Induced-charge electrokinetic phenomena

The field of nonlinear “induced-charge” electrokinetics is rapidly advancing, motivated by potential applications in microfluidics as well as by the unique opportunities it provides for probing fundamental scientific issues in electrokinetics. Over the past few years, several surprising theoretical predictions have been observed in experiments: (i) induced-charge electrophoresis of half-metallic Janus particles, perpendicular to a uniform AC field; (ii) microfluidic mixing around metallic structures by induced-charge electro-osmosis, and (iii) fast, high-pressure AC electro-osmotic pumping by non-planar electrode arrays, and ICEK effects upon the collective behavior of polarizable particle suspensions has been studied theoretically and computationally. A new experimental system enables a clean and direct comparison between theoretical predictions and measured ICEK flows, providing a route to fundamental studies of particular surfaces and high-throughput searches for optimal ICEK systems. Systematic discrepancies between theory and experiment have engendered the search for mechanisms, including new theories that account for electrochemical surface reactions, surface contamination, roughness, and the crowding of ions at high voltage. Promising directions for further research, both fundamental and applied, are discussed.     

The history of electrokinetic phenomena

Electrokinetic phenomena comprise the phenomena where a liquid moves tangentially to a charged surface. Well-known phenomena of this kind are electrophoresis, electro-osmosis, streaming potential and sedimentation potential. A historical review is given here, starting with their discovery by F.F. Reuss in 1808 and continuing with the early investigators including G. Wiedemann, G.Quincke, E. Dorn and U. Saxén. It is also discussed how electrokinetic phenomena gave rise to the concept electrical double layer in colloid science. The development of the theory starting with H. Helmholtz, continuing with M. Smoluchowski is described. Extension of the theory including relaxation and surface conduction is included. Finally the history of other kinds of electrokinetic phenomena such as electroacoustics and diffusiophoresis is treated.     

Applications of electrokinetic phenomena in materials science

The discovery of electrokinetic phenomena by Reuss in 1808 and further investigations that gave rise to the concept of the electrical double layer have played an important role in the understanding of colloidal stability. Electrokinetic phenomena are a family of effects in which a liquid moves tangentially to a charged surface. Well-known phenomena of this kind are electrophoresis, electro-osmosis, streaming potential, and sedimentation potential. A review of parameters involved in the electrochemistry of suspensions is made. The practical applications of these phenomena have become widespread in a broad range of research fields such as biomaterials, biofilms, electrokinetic waste remediation, membranes, nuclear and fossil-fired power plants, adhesive and sealant science, and concrete science. The purpose of this paper is to provide an overview of electrokinetic phenomena and their application to surface modification and characterization in a large number of research fields such as corrosion and protection processes, environmental remediation (soil and sediments, transport processes, inorganic pollutants, solid particle surfaces, filter membranes, and biosorption phenomena), cement-based systems, and biological systems.     

Two-peak phenomena and formation origin in micellar electrokinetic capillary chromatography

Since Terabe et al.[1] developed micellar electrokinetic capillary chromatography (MECC) in 1984, a great number of important advances about separating neutral compounds have been achieved. In MECC mode, micellar of an ionic surfactant can form so-called pseudo stationary phase in the buffer solution when it is above the critical micelle concentration, and some portions of the solute may be distributed into the micellar phase when they are mobilized into the buffer solution from sample zone…     

Role of electrokinetic phenomena in the preparation of supported metal catalysts

A model accounting for the effect of electrokinetic phenomena on the prepartion of supported metal catalysts by wet impregnation is presented. The model is able to explain the dependence of catalyst distribution profile and loading on the concentration of the impregnant, on the pH and ionic strength of the impregnating solution, as well as on the presence of a co-impregnating species.     

Recent advances in colloidal and interfacial phenomena involving liquid crystals

This feature article describes recent advances in several areas of research involving the interfacial ordering of liquid crystals (LCs). The first advance revolves around the ordering of LCs at bio/chemically functionalized surfaces. Whereas the majority of past studies of surface-induced ordering of LCs have involved surfaces of solids that present a limited diversity of chemical functional groups (surfaces at which van der Waals forces dominate surface-induced ordering), recent studies have moved to investigate the ordering of LCs on chemically complex surfaces. For example, surfaces decorated with biomolecules (e.g., oligopeptides and proteins) and transition-metal ions have been investigated, leading to an understanding of the roles that metal-ligand coordination interactions, electrical double layers, acid-base interactions, and hydrogen bonding can play in the interfacial ordering of LCs. The opportunity to create chemically responsive LCs capable of undergoing ordering transitions in the presence of targeted molecular events (e.g., ligand exchange around a metal center) has emerged from these fundamental studies. A second advance has focused on investigations of the ordering of LCs at interfaces with immiscible isotropic fluids, particularly water. In contrast to prior studies of surface-induced ordering of LCs on solid surfaces, LC-aqueous interfaces are deformable and molecules at these interfaces exhibit high levels of mobility and thus can reorganize in response to changes in the interfacial environment. A range of fundamental investigations involving these LC-aqueous interfaces have revealed that (i) the spatial and temporal characteristics of assemblies formed from biomolecular interactions can be reported by surface-driven ordering transitions in the LCs, (ii) the interfacial phase behavior of molecules and colloids can be coupled to (and manipulated via) the ordering (and nematic elasticity) of LCs, and (iii) the confinement of LCs leads to unanticipated size-dependent ordering (particularly in the context of LC emulsion droplets). The third and final advance addressed in this article involves interactions between colloids mediated by LCs. Recent experiments involving microparticles deposited at the LC-aqueous interface have revealed that LC-mediated interactions can drive interfacial assemblies of particles through reversible ordering transitions (e.g., from 1D chains to 2D arrays with local hexagonal symmetry). In addition, recent single-nanoparticle measurements suggest that the ordering of LCs about nanoparticles differs substantially from micrometer-sized particles and that the interactions between nanoparticles mediated by the LCs are far weaker than predicted by theory (sufficiently weak that the interactions are reversible and thus enable self-assembly). Finally, LC-mediated interactions between colloidal particles have also been shown to lead to the formation of colloid-in-LC gels that possess mechanical properties relevant to the design of materials that interface with living biological systems. Overall, these three topics serve to illustrate the broad opportunities that exist to do fundamental interfacial science and discovery-oriented research involving LCs.     

Membrane phenomena in chemotaxis in true slime moldPhysarum polycephalum

Summary A mechanism of how the true slime moldPhysarum polycephalum detects electrolytes (repellents) has been studied by measuring the membrane potential and the motive force of tactic movement with the aid of a double chamber method. It was found that the mold detects electrolytes by utilizing a concentration dependence of the membrane potential. It was also demonstrated that the characteristic features of the membrane potential of the mold closely resembled those of a liquid ion-exchange membrane. This suggested that the membrane potential in chemotaxis of the mold is based not on the mechanism specific to biological membranes, but on the usual physicochemical mechanism.Present results suggested a significance of phenomena at the cell membrane-extracellular solution interface rather than a participation of specific receptor molecule(s) in the detection process of the mold.And a possible detection process was discussed.

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Zusammenfassung Es wurde mit Hilfe der Messungen des Membranpotentials und der treibenden Kraft zur taktischen Bewegung mittels der Doppel-Kammer-Methode studiert, wie der echte SchleimpilzPhysarum polycephalum Elektrolyten (die Ablehnungsmittel) wahrnimmt. Zur Wahrnehmung nützt der Pilz die Abhängigkeit des Membranpotentials von der Elektrolyten-Konzentration aus. Die charakteristischen Merkmale des Membranpotentials des Pilzes sind denen von flüssigen Ionenaustauschmembranen sehr ähnlich.Die Ergebnisse deuten darauf hin, daß für die Erkennung Grenzflächen-Phänomene zwischen der Zellmembran und der extrazellularen Lösung wichtiger sind als spezifische Rezeptormoleküle.

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With 6 figures and 1 table     

Adsorption and electrokinetic phenomena in the system solid — micellar solution of a surface-active substance

Summary It has been found that adsorption isotherms describing the adsorption of sodium 4-hexadecyl-oxytolyl2-sulfonate and sodium dodecyl-benzene-4-sulfonate at different solid surfaces revealed maxima at the range of CMC. Similar maxima also occurred on the curves χ/c. Adsorption of the mentioned surface-active substances from micellar solutions at a solid did not lead to establishment of an equilibrium which would result in an adsorption film containing micelles. The decrease in adsorption at a solid in the range of CMC and above it is discussed on the basis of adsorption equilibrium, established between the micelle, the monomer, and the adsorption film. Postmicellar association of the surfactant can result in a minimum on the adsorption isotherm. During adsorption of a surfactant from micellar solutions, solubilization can take place of reaction products of the surfactant and polyvalent inorganic cations or weakly dissociated molecules, in the rangec _eq≧ CMC.

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Zusammenfassung Die Adsorptionsisothermen von Na-HexadecylOxytolyl-2-Sulfonat und Na-Dodecyl-Benzol-4-Sulfonat an verschiedenen festen Oberfl?chen haben im Bereich der CMC ein Maximum. Dieses Maximum wurde auch bei den Abh?ngigkeiten χ/c gefunden. Die Adsorption der untersuchten grenzfl?chenaktiven Stoffe aus ihren mizellaren L?sungen führt nicht zur Bildung eines Adsorptionsfilmes mit eingelagerten Mizellen. Der Abfall der Adsorption der Tensidmoleküle im Bereich der CMC und darüber hinaus wurde aufgrund der Einstellung des Adsorptionsgleichgewichtes zwischen der Mizelle, dem Monomer und dem Adsorptionsfilm diskutiert. Die postmizellare Assoziation der Tensidmoleküle kann die Entstehung eines Minimums auf der Adsorptionsisotherme zur Folge haben. W?hrend der Adsorption des Tensids aus seinen mizellaren L?sungen k?nnen in Bereichenc _eq ≥ CMC wenig dissoziierte Moleküle oder ein Reaktionsprodukt des Tensids mit den mehrwertigen anorganischen Kationen solubilisiert werden.

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Isothermal oscillations and exotic bifurcation behaviour from second-order schemes involving only three components

The scheme of consecutive reactions ABC (with 3 variables and 4 parameters), where the first step proceeds only through catalysis by the dimer B₂, shows multiple stationarystates and a very rich variety of oscillatory behaviour. At one extreme it yields cubic autocatalysis but its exotic behaviour persists over a very wide range indeed. It is thus a useful prototype for oscillatory systems without cubic kinetics.     

Proper and adsorbed charges on the surfaces of the polysulfonic support of a composite membrane from electrokinetic phenomena

The polysulfonic microporous layer of a composite membrane is studied. The membrane is bathed by several diluted aqueous solutions of NaCl or MgCl₂ at 298 K. The equations of transport through this microporous membrane (Nernst-Planck, Navier-Stokes, Poisson, and charge and mass conservation laws) numerically solved, with the adequate limit conditions, are used to obtain the electrokinetic coefficients. From any measured electrokinetic parameter, here the streaming potential and the membrane conductance per unit area, the adsorbed surface charge density (adsorption isotherm) and the zeta potential can be obtained as functions of concentration for each solute. A heterogeneous adsorption mechanism seems to explain the experimental results, therefore the Freundlich's isotherm is used. It is corrected, in order to take into account the complex structure of the electrical double layer and to introduce the fixed or proper charge density per unit area. The proper charge of the membrane seems to be negligible in our case. While the average adsorption Gibbs' free energy and the maximum number of accessible sites are relatively small, according to the hydrophillic character assumed for the membrane. The distribution function of the adsorption Gibbs' potential is calculated and it is seen to be sharp and concentration independent.     

The use of electrokinetic potential in the interpretation of adsorption phenomena: Adsorption of salicylic acid on hematite

The adsorption of salicylic acid on hematite particles and their electrokinetic mobilities were measured as a function of the pH. The equilibrium in the systems was interpreted by the refined adsorption isotherm which considers the dissociation of the acid in the bulk of the solution and the electrostatic interactions between the surface and the adsorbed ions. The latter was taken into account through the surface potential which was obtained from the electrokinetics using the Gouy–Chapman theory and introducing the concept of slipping plane separation. It was concluded that singly charged anions are the adsorbable species, which occupy a surface area of 87±15 Å² and that the adsorption equilibrium constant is log K°=3.5±0.1. The slipping plane separation from the surface was found to be 15 Å.     

On the use of electrokinetic phenomena of the second kind for probing electrode kinetic properties of modified electron-conducting surfaces

The electrokinetic features of electron-conducting substrates, as measured in a conventional thin-layer electrokinetic cell, strongly depend on the extent of bipolar faradaic depolarisation of the interface formed with the adjacent electrolytic solution. Streaming potential versus applied pressure data obtained for metallic substrates must generally be interpreted on the basis of a modified Helmholtz-Smoluchowski equation corrected by an electronic conduction term-non linear with respect to the lateral potential and applied pressure gradient-that stems from the bipolar electrodic behavior of the metallic surface. In the current study, streaming potential measurements have been performed in KNO(3) solutions on porous plugs made of electron-conducting grains of pyrite (FeS(2)) covered by humic acids. For zero coverage, the extensive bipolar electronic conduction taking place in the plug-depolarized by concomitant and spatially distributed oxidation and reduction reactions of Fe(2+) and Fe(3+) species-leads to the complete extinction of the streaming potential over the entire range of applied pressure examined. For low to intermediate coverage, the local electron-transfer kinetics on the covered regions of the plug becomes more sluggish. The overall bipolar electronic conduction is then diminished which leads to an increase in the streaming potential with a non-linear dependence on the pressure. For significant coverage, a linear response is observed which basically reflects the interfacial double layer properties of the humics surface layer. A tractable, semi-analytical model is presented that reproduces the electrokinetic peculiarities of the complex and composite system FeS(2)/humics investigated. The study demonstrates that the streaming potential technique is a fast and valuable tool for establishing how well the electron transfer kinetics at a partially or completely depolarised bare electron-conducting substrate/electrolyte solution interface is either promoted (catalysis) or blocked (passivation) by the presence of a discontinuous surface layer.     

Cell model calculation for electrokinetic phenomena in concentrated suspensions: an Onsager relation between sedimentation potential and electrophoretic mobility

Cell model calculations for the electrophoretic mobility, electrical conductivity and sedimentation potential in concentrated suspensions of colloidal particles with low zeta potentials are reviewed with particular emphasis on an Onsager relation between sedimentation potential and electrophoretic mobility. A general Onsager relation is derived on the basis of the thermodynamics of irreversible processes. This relation, which involves the ratio of the electrical conductivity K* of the suspension to the conductivity Kinfinity in the absence of the particles, reproduces the Onsager relation derived from cell model calculations at low zeta potentials, where K*/Kinfinity becomes (1 - phi)/(1 + phi/2), phi being the particle volume fraction.     

Membrane characteristics of composite collodion membrane III. Effect of added NaClO4 on transport phenomena

Composite collodion membranes modified by the addition of different amounts (0–1.0 wt.%) of NaClO₄ were prepared keeping a fixed concentration of perfluorobenzoic acid (PFBA) within membrane. The composite membranes indicated relatively high cation transport number and proved to be available as a cation exchange membrane. From volume flow and salt flow studies across membranes, the filtration coefficient, L_p, and the salt permeability, ω, were estimated as a function of added NaClO₄. As a result, while L_p increased gradually in proportion to the amounts of NaClO₄, ω indicated an abrupt increase of around 0.2 wt.% of NaClO₄. The similar critical behavior was found in the measurement of membrane conductance and the result assured the critical change in ω.Furthermore, based on the above membrane parameters such as L_p, ω and Λ_m, the frictional consideration regarding the interactions between water and membrane matrix or between salt and membrane matrix was done and the transport properties were discussed in relation to the membrane structure.     

Micellar electrokinetic chromatography-mass spectrometry

The combination of micellar electrokinetic chromatography (MEKC) with mass spectrometry (MS) is very attractive for the direct identification of analyte molecules, for the possibility of selectivity enhancement, and for the structure confirmation and analysis in a MS-MS mode. The direct coupling of MEKC with MS can be hazardous due to the effect of nonvolatile MEKC surfactants on MS performance, including the loss of analyte sensitivity and ion source contamination. The possibility of off-line coupling between MEKC and matrix-assisted laser desorption/ionization (MALDI)-MS remains to be investigated. Various approaches for on-line coupling MEKC with electrospray ionization (ESI)-MS, including the use of high-molecular-mass surfactant, an electrospray-chemical ionization (ES-CI) interface, a voltage switching and buffer renewal system, partial-filling micellar plug and anodically migrating micelles, are reviewed and evaluated. The use of an ES-CI interface is most promising for routine operation of on-line MEKC-MS under the influence of nonvolatile salts and surfactants. The use of a high-molecular-mass surfactants allows the formation of a micellar phase at very low surfactant concentrations and avoids the generation of a high level of background ions in the low m/z region. Alternatively, the application of a partial-filling micellar plug and anodically migrating micelles eliminate the introduction of MEKC micelles into the ESI-MS system. It is possible to directly transfer the conventional MEKC separations to partial-filling MEKC-ESI-MS and MEKC-ESI-MS using anodically migrating micelles without any instrument modifications.