 mC/m 2, respectively. The surface charge density of 90% chitosan containing films was found to be neutral. The elastic moduli and the water content were found to be decreasing with increasing chitosan concentration. The films with 60%, 80% and 90% chitosan gained 93.5 ± 6.6%, 217.1 ± 22.1% and 396.8 ± 67.5% of their initial weight, respectively. Their elastic moduli were found to be 2.6 ± 0.14 MPa, 1.9 ± 0.27 MPa and 0.93 ± 0.12 MPa, respectively. The trend observed in the mechanical response of these films has been attributed to the combined effect of the concentration of polyelectrolyte complexes (PEC) and the amount of water absorbed. The Fourier transform infrared spectroscopy experiments indicate the presence of higher alginate on the surface of the films compared to the bulk in all films. The presence of higher alginate on surface is consistent with negative surface charge densities of these films, determined from AFM experiments.

Molecular Solvent Model of Spherical Electric Double Layers: A Systematic Study by Monte Carlo Simulations and Density Functional Theory

The structure of spherical electric double layers is studied using Monte Carlo simulation and density functional theory by considering solvent as the third component. In this molecular solvent model (MSM), ions and solvent molecules are considered as charged and neutral hard spheres, respectively, having equal diameter. The macroion is considered as an isolated hard sphere having uniform surface charge density surrounded by the electrolyte and the solvent. The theory is partially perturbative as the hard-sphere contribution to the one particle correlation function is evaluated using suitably averaged weighted density, and the ionic part is obtained through a second-order functional Taylor expansion around the bulk electrolyte. The Monte Carlo simulations have been performed in a canonical ensemble. The system is studied at varying concentrations of electrolytes, and the solvent molecules, at different valences of the electrolyte, at different macroion radii, and at varying surface charge densities. The theory is found to be in good agreement with the simulation results over a wide range of parametric conditions. The excluded volume effects due to the molecular nature of the solvent are shown to have much richer features in diffuse layer phenomena like layering and charge inversion.

Molecular solvent model of cylindrical electric double layers: A systematic study by Monte Carlo simulations and density functional theory

We present the Monte Carlo simulation and density functional study of structure of cylindrical double layers considering solvent as the third component. We have chosen molecular solvent model, where ions and solvent molecules are considered as charged and neutral hard spheres, respectively, having equal diameter. The polyionic cylinder is modeled as an infinite, rigid, and impenetrable charged hard cylinder surrounded by the electrolyte and the solvent spheres. The theory is partially perturbative where the hard-sphere interactions are treated within the weighted density approach, the corresponding ionic interactions have been evaluated through second-order functional Taylor expansion with respect to the bulk electrolyte. The Monte Carlo simulations have been performed in canonical ensemble. The system is studied at varying concentrations of electrolyte ions and the solvent molecules, at different valences of the electrolyte, at different sizes of hard spheres, and at varying surface charge density. The theory and the simulation results are found to be in good agreement at different parametric conditions. The hard-sphere exclusion effects due to molecular nature of the solvent are shown to have special implications in characterizing diffuse layer phenomena such as layering and charge inversion.

A three-scale model for pH-dependent steady flows in 1:1 clays

A new three-scale model to describe the coupling between electro-chemistry and hydrodynamics in non-swelling kaolinite clays in steady-state conditions is proposed. The medium is characterized by three separate nano-micro and macroscopic length scales. At the pore (micro)-scale the portrait of the clay consists of micro-pores saturated by an aqueous solution containing four monovalent ions (Na+, H+, Cl−, OH−) and charged solid particles surrounded by thin electrical double layers. The movement of the ions is governed by the Nernst–Planck equations and the influence of the double layers upon the hydrodynamics is modeled by a slip boundary condition in the tangential velocity governed by the Stokes problem. To capture the correct form of the interface condition we invoke the nanoscopic modeling of the thin electrical double layer based on Poisson–Boltzmann problem with varying surface charge density ruled by the protonation/deprotonation reactions which occur at the surface of the particles. The two-scale nano/micro model is homogenized to the macroscale leading to a precise derivation of effective governing equations. The macroscopic model is discretized by the finite volume method and applied to numerically simulate desalination of a clay sample induced by an external electrical field generated by the placement of electrodes. Numerical results indicate strong pH-dependence of the electrokinetics.

A Singlet Reference Interation Site Model Theory for Solid/Liquid Interfaces Part II: Electrical Double Layers

The previously established singlet reference interaction site model (SRISM) theory for the calculation of the fluid structure in the vicinity of a plane impenetrable interface is renormalized for the application to electrical double layers. In combination with the HNC and KH closures, the equations are solved numerically for a 1 M electrolyte solution adjacent to a charged wall with varying surface charge densities. The wall-solvent and wall-ion density distributions as well as the profiles of the electrical field and the electrical potential are compared to computer simulation results. Reasonable agreement is obtained.

The influence of membrane ion-permselectivity on electrokinetic concentration enrichment in membrane-based preconcentration units

The performance of nanoporous hydrogel microplugs with varying surface charge density is described in concentrating charged analytes electrokinetically in a microfluidic device. A neutral hydrogel plug with a mean pore size smaller than the size of charged analytes acts as a simple size-exclusion membrane. The presence of fixed charges on the backbone of a nanoporous hydrogel creates ion-permselectivity which results in charge-selective transport through the hydrogel. This leads to the development of concentration polarization (CP) in the adjoining bulk electrolyte solutions under the influence of an applied electrical field. CP strongly affects the distribution of the local electrical field strength, in particular, in the vicinity of the hydrogel plug which can significantly reduce the concentration enrichment factors compared to the neutral hydrogel. A theoretical model and simulations are presented, together with experimental data, to explain the interplay of hydrogel or membrane cation-selectivity, electrical field-induced CP, and the distribution of the local electrical field strength with respect to concentration enrichment of negatively charged analytes at the cathodic membrane-solution interface.

The influence of colloidal interactions on fiber network strength

Various forces govern the fiber–fiber interaction in a flowing suspension, causing fibers to create flocs. The aim with this investigation was to examine the influence of colloidal interactions on the fiber network strength by varying surface charge density, electrolyte concentration, and type of counterion. This was accomplished by comparing surface force measurements, utilizing colloidal probe microscopy (CPM), with the apparent yield stress, using a parallel plate rheometer. Results show that by increasing the charge density by grafting carboxymethyl cellulose (CMC) to the surface, a large electrosteric repulsion is created, which gives weaker network strength. Increasing the electrolyte concentration decreases the repulsion. The network strength was, however, not affected by electrolyte concentration for untreated fibers whereas a high electrolyte concentration increased the yield stress for CMC-treated fibers. The change of counterions affect the repulsion, causing a change in network strength due to differences in the surface swelling of cellulose.

Monitoring surface charge migration in the spectral dynamics of singleCdSe∕CdSnanodot/nanorod heterostructures

Spherical CdSe nanocrystals capped by a CdS rod-like shell exhibit interesting spectral dynamics on the single particle level. Spectral boxcar averaging reveals a high degree of correlation between the emission energy, spectral linewidth, phonon coupling strength, and emission intensity of the single nanocrystal. The results can be described in terms of a spatially varying surface charge density in the vicinity of the exciton localized in the CdSe core, leading to a quantum confined Stark effect which modifies the transition energy and the radiative rate. Whereas internal charging of the particle results in a change in the nonradiative rate, surface charges primarily influence the radiative rate. Additionally, we observe characteristic spectral dynamics in frequency space, the magnitude of which depends slightly on temperature and strongly on excitation density. By distinguishing between continuous spectral jitter and discrete spectral jumping associated with a reversible particle ionization event, we can attribute the spectral dynamics to either a slowly varying surface charge density or a rapidly occurring polarization change due to a reversible expulsion of a charge carrier from the semiconductor nanostructure. Whereas the former exhibits universal Gaussian statistics, the latter is best characterized by a Lorentzian noise spectrum. The Gaussian spectral noise increases with spectral redshift of the emission and with increasing proximity of the surface charge to the localized exciton. The observation of a high degree of correlation between peak position and linewidth right up to room temperature suggests applications of the nanocrystals as extremely sensitive single charge detectors in both solid state devices and in biomolecular labeling, where highly local measurements of the dielectric environment are required. Nanoscale control of the physical shape of nanocrystals provides a versatile test bed for studying electronic noise, making the approach relevant to a wide range of conducting and emissive solid state systems.

Flocculation of Latex Particles of Varying Surface Charge Densities by Chitosans

A series of well-characterized chitosan samples spanning a wide range of molecular weights, having been homogeneously acetylated to produce varying degrees of linear charge density, were used to fl...

Surface charge induced modifications of the structure and dynamics of mixed dipolar liquids at solid–liquid interfaces: A molecular dynamics simulation study

Molecular dynamics simulations are carried out to investigate the structural and dynamical properties of binary Stockmayer liquids near charged solid surfaces at varying surface charge density. The two solvent components differ widely in their polarity. The dipolar mixtures are formed at varying composition and the properties of the interfacial molecules are calculated in terms of several equilibrium and dynamical quantities such as the number density and polarization profiles, electrostriction at surfaces, linear and angular velocity autocorrelation functions, perpendicular (z) and parallel (x,y) components of translational diffusion tensors and rotational diffusion coefficients. The extent of selective adsorption of one species against the other at the surfaces is investigated as a function of surface charge density and composition and its effects on translational and rotational diffusion of interfacial molecules are discussed. The dynamical properties of the interfaces are also compared with those of the bulk.

Interfacial structure of a mixed dipolar liquid in contact with a charged solid surface

We develop a nonlinear theory for the calculation of interfacial structural properties of a dipolar mixture in contact with a charged solid surface. Both the molecular sizes and the dipole moments of various species can be unequal. Explicit numerical results are obtained for the interfacial structure of a binary dipolar liquid in contact with a charged surface of varying surface charge density. The dipole moments of the two species are also varied. The density profiles of both the species are found to be highly inhomogeneous and oscillatory near the solid surface. The more polar species is found to exhibit a positive electrostriction at the surface with an increasing surface electrostatic field. An opposite behavior is observed for the less polar species. The polarization profiles reveal pronounced orientational order of the solvent molecules of both the species near the charged surface. The contact polarizations of the more polar and the less polar species show, respectively, a more than linear and a less than linear increase with increasing surface charge density. The predictions of the present theory for a given set of parameter values are compared with the results of Monte Carlo simulation of the same system and a good agreement is found for the inhomogneous density and polarization profiles of both the species.

Alkyl chain order in a lamellar lyotropic liquid crystal with varying surface charge density

Abstract The alkyl chain order in the lamellar liquid-crystalline phase formed in the octylamine/octylammonium chloride/water system has been investigated by 2H N.M.R. at constant mole ratio water/amphiphile and varying charge of the lamellae. The results are correlated to the dimensions of the aggregate obtained from X-ray low angle diffraction measurements.

Characterization of Liquid Developers for Continuous Tone Electrography

The relatively small particle sizes and lack of threshold development forces make liquid development a particularly suitable candidate for the development of continuously varying surface charge densities in pictorial electrography. The characterization of such developers has been pursued by studying their response to continuous and discrete surface charge density wedges on a computer-controlled test fixture. Results have been interpreted in terms of standard models of liquid development, and in most cases allow the extraction of the effective area coverage provided by the developer per unit of input surface charge density (the imagewise pattern of charge deposited by whatever electrographic writing device is employed). Examples of materials displaying linear and nonlinear responses to varying surface charge densities will be shown.