Nernstian and non-Nernstian potential differences at aqueous interfaces

A simplified general formalism of the physics of the charge-potential interdependence of the electrical double layer at ionisable group surfaces is presented. The equilibrium total double layer potential is obtained graphically and analytically for surfaces with acidic and basic sites, with amphoteric sites and with single acid sites. The deviations from Nernstian behaviour are given as a function of the acid ionization constants of the surface groups.     

The influence of electrostatic forces on protein adsorption

In this paper we investigate the importance of electrostatic double layer forces on the adsorption of human serum albumin by UV-ozone modified polystyrene. Electrostatic forces were measured between oxidized polystyrene surfaces and gold-coated atomic force microscope (AFM) probes in phosphate buffered saline (PBS) solutions. The variation in surface potential with surface oxygen concentration was measured. The observed force characteristics were found to agree with the theory of electrical double layer interaction under the assumption of constant potential. Chemically patterned polystyrene surfaces with adjacent 5 microm x 5 microm polar and non-polar domains have been studied by AFM before and after human serum albumin adsorption. A topographically flat surface is observed before protein adsorption indicating that the patterning process does not physically modify the surface. Friction force imaging clearly reveals the oxidation pattern with the polar domains being characterised by a higher relative friction compared to the non-polar, untreated domains. Far-field force imaging was performed on the patterned surface using the interleave AFM mode to produce two-dimensional plots of the distribution of electrostatic double-layer forces formed when the patterned polystyrene surfaces is immersed in PBS. Imaging of protein layers adsorbed onto the chemically patterned surfaces indicates that the electrostatic double-layer force was a significant driving force in the interaction of protein with the surface.     

Electrostatic interactions between double layers: influence of surface roughness, regulation, and chemical heterogeneities

Electrostatic interactions between two surfaces as measured by atomic force microscopy (AFM) are usually analyzed in terms of DLVO theory. The discrepancies often observed between the experimental and theoretical behavior are usually ascribed to the occurrence of chemical regulation processes and/or to the presence of surface chemical or morphological heterogeneities (roughness). In this paper, a two-gradient mean-field lattice analysis is elaborated to quantifying double layer interactions between nonplanar surfaces. It allows for the implementation of the aforementioned sources of deviation from DLVO predictions. Two types of ion-surface interaction ensure the adjustment of charges and potentials upon double layer overlap, i.e., specific ionic adsorption at the surfaces and/or the presence of charge-determining ions for the surfaces considered. Upon double layer overlap, charges and potentials are adjusted via reequilibrium of the different ion adsorption processes. Roughness is modeled by grafting asperities on supporting planar surfaces, with their respective positions, shapes, and chemical properties being assigned at will. Local potential and charge distributions are derived by numerically solving the nonlinear Poisson-Boltzmann equation under the boundary conditions imposed by the surface profiles and regulation mechanism chosen. Finite size of the ions is taken into account. A number of characteristic situations are briefly discussed. It is shown how the surface irregularities are reflected in the Gibbs energy of interaction.     

Electrostatic free energy of interacting ionizable double layers

The electrostatic contribution to the interaction free energy of charge-regulating materials, similar as well as dissimilar, contains electric work as well as chemical work and can be obtained from an integration over the diffuse part of the double layer together with a summation of the surface contribution to the free energy over the two surfaces. Examples for the surface contribution are given for acid, base, zwitterionic, and amphoteric (1-pK and 2-pK) materials for a diffuse double layer and for the Stern-Gouy-Chapman model, with and without ion adsorption. For charge-regulating materials, the electrostatic contribution to the interaction free energy at contact (adhesion force of curved surfaces, or particles) is always finite and can be obtained from a simple calculation.     

Effects of ionic strength and surface charge on protein adsorption at PEGylated surfaces

PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers, allowing control of the PEG surface density, as well as the surface charge. PEG (MW 2 kDa) surface densities between 0 and 0.5 nm(-2) were obtained by changing the PEG to lysine-mer ratio in the PLL-g-PEG polymer, resulting in net positive, negative and neutral surfaces. Colloid probe atomic force microscopy (AFM) was used to characterize the interfacial forces associated with the different surfaces. The AFM force analysis revealed interplay between electrical double layer and steric interactions, thus providing information on the surface charge and on the PEG layer thickness as a function of copolymer architecture. Adsorption of the model proteins lysozyme, alpha-lactalbumin, and myoglobin onto the various PEGylated surfaces was performed to investigate the effect of protein charge. In addition, adsorption experiments were performed over a range of ionic strengths, to study the role of electrostatic forces between surface charges and proteins acting through the PEG layer. The adsorbed mass of protein, measured by optical waveguide lightmode spectroscopy (OWLS), was shown to depend on a combination of surface charge, protein charge, PEG thickness, and grafting density. At high grafting density and high ionic strength, the steric barrier properties of PEG determine the net interfacial force. At low ionic strength, however, the electrical double layer thickness exceeds the thickness of the PEG layer, and surface charges "shining through" the PEG layer contribute to protein interactions with PLL-g-PEG coated surfaces. The combination of AFM surface force measurements and protein adsorption experiments provides insights into the interfacial forces associated with various PEGylated surfaces and the mechanisms of protein resistance.     

The effects of unequal ionic sizes for an electrolyte in a charged slit

The modified Gouy-Chapman (MGC) theory has been used to study the electrical double layer near two charged plates immersed in a model electrolyte. The effects of assigning to the cations and anions different distances of closest approach to the charged surfaces are examined. The dependence of overcharging and charge reversal on the system parameters such as concentration, ion size and valence, is investigated both inside and outside the charged slit.     

The interaction of diphosphonates with calcitic surfaces: understanding the inhibition activity in marble dissolution

The rates of dissolution of calcitic Carrara marble have been reported to be significantly reduced in alkaline pH (pH 8.25) at 25 degrees C in the presence of (1-hydroxyethylidene)-1,1 diphosphonic acid (HEDP). The adsorption takes place at the calcite/water interface at the double layer through the interaction of charged surface species with the charged solution species of the adsorbate. The present work focused on obtaining a better understanding of the interaction of the calcite surface with HEDP. Calculations were performed according to the triple layer model, assuming the formation of surface complexes between the charged surface species of calcite and the species of HEDP dominant at pH 8.25. According to the model, the adsorbed species are located at the inner Helmholtz plane of the electrical double layer. Strong lateral interactions between the adsorbed species were suggested and were corroborated from the calculation of the respective energy, which was equal to 69 kJ mol(-1). The adsorption isotherm was consistent with the proposed model at low surface coverage values, while discrepancies between the values experimentally measured and the predicted were found at higher adsorbate concentrations. The deviations from the predicted values were attributed to the fact that HEDP adsorption on calcite resulted in the formation of multiple layers. The model explained adequately the changes in the zeta-potential values of calcite in the presence of HEDP in the solution which resulted in charge reversal upon adsorption.     

Adsorption of the fluoride ion on the surface of various faces of a single crystal of silver: a quantum-chemical study

The fluoride ion adsorption from a gas phase on various faces of a single crystal of silver is studied by a density functional method within a cluster model for metal. The adsorption bond energy is found to increase in the series Ag(100) < Ag(111) < Ag(311) < Ag(110). A substantial structural and energetic heterogeneity of various adsorption sites is revealed. The results are utilized to simulate the electrochemical interface between individual faces of a single crystal of silver and aqueous solutions containing the fluoride ion. It is assumed that the adsorption potential may be represented as the sum of two contributions, one of which describes the metal–ion interaction and the other, the ion solvation energy. The plotted adsorption terms take into account partial degradation of the fluoride ion when adsorbed from an aqueous solution. Estimates of discreteness of the electrical double layer are presented. A conclusion on the maximum manifestation of specific adsorption of the fluoride ion for the Ag(100) face is made.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 232–238.Original Russian Text Copyright © 2005 by Nazmutdinov, Zinkicheva.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

The diamond/aqueous electrolyte interface: an impedance investigation

We have investigated the electrochemical interface between diamond electrodes and aqueous electrolytes using electrochemical techniques such as cyclic voltammetry and ac impedance spectroscopy. High-quality CVD-grown boron-doped polycrystalline diamond electrodes and IIa single crystalline natural diamond electrodes have been used in this study. In the case of hydrogen-terminated diamond electrodes, the electrochemical interface is dominated by the electrochemical double layer. Frequency-dependent impedance spectroscopy reveals a potential regime in which the contribution of ion adsorption becomes relevant. We have conducted experiments to evaluate the effect of pH and ionic strength on the double layer. Our results suggest that only ions resulting from water auto-dissociation, i.e., hydroxide and hydronium ions, are responsible for ion adsorption and, thus, able to modify the charge at the double layer. In contrast, no effect of the adsorption of several dissolved ions (such as Na+, K+, Cl-) has been observed On the basis of the electrochemical characterization of H-terminated diamond surfaces, we also discuss the phenomenon of the surface conductivity in diamond, as well as the pH sensitivity of the diamond surface. The influence of the O2/OH- and H2/H3O+ redox couples on the origin of the surface conductivity is discussed.     

Structure of the electrical double layer on the aqueous solution side of the polarized interface between water and a room-temperature ionic liquid, tetrahexylammonium bis(trifluoromethylsulfonyl)imide

The structure of the electrical double layer on the aqueous solution side has been studied by measuring electrocapillary curves at the polarized interface between a room-temperature ionic liquid (RTIL), tetrahexylammonium bis(trifluoromethylsulfonyl)imide, and water (W) at different concentrations of LiCl. Thermodynamic analysis of the electrocapillary curves indicates that Li+ ions negatively adsorb at the point of zero charge. The adsorption of Li+ and Cl- ions in the polarized potential window of about 200 mV can be explained by the Gouy's double layer model, and the specific adsorption of Li+ and Cl- ions at the RTIL|W interface is negligible within the polarized potential window.     

A general Poisson-Boltzmann model with position-dependent dielectric permittivity for electric double layer analysis

In this paper, we propose a general Poisson-Boltzmann model for electric double layer (EDL) analysis with the position dependence of dielectric permittivity considered. This model provides physically reasonable property profiles in the EDL region, and it is then utilized to investigate the depletion layer effect on EDL structure and interaction near hydrophobic surfaces. Our results show that both the electric potential and the interaction pressure between surfaces decrease due to the lower permittivity in the depletion layer. The reduction becomes more profound at larger variation magnitude and range. This trend is in general agreement with that observed from the previous stepwise model; however, that model has overestimated the influence of permittivity variation effect. For the thin depletion layer and the relative thick EDL, our calculation indicates that the permittivity variation effect on EDL usually can be neglected. Furthermore, our model can be readily extended to study the permittivity variation in EDL due to ion accumulation and hydration in the EDL region.     

电解液离子与炭电极双电层电容的关系

以酚醛树脂基纳米孔玻态炭(NPGC)为电极, 通过微分电容伏安曲线的测试, 研究了水相体系电解液离子与多孔炭电极双电层电容的关系. 结果表明, 稀溶液中, 多孔炭电极的微分电容曲线在零电荷点(PZC)处呈现凹点, 电容降低, 双电层电容受扩散层的影响显著；若孔径小, 离子内扩散阻力大, 电容下降更为迅速, 扩散层对双电层电容的影响增大. 而增大炭材料的孔径或电解液浓度, 可明显减弱甚至消除扩散层对电容的影响. 炭电极的单位面积微分电容高, 仅表明孔表面利用率高, 如欲获得高的电容量, 还要有大的比表面积. 离子水化对炭电极的电容产生不利影响, 选用大离子和增大炭材料的孔径, 可有效降低离子水化对炭电极电容性能的影响.     

Electric double layer and electrostatic interaction of hydrophobic particles

An hypothesis regarding the impact of water density near hydrophobic surfaces on the electrostatic component of their interaction was offered. A theoretical model of the electric double layer and the interparticle interaction under conditions of the variable density and, consequently, variable dielectric permittivity of water has been developed. It was shown that reduction of the dielectric permittivity near interfaces determined by their hydrophobicity resulted in compression of double electrical layers and weakening of their overlapping. This, in its turn, results in reduction of the electrostatic repulsion of hydrophobic disperse particles as compared with nonhydrophobic ones.     

A discussion on ion conductivity at cation exchange membrane/solution interface

By Gouy–Chapman–Stern–Grahame (CGSG) model, the electric double layer at ion exchange membrane/solution interface consists of two parts: the Stern layer and the diffusion layer. The ions in Stern layer are compacted and considered to be immobile. The relation of diffusion layer mean conductivity K with outer Stern layer potential φ₀, the boundary potential φ_δ and the electrolyte concentration C₀ is educed for symmetric electrolyte system. The results show that K is higher than that of the bulk solution and is greatly influenced by φ₀, φ_δ and C₀.The examination of PE01 cation exchange membrane/solution interface resistance R_e measured by ac impedance technique, shows that R_e value decreases quickly as the KCl electrolyte concentration rises. The effect of electrolyte concentration on the resistance of EDL can be explained by the electrical interactions between ions and charged groups of the membrane. Since the membrane/solution interface resistance is much higher than that of bulk solution, therefore, a further analysis based on the theory developed in this study proves that the ion transfer resistance R_e of membrane–solution interface predominantly occurs at Stern layer as a result of static electrical interaction.     

Atomic force microscopy study of cellulose surface interaction controlled by cellulose binding domains

Colloidal probe microscopy has been used to study the interaction between model cellulose surfaces and the role of cellulose binding domain (CBD), peptides specifically binding to cellulose, in interfacial interaction of cellulose surfaces modified with CBDs. The interaction between pure cellulose surfaces in aqueous electrolyte solution is dominated by double layer repulsive forces with the range and magnitude of the net force dependent on electrolyte concentration. AFM imaging reveals agglomeration of CBD adsorbed on cellulose surface. Despite an increase in surface charge owing to CBD binding to cellulose surface, force profiles are less repulsive for interactions involving, at least, one modified surface. Such changes are attributed to irregularity of the topography of protein surface and non-uniform distribution of surface charges on the surface of modified cellulose. Binding double CBD hybrid protein to cellulose surfaces causes adhesive forces at retraction, whereas separation curves obtained with cellulose modified with single CBD show small adhesion only at high ionic strength. This is possibly caused by the formation of the cross-links between cellulose surfaces in the case of double CBD.     

Adsorption of monoclonal IgG on polystyrene microspheres

In this paper the adsorption of a monoclonal antibody IgG-1 isotype against HBsAg onto positively and negatively charged polystyrene beads has been studied. To determine the role played by electrostatic forces in the adsorption process different pH values were used. It was confirmed that the affinity of adsorption isotherms depends on the electrostatic interaction between protein and polymer surface. The maximum adsorption amount is located around the i.e.p. of the dissolved protein, and decreases markedly as pH moves away. Thus, the major driving force for adsorption of monoclonal antibodies on polystyrene beads comes from the hydrophobic interaction between the antibody molecules and the adsorbent surface. Desorption of preadsorbed IgG molecules by increasing ionic strength has shown that the positively charged polystyrene is also more hydrophobic in character than the negatively charged one. Finally, electrokinetic experiments have determined that the electric double layer (e.d.l.) of monoclonal antibody changes as the consequence of adsorbing on charged polymer surfaces.     

Overcharging in Colloids: Beyond the Poisson–Boltzmann Approach

A broad range of manufactured products and biological fluids are colloids. The ability to understand and control the processes (of scientific, technological and industrial interest) in which such colloids are involved relies upon a precise knowledge of the electrical double layer. The traditional approach to describing this ion cloud around colloidal particles has been the Gouy–Chapman model, developed on the basis of the Poisson–Boltzmann equation. Since the early 1980s, however, more sophisticated theoretical treatments have revealed both quantitative and qualitative deficiencies in the Poisson–Boltzmann theory, particularly at high ionic strengths and/or high surface charge densities. This review deals with these novel approaches, which are mostly computer simulations and approximate integral equation theories based on the so‐called primitive model. Special attention is paid to phenomena that cannot be accounted for by the classic theory as a result of neglecting ion size correlations, such as overcharging, namely, the counterion concentration in the immediate neighborhood of the surface is so large that the particle surface is overcompensated. Other illustrative examples are the nonmonotonic behavior of the electrostatic potential and attractive interactions between equally charged surfaces. These predictions are certainly remarkable and, on paper, they can have an effect on experimentally measurable quantities (for instance, electrophoretic mobility). Even so, these new approaches have scarcely been applied in practice. Thus a critical survey on the relevance of ion size correlation in real systems is also included. Overcharging of macroions can also be brought about by adsorption of oppositely charged polyelectrolytes. Noteworthy examples and theoretical approaches for them are also briefly reviewed.     

聚苯乙烯胶乳的表面电性质与表面活性离子的吸附

根据电泳与电导的测量得出,聚苯乙烯胶乳质点的ξ电势随电解质浓度增加而变大,主要是质点表面基团与溶液间离子交换的结果。根据Langmuir吸附公式与Stern双电层模型,由电泳数据求出了表面活性阳离子在聚苯乙烯胶乳上的吸附自由能与吸附位数。增大电解质浓度使质点表面吸附位数增加,表面活性阳离子的吸附量也因此变大。     

Charge dependence of adsorption energy of ions at electrodes

Analysis of the energetic and geometric characteristics of the inner part of the electrical double layer has been carried out in the presence of the specific adsorption of ions accompanied by change in the dimensions and dielectric properties of the inner layer. On the basis of the formulae for the electrochemical potential of the adsorbed ion on the electrode surface and for the components of the inner layer capacity, the equations of the Frumkin isotherm and of its parameters have been derived. It has been shown that, in general, the Frumkin isotherm is non-linear at the given electrode charge and the charge dependence of the adsorption equilibrium constant has a parabolic form. The results of the corresponding theoretical calculations have been compared with the experimental data obtained by the study of the specific adsorption of tetra-alkylammonium cations on a bismuth electrode in ethanolic and aqueous solutions. Good agreement of the experimental results with those calculated theoretically confirm the validity of the equations derived.