J. Lyklema

Measurement and interpretation of electrokinetic phenomena - (IUPAC technical report)

In this report, the status quo and recent progress in electrokinetics are reviewed. Practical rules are recommended for performing electrokinetic measurements and interpreting their results in terms of well-defined quantities, the most familiar being the z-potential or electrokinetic potential. This potential is a property of charged interfaces, and it should be independent of the technique used for its determination. However, often the z-potential is not the only property electrokinetically characterizing the electrical state of the interfacial region; the excess conductivity of the stagnant layer is an additional parameter. The requirement to obtain the z-potential is that electrokinetic theories be correctly used and applied within their range of validity. Basic theories and their application ranges are discussed. A thorough description of the main electrokinetic methods is given; special attention is paid to their ranges of applicability as well as to the validity of the underlying theoretical models. Electrokinetic consistency tests are proposed in order to assess the validity of the z-potentials obtained. The recommendations given in the report apply mainly to smooth and homogeneous solid particles and plugs in aqueous systems; some attention is paid to nonaqueous media and less ideal surfaces.

DLVO-theory : a dynamic re-interpretation

Abstract As a tribute to the celebration of DLVO theory we reconsider particle interaction from a dynamic point of view. Special attention is paid to identifying various currents occurring between interacting colloids, with their time of relaxation estimated. The relation with electrokinetic phenomena is reconsidered and the information obtained integrated into the stability phenomena. Moreover, a dynamic interpretation of the slip process is offered.

Hydrolysis versus ion correlation models in electrokinetic charge inversion: establishing application ranges.

In this article, we investigate experimentally a wide range of situations where charge inversion (i.e., overcompensation of the surface charge of a colloidal particle by the countercharge) can occur. To that end, the electrophoretic mobility of sodium montmorillonite, silica, and polystyrene latex as functions of pH and concentration of different salts is presented, and conditions are established where charge inversion occurs. The reason for this study is to provide experimental evidence for distinguishing between two existing models for the explanation of charge inversion. One of these is the specific adsorption of ions located in the Stern layer in combination with a Gouy-Chapman diffuse part of the double layer. The other ion-correlation theories explain the phenomenon in terms of purely physical arguments based on Coulombic pair interactions between ions and surface charges and on excluded volume effects. In distinguishing between these two interpretations, the influence of the pH plays a central role because of its effect on the hydrolysis of multivalent cations. In our experiments, it is found that although 1-2 and 2-2 electrolytes provoke a decrease in the absolute values of the electrophoretic mobilities when their concentration in solution is increased, they never lead to charge inversion, whatever the surface charge or the pH. However, in the case of salts of trivalent cations, electrokinetic charge reversal is often observed above a certain critical electrolyte concentration. In addition, the extent of overcharging increases when the concentration is raised above the critical value. This trend occurs for any system in which the surface charge is pH-independent, as in polystyrene latex and montmorillonite. Most of the results presented here are compatible with the specific adsorption of hydrolyzed metal ions as the main driving force for charge inversion. At low pH, when the hydrolysis of trivalent cations is likely to be absent, overcharging can be attributed to ion correlation effects.

Preparation and characterization of anatase powders

Anatase powders have been prepared by precipitation and by sol–gel methods. In the former, titania was continuously precipitated at 25°C, pH 1.97 in a stirred reactor by mixing TiOSO4 and potassium hydroxide so as to keep solution supersaturation constant throughout the precipitation process. The preparations were performed in the absence and in the presence of Li+, Nb5+ and W6+ ions. In all cases anatase was the sole phase forming. The presence of these metal ions (1–7×10-5M) did not influence the precipitation kinetics, which was controlled by surface integration. XPS analysis showed that the dopant ions were incorporated into the anatase lattice for the preparations in supersaturated solutions. Microelectrophoresis experiments did not show any differentiation of the electric charge of the preparations in the presence and in the absence of these ions. The relatively high specific surface area (SSA) of the anatase obtained (135 g-1) increased (up to 250 g-1) by the incorporation of the dopant ions. In the sol–gel preparations the process was found to depend on the supersaturation of the sol with respect to the solid phase forming. A threshold sol composition corresponding to a total titanium concentration of CTi=0.06 M (25°C, pH=2.5–3.0) was found to be critical for the formation of the gel. Anatase was exclusively formed both in the absence and in the presence of dopant ions in the gel. The SSA obtained was low although it increased in the presence of dopant metal ions. Electrokinetic measurements of the solids formed by the sol–gel method, suggested that the dopant ions tend to accumulate on the surface of the anatase particles.

Streaming potentials and conductivities of porous silica plugs.

Abstract Streaming potentials and conductivities of close-packed plugs of spherical Stober-silica particles were measured. Stober silica is highly porous; more than 98% of the countercharge may be situated inside the particle. Only the countercharge in the double layer region outside the porous particle contributes to the plug conductivity and the streaming potential. The countercharge obtained from plug conductivities is less than the double layer charge obtained from titrations. The countercharge obtained from streaming potentials is less than obtained from plug conductivities, which indicates the occurrence of conduction behind the shear plane. It is essential to take this conduction into proper account in order to arrive at more realistic ζ -potentials.

Electro-kinetic measurements on plugs of doped titania.

Abstract The z-potential and the surface conductivity of pure anatase and anatase doped with Li+, Nb5+ and W6+ prepared with both the precipitation (continuous crystallization at constant supersaturation) and the sol–gel method have been determined at two pH values (3.3. and 6.0 or 6.5). This was carried out by measuring the streaming potential and electrical conductivity of a porous plug containing the anatase particles. The sol–gel preparations exhibited a higher density than those obtained by precipitation. Plots of the plug conductivity as a function of the conductivity of the solution are essentially linear, allowing for the establishment of the surface conductivity, Kσ. The main contribution to Kσ stemmed from the counter charge behind the slip plane. At pH 3.3, i.e. much lower than the i.e.p., doping with Nb5+ and Li+ caused a considerable increase of the surface conductivity in both preparations, whereas at a pH 6.0 or 6.5, i.e. near the i.e.p., doping led to a slight decrease of the relatively high surface conductivity of the sample prepared by precipitation. At pH 6.5 sol–gel preparations showed negligible surface conductivity, independent of the doping, with the exception of the preparation with a high concentration of W6+. Moreover, doping with W6+ did not considerably change the surface conductivity. There is some indirect correlation between the surface and the bulk conductivity of the various samples. Tangential mobilities of Cl− ions in the stagnant layers are of the same order of magnitude as those in the bulk solution.

Interfacial Electrostatics and Electrodynamics in Disperse Systems

Although there are physically relevant analogies between the properties of electrical double layers on electrodes and on dispersed matter, research in these two domains has to a large extent followed disparate paths. The lack of interaction is perhaps understandable but it is also a pity.