Diffusioosmosis and electroosmosis in a capillary slit with surface charge layers

This study analytically examines the steady diffusioosmotic and electroosmotic flows of an electrolyte solution in a fine capillary slit with each of its inside walls covered by a layer of adsorbed polyelectrolytes. In this solvent-permeable and ion-penetrable surface charge layer, idealized polyelectrolyte segments are assumed to distribute at a uniform density. The electric double layer and the surface charge layer may have arbitrary thicknesses relative to the gap width between the slit walls. The electrostatic potential distribution on a cross section of the slit is obtained by solving the linearized Poisson–Boltzmann equation, which applies to the case of low potentials or low fixed-charge densities. Explicit formulas for the fluid velocity profile due to the imposed electrolyte concentration gradient or electric field through the slit are derived as the solution of a modified Navier–Stokes/Brinkman equation. The results demonstrate that the structure of the surface charge layer can lead to an augmented or a diminished electrokinetic flow (even a reversal in direction of the flow) relative to that in a capillary with bare walls, depending on the characteristics of the capillary, of the surface charge layer, and of the electrolyte solution. For the diffusioosmotic flow with an induced electric field, competition between electroosmosis and chemiosmosis can result in more than one reversal in direction of the flow over a range of the Donnan potential of the adsorbed polyelectrolyte in the capillary.     

Ion specific surface forces between membrane surfaces

Entities such as ion distributions and forces between lipid membranes depend on effects due to the intervening salt solution that have not been recognized previously. These specific ion or Hofmeister effects influence membrane fusion. A typical illustrative example is this: measurements of forces between double-chained cationic bilayers adsorbed onto molecularly smooth mica surfaces across different 0.6-2 mM salt solutions have revealed a large degree of ion specificity [Pashley et al. J. Phys. Chem. 1986, 90, 1637]. This has been interpreted in terms of very specific anion "binding" to the adsorbed bilayers, as it would too for micelles and other self-assembled systems. However, we show here that inclusion of nonelectrostatic (NES) or ionic dispersion potentials acting between ions and the two surfaces explains such "ion binding". The observed Hofmeister sequence for the calculated pressure without any direct ion binding is given correctly. This demonstrates the importance of a source of ion specificity that has been ignored. It is due to ionic physisorption caused by attractive NES ionic dispersion potentials. There appear to be some far reaching consequences for interpretations of membrane intermolecular interactions in salt solutions.     

Recoil spectrometry: Ion accelerator based elemental characterisation of surface layers

Recoil Spectrometry covers a group of techniques that are very similar to the well known Rutherford backscattering Spectrometry technique, but with the important difference that one measures the recoiling target atom rather than the projectile ion. This makes it possible to determine both the identity of the recoil and its depth of origin from its energy and velocity, using a suitable detector system. The incident ion is typically high-energy (30–100MeV)³⁵C1,⁸¹Br or¹²⁷I. Low concentrations of light elements such as C, O and N can be profiled in a heavy matrix such as Fe or GaAs. Here we present an overview of mass and energy dispersive recoil Spectrometry and illustrate its successful use in some typical applications.     

An algorithm for the calculation of the electrostatic repulsion between surface coated with a charge membrane

We propose a numerical procedure for the calculation of the electrostatic repulsion force between two identical, parallel surfaces immersed in anab electrolyte solution. These surfaces are coated with an ion-penetrable membrane carrying fixed charges. The amount of fixed charges is governed by the dissociation of the functional groups in the membrane phase. The effect of pH on the degree of dissociation of these functional groups is taken into account. The difficulty of extensive use of Jacobi elliptic function in the numerical treatment of Poisson-Boltzmann equation can be circumvented by resorting to the present algorithm.     

Electrophoresis of large colloidal particles with surface charge layers. Position of the slipping plane and surface layer thickness

A theory is proposed for the electrophoresis of a large colloidal particle with a surface charge layer. The slipping plane is assumed to be located within the surface layer but may not be located at the boundary between the surface layer and the particle core. In previous studies, the depth of the slipping plane is assumed to coincide with the surface layer thickness. The present theory makes it possible to examine the separate dependence of the electrophoretic mobility on the position of the slipping plane and on the surface layer thickness. It is shown that, at constant amount of particle-fixed charges in the surface layer, the mobility increases as the depth of the slipping plane (d _s ) increases, while it decreases as the surface layer thickness (d _c ) increases, causing a mobility maximum in some cases ifd _s =d _c . Several approximate analytic expressions for the mobility are presented.     

Effect of the surface charge discretization on electric double layers: a Monte Carlo simulation study

The structure of the electric double layer in contact with discrete and continuously charged planar surfaces is studied within the framework of the primitive model through Monte Carlo simulations. Three different discretization models are considered together with the case of uniform distribution. The effect of discreteness is analyzed in terms of charge density profiles. For point surface groups, a complete equivalence with the situation of uniformly distributed charge is found if profiles are exclusively analyzed as a function of the distance to the charged surface. However, some differences are observed moving parallel to the surface. Significant discrepancies with approaches that do not account for discreteness are reported if charge sites of finite size placed on the surface are considered.     

Ion transport in materials with a developed surface

The experimental data on surface charge and mass transfer in ionic solids (ceramics, composites, nanostructures, and heterostructures) are generalized. Interphase and intergrain boundaries are shown to play an important role in transfer processes. The influence of the wetting phase transition on the transport properties of solids is considered.     

Intrinsic charge and Donnan potentials of grafted polyelectrolyte layers determined by surface conductivity data

In order to characterize grafted polyelectrolyte layers based on electrokinetic measurements a theory of the surface conductivity Ksigma was developed, starting from the model of thick polyelectrolyte layers with uniform segment distribution and dissociable groups with an unknown pK value. According to this model the inner part of the polyelectrolyte layer adjacent to the substrate is considered to be isopotential while the potential decay occurs in a zone near the solution side of the layer. A simple equation for the Donnan potential psiD as a function of pH, pK, electrolyte concentration C0, and volume charge density rho was obtained. In the derived equation Ksigma is directly related to psiD while the other terms have less influence on the magnitude of Ksigma and can be accounted for in a second approximation using psiD as determined from the measured Ksigma. Evaluation of the suggested model indicates that Ksigma measurements provide an effective method to characterize polyelectrolyte layers by analyzing the dependence of psiD on pH and C0: The magnitude of Ksigma yields information about the surface charge at complete dissociation of the ionizable groups. The dependence of Ksigma on pH and C0 can be used for the determination of the pK value of the dissociating functions and the segment volume fraction of the polyelectrolyte can be estimated using the measured value of rho.     

Fixed charge density effective to membrane phenomena

Summary Mobilities and activity coefficients of small ions in membranes were determined experimentally for collodion based polystyrene sulfonic acid membrane in aqueous KCl solution in a wide range of concentrations. The activity coefficients were measured by analyzing the amounts of co- and counter-ions taken into the membranes, while the mobilities were determined by a proper combination of the membrane potential and ion permeability data in conjunction with the analytical data of small ions in the membranes. It was found that the mobility of the coions was not very different from that in bulk solution in the range of KCl concentrations studied, while the mobility of counterions decreased very much in comparison with that in bulk solution. The concentration dependence of the product of mobilities of positive and negative ions in the membrane was the same as that of activity coefficients of small ions in the membrane. The depressions of mobility and activity coefficient of the counterions in the membrane were represented by the similar formulas as that in the polyelectrolyte solutions, i.e. the empirical additivity rule held formally, although the effective charge density of the membrane or fraction of counterions not bounded in the vicinity of fixed charges of the membrane skeleton was found to be dependent upon the salt concentration of the external solution when the concentration was lower than the stoichiometric density of charges fixed in the membrane.

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Zusammenfassung Beweglichkeiten und Aktivitätskoeffizienten kleiner Ionen in Membranen wurden experimentell für auf der Basis Polystyrene-Polystyrolsulfosäure hergestellte kolloide Membranen in wäßriger KCl-Lösungineinem weiten Bereich der Konzentrationen bestimmt. Die Aktivitätskoeffizienten wurden durch analytische Bestimmung der Beträge von Ko- und Gegenionen, die in den Membranen enthalten waren, gemessen, während die Beweglichkeiten durch geeignete Kombinationen von Daten des Membranpotentials und der Ionenpermeabilität in Verbindung mit analytischen Daten kleiner Ionen in den Membranen vermittelt wurden. Es wurde gefunden, daß die Beweglichkeit des Ko-Ions nicht sehr verschieden von derjenigen der reinen Lösung im Bereich der untersuchten KCl-Konzentrationen ist, während die Mobilität des Gegenions sehr stark im Vergleich zu der in der reinen Lösung abnahm. Die Konzentrationsabhängigkeit des Produktes von Beweglichkeiten positiver und negativer Ionen in Membranen war dieselbe wie die der Aktivitätskoeffizienten kleiner Ionen in den Membranen. Die Herabsetzung der Beweglichkeit und des Aktivitätskoeffizienten des Gegenions in der Membran werden durch ähnliche Formeln dargestellt wie die in Polyelektrolyt-Lösungen. Zum Beispiel gilt formal die empirisch Additivitätsregel, obgleich die effektive Ladungsdichte in der Membran der Nachbarschaft der fixierten Ladungen am Membranskelett als abhängig von der Salzkonzentration in der äußeren Lösung gefunden wurde, wenn die Konzentration geringer war als die stöchiometrische Dichte der fixierten Ladung an der Membran.

     

Ion exchange at a surface monolayer

Total reflection total conversion electron yield X-ray absorption fine structure is applied to the evaluation of ion exchange occurring at the surface monolayer of two-tailed ammonium ions. X-ray absorption measurements at a Br K-edge allow us to detect ion-exchange equilibria between Br- and Cl- added in a subphase. The ion-exchange selectivity of Br- over Cl- basically increases as the monolayer is compressed, indicating that Cl- is selectively squeezed out by compression because of its larger hydrated ionic radius.     

Label-free chiral detection of melittin binding to a membrane

The study presented here describes an innovative approach for the detection of surface-confined proteins using chiral second harmonic generation (C-SHG). A unique optical geometry has been employed which allows for the separation of the chiral and achiral nonlinear response. By utilizing this optical arrangement, the detection of chirality originating from melittin adsorbed to a planar supported lipid bilayer has been performed for the first time by C-SHG. Melittin binding to the membrane was monitored as a function of bulk concentration through detection of the C-SHG signal. Analysis of the C-SHG adsorption isotherms reveals Frumkin adsorption behavior with a positive interaction energy. The binding constant (Ka) obtained was determined to be (8.3 +/- 1.0) x 105 M-1. The results of these studies have far-reaching implication in the use of C-SHG for the label-free detection of protein association to surfaces and in the analysis of protein interfacial phenomena.     

Ion pair chromatography on reversed-phase layers

The results of investigations with ion pair chromatography on RP₂, RP₈ and RP₁₈ thin layers are described. Heptane sulphonic acid is used as ion pair former. The best results are achieved on RP₁₈-layers with the counter ion in the mobile phase.     

The role of dimension in multivalent binding events: structure-activity relationship of dendritic polyglycerol sulfate binding to L-selectin in correlation with size and surface charge density

L ‐, P ‐, and E ‐Selectin are cell adhesion molecules that play a crucial role in leukocyte recruitment from the blood stream to the afflicted tissue in an acute and chronic inflammatory setting. Since selectins mediate the initial contact of leukocytes to the vascular endothelium, they have evolved as a valuable therapeutic target in diseases related to inflammation by inhibition of the physiological selectin–ligand interactions. In a previous study, it was demonstrated that dPGS, a fully synthetic heparin analogue, works as an efficient inhibitor towards L ‐ and P ‐selectin in vitro as well as in vivo. Herein, the focus is directed towards the effect of size and charge density of the polyanion. The efficiency of L ‐selectin inhibition via an SPR‐based in vitro assay and a cell‐based flow chamber assay is investigated with dPGS ranging from approximately 4 to 2000 kDa. SPR measurements show that the inhibitory potential of highly sulfated dPGS increases with size and charge density. Thereby, IC₅₀ values from the micromolar to the low picomolar range are determined. The same tendency could be observed in a cell‐based flow chamber assay with three representative dPGS samples. This structure–affinity relationship of dPGS suggests that the strong inhibitory potential of dPGS is not only based on the strong electrostatic interaction with areas of cationic surface potential on L ‐selectin but is also due to a steric shielding of the carbohydrate binding site by large, flexible dPGS particles.

     

Electrophoretic mobility of a colloidal particle with constant surface charge density

When the electrophoretic mobility of a particle in an electrolyte solution is measured, the obtained electrophoretic mobility values are usually converted to the particle zeta potential with the help of a proper relationship between the electrophoretic mobility and the zeta potential. For a particle with constant surface charge density, however, the surface charge density should be a more characteristic quantity than the zeta potential because for such particles the zeta potential is not a constant quantity but depends on the electrolyte concentration. In this article, a systematic method that does not require numerical computer calculation is proposed to determine the surface charge density of a spherical colloidal particle on the basis of the particle electrophoretic mobility data. This method is based on two analytical equations, that is, the relationship between the electrophoretic mobility and zeta potential of the particle and the relationship between the zeta potential and surface charge density of the particle. The measured mobility values are analyzed with these two equations. As an example, the present method is applied to electrophoretic mobility data on gold nanoparticles (Agnihotri, S. M.; Ohshima, H.; Terada, H.; Tomoda, K.; Makino, K. Langmuir 2009, 25, 4804).     

Effect of surface-active substances on the surface charge density of purple membrane fragments

The surface charge density of purple membrane fragments and its alteration upon treatment of purple membranes with several surface-active substances [sodium dodecyl sulphate (SDS), cetylpyridinium chloride (CPC) and 3-[(3-cholamidopropyl) dimethylammonio]-1-propane-sulphonate (CHAPS) were examined by use of 9-amino-acridine fluorescence.The value of the surface charge density of native purple membrane fragments (0.8 electric charges/nm²) obtained by this method is comparable to previously reported values and in agreement with the structural model of the purple membrane.An increase followed by a decrease in the negative surface charge density was observed after treatment of purple membranes with the negatively charged surfactant SDS within the concentration range 0–5 mM, whereas treatment with the positively charged surfactant CPC and zwitterionic derivative of cholic acid (CHAPS) led to a decrease in the surface charge density. The large reduction of the surface charge density after treatment of purple membranes with CHAPS (i.e. partial delipidation of purple membranes) proves the significant contribution of the negative charges of the lipid polar head groups to the negative surface charge of purple membranes.     

Nonbonded interactions in membrane active cyclic biopolymers. II. Ion binding by valinomycin

A possible mode for the capture of a cation by the carrier molecule valinomycin has been simulated. The intramolecular nonbonded energy and the binding energy for a centrally located potassium ion at different stages in the capture process have been evaluated using a method proposed earlier. Contrary to intuitive expectations the open conformation of valinomycin is also found to have a strong affinity for K⁺. There appears to be a rather high potential barrier between the open and closed symmetric conformations.     

A dynamic method of measuring surface potential change due to binding of bitter substance at monolayer-coated liquid membrane surface.

A dynamic method of determining the membrane surface potential change due to a binding of a hydrophobic ion has been presented. The surface potential was determined from the time course of membrane potential under zero electric current during a transition between two steady states in a membrane filter impregnated with a phospholipid and 1-octanol. One of the alkaloids, quinine hydrochloride, was used as a hydrophobic electrolyte. Surface charge density and equilibrium constant for binding of quinine ions with ionizable groups of the phospholipids at the membrane surface were determined from the surface potential according to the Poisson-Boltzmann equation.     

535—The surface compartment model (SCM): Role of surface charge in membrane permeability changes

Small shifts in the surface charge of excitable membranes precede the ion permeability changes that are the basis of the action potential. If we assume that an increase in ion permeability is due to the opening of a molecular pore, we can model this process like subunit dissociation in oligomeric proteins. (The dissociation of hemoglobin tetramers into dimers is sensitive to changes in charge and occurs when the surface charge is approximately the same as on the inner face of the squid axon membrane.) Using the relation between the hemoglobin dissociation constant and charge, together with the estimated number of pores in the axon membrane, we have derived a relation between the fraction of pores open and the surface charge density. When this latter relation is incorporated into the surface compartment model (SCM), the gating currents give rise to voltage (polarization) dependent changes in ion permeability.