Double electrical layer electrokinetic potential

Two parameters were introduced into the description of double electrical layer. One of them is the point of zero charge (PZC) which according to lUPAC definition [101] can be expressed as concentration of potential-determining ions PDI at which the surface charge is equal to zero ( o = 0), as well as the surface potential (V>o = 0). Another parameter is isoelectric point (lEP) defined [101] as concentration of PDI at which the electrokinetic potential is equal to zero (( = 0). 

Kondo et al. [76] discuss the effect of different salts on the CjS hydration in relation to the mobility of ions and conclude that the accelerating effect of added electrolyte is determined by the rate of diffusion of particular ions. The addition of added electrolyte means the simultaneous impact of both anions and cations on the double electric layer. The ions strongly accelerating the hydration reduce the double layer thickness and the electrokinetic potential in such a way the coagulation of colloid C-S-H is facilitated. The formation of gel of higher density and smaller pores thus of lower permeability is therefore favoured. 

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

A further electrokinetic phenomenon is the inverse of the former according to the Le Chatelier-Brown principle if motion occurs under the influence of an electric field, then an electric field must be formed by motion (in the presence of an electrokinetic potential). During the motion of particles bearing an electrical double layer in an electrolyte solution (e.g. as a result of a gravitational or centrifugal field), a potential difference is formed between the top and the bottom of the solution, called the sedimentation potential. 

The potential governing these electrokinetic effects is clearly at the boundary (the face of shear) between the stationary phase (the fixed double layer) and the moving phase (the solution). This potential is called the electrokinetic potential or the zeta potential. An electrokinetic phenomenon in soil involves coupling between electrical, chemical, and hydraulic gradients. 

The electrokinetic potential (zeta potential, Q is the potential drop across the mobile part of the double layer (Fig. 3.2c) that is responsible for electrokinetic phenomena, for example, elecrophoresis (= motion of colloidal particles in an electric field). It is assumed that the liquid adhering to the solid (particle) surface and the mobile liquid are separated by a shear plane (slipping plane). The electrokinetic charge is the charge on the shear plane. 

If the solid diaphragm material adsorbs both hydrogen and hydroxyl ions it is evident that electric endosmose will cease when equal ionic adsorption has taken place, the double layer potential or electrokinetic potential being at this point zero and the diaphragm is at the isoelectric point. 

The electrical double layer at the metal oxide/electrolyte solution interface can be described by characteristic parameters such as surface charge and electrokinetic potential. Metal oxide surface charge is created by the adsorption of electrolyte ions and potential determining ions (H+ and OH-).9 This phenomenon is described by ionization and complexation reactions of surface hydroxyl groups, and each of these reactions can be characterized by suitable constants such as pKa , pKa2, pKAn and pKct. The values of the point of zero charge (pHpzc), the isoelectric point (pH ep), and all surface reaction constants for the measured oxides are collected in Table 1. 

At the beginning, the electric double layer at the solid-aqueous electrolyte solution interface was characterized by the measurements of the electrokinetic potential and stability of dispersed systems. Later, the investigations were supported by potentiometric titration of the suspension, adsorption and calorimetric measurements [2]. Now, much valuable information on the mechanism of the ion adsorption can be obtained by advanced spectroscopic methods (especially infrared ATR and diffuse spectroscopy) [3], Mosbauer spectroscopy [4] and X-ray spectroscopy [5]. Some data concerning the interface potential were obtained with MOSFET [6], and AFM [7]. An enthalpy of the reaction of the metal oxide-solution systems can be obtained by... 

Zeta potential — The electrical -> potential difference between the bulk solution and the shear plane or outer limit of the rigid part of the double layer (the limits of the diffuse - double layer) is the electrokinetic potential , often called the Zeta potential ((or more precisely the Zeta potential difference (). 

Many more-sophisticated models have been put forth to describe electrokinetic phenomena at surfaces. Considerations have included distance of closest approach of counterions, conduction behind the shear plane, specific adsorption of electrolyte ions, variability of permittivity and viscosity in the electrical double layer, discreteness of charge on the surface, surface roughness, surface porosity, and surface-bound water [7], Perhaps the most commonly used model has been the Gouy-Chapman-Stem-Grahame model 8]. This model separates the counterion region into a compact, surface-bound Stern" layer, wherein potential decays linearly, and a diffuse region that obeys the Poisson-Boltzmann relation. 

Zeta Potential Properly called the electrokinetic potential, the zeta potential refers to the potential drop across the mobile part of the electric double layer. Any species undergoing electrophoretic motion moves with a certain immobile part of the electric double layer that is assumed to be distinguished from the mobile part by a sharp plane, the shear plane. The zeta potential is the potential at that plane. 

The pH value at which the oxide surface carries no fixed charge, i.e. Oj = 0, is defined as the point of zero charge (PZC) . A closely related parameter, the isoelectric point (lEP), obtained from electrophoretic mobility and streaming potential data, refers to the pH value at which the electrokinetic potential equals to zero The PZC and lEP should coincide when there is no specific adsorption in the iimer region of the electric double layer at the oxide-solution interface. In the presence of the specific adsorption, the PZC and lEP values move in opposite directions as the concentration of supporting electrolyte is increased. ... 

In another mode of presentation of experimentally determined surface charging data, values of parameters of a certain model of an electrical double layer, adjusted to the experimentally determined results, are reported rather than the PZC. Usually, this information is sufficient to calculate the PZC, and the result of such a calculation (rounded to the nearest one-tenth of pH unit) is used in the present compilation when the PZC is not explicitly reported in the original publication. A few studies report the results (usually electrokinetic potentials) for... 

Gorichev, I.G., Batrakov, V.V., and Dorofeev, M.V., The calculation from electrokinetic potentials of electrical double-layer parameters and acid-base equihbria constants for an oxide electrolyte interface, Russ. J. Electrochem., 30,105,1994. 

In many flotation systems, the electrical nature of the mineral/water interface controls the adsorption of collectors. The flotation behavior of insoluble oxide minerals, for example, is best understood in terms of electrical double-layer phenomena. A very useful tool for the study of these phenomena in mineral/water systems is the measurement of electrokinetic potential, which results from the interrelation between mechanical fluid dynamic forces and interfacial potentials. Two methods most commonly used in flotation chemistry research for evaluation of the electrokinetic potential are electrophoresis and streaming potential. 

By increasing the electrolyte concentration, the electric double layer is compressed and the compensation ions are situated tightly above the charging layer. In the case of a low electrolyte concentration, or in the absence of ions of a low valency, the thickness of the compensation layer increases, the compensation ions dissociate and they are separate from the charging layer by diffusion. This results in the formation of a micelle charge and increase of the value of the electrokinetic potential. The electrokinetic po-... 

The amount of Si ions dissolution is found to be dependent on surface modification, which was confirmed by induchvely coupled plasma-atomic emission spectrometer (ICP-AES) analysis. Table 2.2 shows the dissolution amount of Si ions with and without surface modification of fumed silica slurry. Without surface modification, the amount of Si dissoluhon was 1.370 0.002 mol/L, whereas surfaces modified with poly(vinylpyrrolidone) (PVP) polymer yielded a dissoluhon of 0.070 0.001 mol/L, almost 20 hmes less than the unmodified surface. Figure 2.6 represents the electro-kinetic behavior of silica characterized by electrosonic amplitude (ESA) with and without surface modification. When PVP polymer modified the silica surface, d5mamic mobility of silica particles showed a reduchon from -9 to -7 mobility units (10 m /Vxs). Dynamic mobility of silica particles lacking this passivation layer shows that silica suspensions exhibit negative surface potentials at pH values above 3.5, and reach a maximum potential at pH 9.0. However, beyond pH 9.0, the electrokinetic potential decreases with an increasing suspension pH. This effect is attributed to a compression of the electrical double layer due to the dissolution of Si ions, which resulted in an increase of ionic silicate species in solution and the presence of alkali ionic species. When the silica surface was modified by... 

There are four electrokinetic phenomena (electrophoresis, electroosmosis, streaming potential, and sedimentation potential), all of which involve both the theory of the electric double layer and that of liquid flow. Among them electrophoresis has the greatest practical applicability to the study of biomolecules and biocell surface porperties. In this section, the relation between electrophoretic mobility and its related electrokinetic potential C will be discussed. 

Zeta potential refers to the electrokinetic potential at the shear surface of the electric double layer. Along with surface conductance, zeta potential is an important parameter in the study and modeling of a variety of electrokinetic... 

When a (solid) surface moves in a liquid, or vice versa, there is always a layer of liquid adjacent to the surface that moves with the same velocity as the surface. The distance from the surface over which this stagnant liquid layer extends or, in other words, the location of the boundary between the mobile and the stationary phases, the so-called plane of shear or slip plane, is not exactly known. For smooth surfaces, the plane of shear is within a few liquid (water) molecules from the surface (see Figure 9.4), that is, well within the electrical double layer. The stagnant layer is probably somewhat thicker than the Stern layer, so that the plane of shear is located in the diffuse part of the electrical double layer. It follows that the potential at the plane of shear, that is, the electrokinetic potential or the zeta potential is somewhat lower than the Stern potential /j. Because the largest part of the potential drop in the... 

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