Flexoelectricity of lipid bilayers

Abstract

The direct flexoeffect in single lipid bilayers in the form of black lipid membranes has been investigated experimentally by the oscillating pressure technique in the regime of voltage measurement. Black lipid membranes of various composition have been studied in order to check the effect of lipid surface charge on the curvature-electric response and its frequency dependence; these include egg yolk lecithin (low negative charge); egg yolk lecithin plus phosphatidyl serine (high negative charge); egg yolk lecithin with surface adsorbed ions of uranyl acetate (high positive charge). An increase of the response has been found by increasing the surface charge and a reversal of the sign of the flexoelectric coefficient from positive to negative has been obtained by changing the sign of the surface charge from negative to positive. These results underline the leading role of the contribution of the surface charge to the flexoelectricity of lyotropics. Their theoretical interpretation provides further insight into the molecular mechanism of this phenomenon.     

Subsurface Space‐Charge Dopant Segregation to Compensate Surface Excess Charge in a Perovskite Oxide

Since the first prediction by Frenkel, many follow‐up studies have been carried out to show the presence of subsurface space‐charge layers having the opposite sign to that of the excess charge at the surface, producing overall neutrality in ionic crystals. However, no precise experimental evidence demonstrating how the aliovalent solutes segregate in the space‐charge region beneath the surface has been provided over the past several decades. By utilizing atomic‐scale imaging and chemical probing in a perovskite oxide, the origin of the surface excess charge at the topmost surface and the position of segregated dopants in the space‐charge region is precisely determined. The impact of the space‐charge contribution to the dopant distribution near the surface in oxide crystals is explored.     

Simultaneous investigation of the influence of topography and charge on protein adsorption using artificial nanopatterns

The combined influence of surface topography and charge of a polymer surface on the adsorption of the protein avidin has been investigated. Atomic force microscopy contact mode imaging and charge writing were used to create defined topographical roughness and electrostatic charge patterns on the surface of polystyrene. Increased avidin adsorption was found on nanometer-size topographical patterns, but the adsorption remained unaffected by electrostatic patterns.     

Effect of thermodiffusion on pH‐regulated surface charge properties of nanoparticle

Surface properties of nanoparticle are of high importance in the field of biotechnology, drug delivery and micro/nanofabrication. In this article, we developed a comprehensive theoretical model and subsequently solved that numerically to study the effect of thermodiffusion of ions on surface charge properties of nanoparticle. The theoretical study has been done considering silica nanoparticle for two aqueous solutions NaCl and KCl. The effect of solution pH in conjunction with nanoparticle temperature on surface charge density has been obtained for different salt concentrations (1, 10 and 100 mM) and nanoparticle size (diameter of 2 and 100 nm). It is observed from the results that with increasing temperature of the nanoparticle, the negative surface charge density gets higher due to increasing thermodiffusion effect. It is also found out that the magnitude of surface charge density is higher for KCl solution than NaCl solution under same condition which is attributed mostly due to less thermodiffusion of counterions for KCl than NaCl. Present study also shows that magnitude of surface charge density decreases with increasing nanoparticle size until it reaches a limiting value (called critical size) above which the effect of nanoparticle size on surface charge density is insignificant.     

Electrochemical Quantitative Evaluation of the Surface Charge of a Poly(1-Vinylimidazole) Multilayer Film and Application to Nanopore pH Sensor

Nanopore pH sensing is based on the interaction between the surface charge of the nanopore and ions passing through the nanopore. The nanopore surface charge can be derived from the acid-base dissociation equilibrium of the modified polyelectrolyte. Various polyelectrolytes have been selected based on the acid dissociation constant of the monomer units, and various techniques have been applied to modify nanopores. However, they have been developed without clear guidelines for characterizing the surface modification status or surface charge. One reason has been the difficulty in accurately estimating the surface charge of nanopores in solution. Thus, in this study, the dissociation constant (pKa_app) of the surface charge of a modified polyelectrolyte nanopore was quantitatively estimated via electrochemical measurements. Previously, the modification status of nanopores has been evaluated using the ion current response. In addition, we monitored in real-time the polyelectrolyte modification status using a quartz crystal microbalance (QCM). Some polyelectrolytes were difficult to immobilize directly on the nanopore surface, and those polymers could be effectively modified by the layer-by-layer (LbL) technique. Therefore, we produced a guideline for the fabrication of a nanopore sensor for pH measurements under physiological conditions by quantitative evaluation of the pKa_app via electrochemical methods, the monitoring of the modification status by QCM, and the development of an effective modification method via the LbL technique.     

Enhancing the inter-fiber bonding properties of cellulosic fibers by increasing different fiber charges

Cellulosic fiber has been increasingly used in many fields. The fiber charge, including the surface charge and inner charge, affects the properties of cellulosic fiber and fiber-based materials significantly. In this study, the cellulosic fiber was subjected to different treatments, including 2,2,6,6-tetramethyl-piperidine-1-oxyl radical-mediated oxidation, carboxymethyl cellulose attachment and mechanical refining, to alter the fiber charge selectively. The effects of the fiber surface charge and inner charge on fiber performances and inter-fiber bonding strength for improving the high-value application of cellulosic fibers, respectively, were discussed. The results showed that the performances of cellulosic fiber can be improved with the increase of either surface or inner fiber charges, including the increased water retention value, flexibility and inter-fiber bonding strength, but with slightly decreased drainability. An increasing bulk fiber charge showed more significant enhancement of the inter-fiber bonding strength than only an increase of the fiber surface charge on cellulosic fiber. This was because the fiber inner charge contributed to the increase of fiber flexibility and deformability, which could benefit the inter-fiber bonding indirectly. As a consequence, the bulk fiber charge enhancement was better for tensile strength improvement of handsheets (fiber-based material) than only fiber surface charge enhancement. Increasing both the surface charge and inner charge improved the tensile strength effectively with less change of the bulky fiber network than the refining treatment.     

Effect of low-molecular-weight organic anions on surface charge of variable charge soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes. In the present paper, the effect of the anions of four low-molecular-weight organic acids on the surface charge of three variable charge soils was investigated. The results showed that the presence of organic anions led to an increase in negative charge and a decrease in positive charge. Positive charges decreased to a larger extent than negative charges. The effect of different anions on surface charge followed the order citrate > malate > oxalate > acetate. For hyper-rhodic ferrasol and rhodic ferrasol, the change of positive charge decreased with the increase in pH, while that of negative charge increased with the increase in pH. Among different soils the extent of change in surface charge was related to their iron oxides content. When free iron oxides were removed from the soil, the effect of organic anions on surface charge decreased sharply. These findings may be of practical significance for variable charge soils low in nutrient-retaining capacity by increasing the retaining capacity for cations such as potassium and calcium considerably but decreasing that for anions such as nitrate remarkably.     

The surface charge of regenerated cellulose fibres

In this paper the influence of charged species on the sheet strength of viscose fibres was investigated. Four samples of chemical modified viscose fibres, as well as a reference fibre were studied. The swelling of these viscose fibres and the breaking length of hand sheets have been determined. Comparing the results, the influence of both, swelling and surface charge on the bonding force, is evident. The allocation of the charges, induced by cationic starch and Carboxmethylcellulose, has been analyzed by Titration, attenuated total reflection spectroscopy (ATR) and X-ray photoelectron spectroscopy (XPS). Titration was used to make a first estimation of the charge distribution within the fibre. Using ATR and XPS, more detailed information about the surface charge has been achieved. All measurement methods showed a significant amount of charge on the fibre surface.     

Effect of discrete macroion charge distributions in solutions of like-charged macroions

The effect of replacing the conventional uniform macroion surface charge density with discrete macroion charge distributions on structural properties of aqueous solutions of like-charged macroions has been investigated by Monte Carlo simulations. Two discrete charge distributions have been considered: point charges localized on the macroion surface and finite-sized charges protruding into the solution. Both discrete charge distributions have been examined with fixed and mobile macroion charges. Different boundary conditions have been applied to examine various properties. With point charges localized on the macroion surface, counterions become stronger accumulated to the macroion and the effect increases with counterion valence. As a consequence, with mono- and divalent counterions the potential of mean force between two macroions becomes less repulsive and with trivalent counterions more attractive. With protruding charges, the excluded volume effect dominates over the increased correlation ability; hence the counterions are less accumulated near the macroions and the potential of mean force between two macroions becomes more repulsive/less attractive.     

X-ray photoelectron spectroscopic studies of charge transfer interactions in electroactive polyaniline

Four modes of charge transfer interactions in polyaniline (PAN), viz. acid protonation, self-doping, charge transfer interactions with organic acceptors, and charge transfer interactions with surface grafted functional polymers have been studied by X-ray photoelectron spectroscopy (XPS). In the case of acid protonation, the protonation behavior of volatile and non-volatile acid differs. The structures of sulfonated leucomeraldine (LM) and nigraniline (NA) are similar to those of sulfonated and self-protonated emeraldine (EM). A substantially higher degree of charge transfer interaction with the organic acceptors is observed for EM film that has been subjected to one cycle of acid/base treatment. The charge transfer interactions with the organic acceptors have proceeded further than the pure formation of molecular complexes. Both pristine and Ar plasma or O₃ pretreated EM films are susceptible to surface modifications by graft copolymerization. The protonic acid functional groups of the graft readily give rise to a self-protonated EM surface.     

Preparation and evaluation of effective fixed charge density of titanium phosphate membranes for uni-univalent electrolytes in aqueous solutions

The preparation of polystyrene-based titanium phosphate membranes at different pressures with varying amounts of material has been explained. The membrane potentials of inorganic membranes were measured with uni-univalent electrolytes (KCl, NaCl and LiCl) solution using saturated calomel electrodes (SCEs). The TMS method was used for the evaluation of the effective fixed charge density of these membranes with increasing pressure, and the surface charge density of membrane appeared to be increased due to gradually diminution in surface opening channels. The order of surface charge density for electrolytes used is found to be KCl > NaCl > LiCl. In addition to the effective fixed charge density, distribution coefficient, transport numbers, charge effectiveness and other related parameters were calculated for characterizing the ion exchange membranes by utilizing the TMS method. The theoretical prediction is consistent well with the experimental data. The SEM of these membranes at various pressures has been presented.     

Heteroaggregation in binary mixtures of oppositely charged colloidal particles

Heteroaggregation (or heterocoagulation) rate constants have been measured in mixtures of well-characterized colloidal particles of opposite charge with multiangle static and dynamic light scattering. This technique permits routine measurements of absolute heteroaggregation rate constants, also in the presence of homoaggregation. Particularly with multiangle dynamic light scattering, one is able to estimate absolute heteroaggregation rate constants accurately in the fast aggregation regime for the first time. Heteroaggregation rate constants have also been measured over a wide range of parameters, for example, ionic strength and different surface charge densities. Amidine latex particles, sulfate latex particles, and silica particles have been used for these experiments, and they were well characterized with respect to their charging and homoaggregation behavior. It was shown that heteroaggregation rate constants of oppositely charged particles increase slowly with decreasing ionic strength, and provided the surface charge is sufficiently large, the rate constant is largely independent of the surface charge. These trends can be well described with DLVO theory without adjustable parameters.     

On the role of electrostatic forces in the adhesion of polymer particles to solid surfaces

Simultaneous measurements have been made of the adhesive force and double electric charge of particles after their removal from a metal surface. For the systems investigated, the adhesive force and charge on the particles increase with particle diameter according to a power law with an exponent close to 2. Such dependence can be explained on the basis of the electrostatic nature of the adhesive forces. A double electric layer exists at the interface between the particles and the metal surface. A calculation was made of the surface density of charge for the polyvinyl chloride particle-steel system.     

Direct correlation of the crystal structure of proteins with the maximum positive and negative charge states of gaseous protein ions produced by electrospray ionization

Electrospray mass spectrometric studies in native folded forms of several proteins in aqueous solution have been performed in the positive and negative ion modes. The mass spectra of the proteins show peaks corresponding to multiple charge states of the gaseous protein ions. The results have been analyzed using the known crystal structures of these proteins. Crystal structure analysis shows that among the surface exposed residues some are involved in hydrogen-bonding or salt-bridge interactions while some are free. The maximum positive charge state of the gaseous protein ions was directly related to the number of free surface exposed basic groups whereas the maximum negative charge state was related to the number of free surface exposed acidic groups of the proteins. The surface exposed basic groups, which are involved in hydrogen bonding, have lower propensity to contribute to the positive charge of the protein. Similarly, the surface exposed acidic groups involved in salt bridges have lower propensity to contribute to the negative charge of the protein. Analysis of the crystal structure to determine the maximum charge state of protein in the electrospray mass spectrum was also used to interpret the reported mass spectra of several proteins. The results show that both the positive and the negative ion mass spectra of the proteins could be interpreted by simple consideration of the crystal structure of the folded proteins. Moreover, unfolding of the protein was shown to increase the positive charge-state because of the availability of larger number of free basic groups at the surface of the unfolded protein.     

TiO2在水及丙二醇介质中表面电性质的研究

0引言 固体颗粒在液相介质中的分散是基础研究领域和工业技术部门普遍遇到的问题 ?在化学工业领域,如涂料 ? 染料 ? 油墨 ? 化妆品等,固体颗粒的分散及分散稳定性直接影响着产品的质量和性能 ?TiO 2 颗粒的水基分散体系广泛应用于涂料 ? 油墨以及化妆品中,也是陶瓷制备过程中重     

The ion sensitivity of surface conductive single crystalline diamond

Charge build-up at the solid/aqueous interface is a ubiquitous phenomenon that determines the properties of interfacial electrical double layers. Due to its unique properties, the surface of diamond offers an attractive platform to investigate charging mechanisms in aqueous solutions. We investigate the surface charge by studying the ion sensitivity of H-terminated single crystalline diamond surface conductive layers. The effect of monovalent and divalent salts has been probed at different pH values. For a pH above 3.5, increasing the ionic strength results in a decrease of the surface conductivity, in contrast to the results obtained for pH below 3.5. Electrokinetic experiments are in good agreement with the surface conductivity measurements, showing an isoelectric point at pH 3.5 for the H-terminated diamond surface. We discuss the results in terms of the Coulombic screening by electrolyte ions of the surface potential, which is induced by a pH-dependent surface charge. The origin of this surface charge is discussed in terms of charge regulation by amphoteric hydroxyl surface groups and unsymmetrical adsorption of hydroxide and hydronium ions induced by the hydrophobic nature of the H-terminated diamond surface. This surface charge can have important consequences for processes governed by the diamond/aqueous interface, such as electron transfer to charged redox molecules, adsorption of charged molecules and proteins, and ion sensitivity.     

Colloido-chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 2. Equilibrium electrochemical characteristics of membranes

The regularities of variations in the electrokinetic potential and surface charge of nanoporous glass membranes with different compositions have been studied as depending on the type of an electrolyte (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium chlorides) and the structure of pore space. It has been shown that, in solutions containing specifically sorbed organic counterions, the range of positive values of electrokinetic potential arises due to the superequivalent absorption of counterions in the Stern layer. It has been found that the influence of the specific adsorption of counterions on the electrokinetic potential of porous glasses increases with the amount of secondary silica in the pore space. The effects of the counterion specificity, pore channel sizes, and composition of a porous glass on the value of the surface charge have been analyzed. The absolute value of the surface charge has been shown to significantly increase in the presence of organic counterions in comparison with inorganic ions throughout the examined range of background electrolyte concentrations.     

Determination of the surface potential for hollow-fiber membranes by the streaming-potential method

A procedure has been proposed for measuring the surface potential of hollow-fiber membranes by the streaming-potential method under the conditions of a tangential flow of a solution. The zeta-potential and surface charge of nanofiltration hollow-fiber polyacrylonitrile membranes have been measured. The measurements have been performed for membranes with different porosities, which were obtained by partial drying of initial humid membranes. The porosity has been determined from the electrical conductivity of a membrane. An equation has been proposed for calculating the charge transfer by a solution flow in a porous layer. It has been shown that the use of the proposed equation makes it possible to obtain more correct values of the membrane surface potential.     

On the Applicability of DLVO Theory to the Prediction of Clay Colloids Stability

The stability behavior of Na-montmorillonite colloids has been studied by combining the analysis of their surface charge properties and time-resolved dynamic light scattering experiments. The chemical surface model for several types of clays, including montmorillonite, has to take into account the double surface charge contribution due to their permanent structural charge and to their pH-dependent charge, which is developed at the edge sites, therefore, these stability studies were carried out as a function of both ionic strength and pH. DLVO theory is largely applied for the prediction of the stability of many colloidal systems, including the natural ones. This work shows that the stability behavior of Na-montmorillonite colloids cannot be satisfactorily reproduced by DLVO theory, using the surface parameters experimentally obtained. Particularly, this theory is unable to explain their pH-dependent stability behavior caused by the small charge at the edge sites. Based on these results, a literature review of DLVO stability prediction of clay colloids was performed. It confirmed that this theory is not capable of taking into account the double contribution to the total surface charge and, at the same time, pointed out the main uncertainties related to the appropriate use of the input parameters for the calculation as, for example, the Hamaker constant or the surface potential. Copyright 2000 Academic Press.     

The Dielectrophoretic Behavior of Submicron Latex Spheres: Influence of Surface Conductance

An analysis of the dielectrophoretic crossover frequency as a function of medium conductivity has been made for submicron spheres with different surface functionalities. It is shown that the AC electrokinetic behavior of the particles can be explained by modeling the surface conductivity of the particle as the sum of two surface conductance components: one due to charge motion behind the slip plane (the Stern layer) and the other due to charge motion in front of the slip plane.