Potential electrokinetic

Theoretically, electrokinetic processes should also occur in nondisperse systems, but then additional factors arise (vortex formation in the liquid, settling of the particles, etc.) which produce a strong distortion. Hence, electrokinetic processes can be regarded as one of the aspects of the electrochemistry of disperse heterogeneous systems. 

Electrokinetic processes only develop in dilute electrolyte solutions. The second phase can be conducting or nonconducting. Processes involving insulators are of great importance, since they provide the only way of studying the structure and electrical properties of the surface layer of these materials when they are in contact with the solution. Hence, electrokinetic processes can also be discussed as one of the aspects of insulator electrochemistry. 

Auxiliary electrodes are placed into the solution to set up the electric field that is needed to produce electrophoresis or electroosmosis. Under these conditions an electric current passes through the solution and the external circuit its value depends on the applied voltage and on solution conductivity. The lower this conductivity, the higher will be the electric field strength E (or ohmic voltage drop) in the solution that can be realized at a given value of current. 

Transport processes of this type are called nonfaradaic transport. The nonfaradaic transport considered here is a steady-state process, in contrast to nonfaradaic currents mentioned previously that were due, for example, to charging of the electric double layer. Electrokinetic processes are of great practical significance, as discussed in Section 31.3. 

The electrokinetic processes have electrostatic origins they are linked to the charges present on both sides of the slip plane close to the phase boundary. The charge and potential distribution in the surface layer can be described by the relations and laws outlined in Chapter 10. 

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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. 

The charge on the surface of colloid particles is an important parameter, and DNA/cation self-assembled complexes are no exception. It can be measured experimentally as the -potential or electrokinetic potential (the potential at the surface of shear be-... 

As the particle moves relative to the electrolyte solution, the layer of water mol-ecnles that is directly adjacent to the particle surface is strongly bonnd and will be pnlled along. The thickness of this bonnd layer is approximately one or two diameters of a water molecule. We shall write x, for the x-coordinate of this layer s outer boundary, which is the slip plane. The electrostatic potential at this plane relative to the potential in the bulk solution is designated by the Greek letter and called the zeta potential or electrokinetic potential of the interface discussed. This potential is a very important parameter characterizing the electrokinetic processes in this system. 

Consider a solid surface in contact with a dilute electrolyte solution. The plane where motion of the liquid can commence is parallel to the outer Helmholtz plane but shifted in the direction into the bulk of the solution. The electric potential in this plane with respect to the solution is termed the electrokinetic potential ( = 02 ). 

If the electric field E is applied to a system of colloidal particles in a closed cuvette where no streaming of the liquid can occur, the particles will move with velocity v. This phenomenon is termed electrophoresis. The force acting on a spherical colloidal particle with radius r in the electric field E is 4jrerE02 (for simplicity, the potential in the diffuse electric layer is identified with the electrokinetic potential). The resistance of the medium is given by the Stokes equation (2.6.2) and equals 6jtr]r. At a steady state of motion these two forces are equal and, to a first approximation, the electrophoretic mobility v/E is... 

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 electrical potential in the layer of the liquid moving only negligibly at the surface of the pores is equal to the electrokinetic potential (see page 242). At the middle of the pore, the electric potential and its... 

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. 

In practice we use the electrokinetic potential, , in place of the surface potential as it is readily measurable and will reflect the changes to the surface as a result of adsorbed ionic species. 

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system. Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity. 

In addition to the interphase potential difference V there exists another potential difference of fundamental importance in the theory of the electrical properties of colloids namely the electro-kinetic potential, of Freundlich. As we shall note in subsequent sections the electrokinetic potential is a calculated value based upon certain assumptions for the potential difference between the aqueous bulk phase and some apparently immobile part of the boundary layer at the interface. Thus represents a part of V but there is no method yet available for determining how far we must penetrate into the boundary layer before the potential has risen to the value of the electrokinetic potential whether in fact f represents part of, all or more than the diffuse boundary layer. It is clear from the above diagram that bears no relation to V, the former may be in fact either of the same or opposite sign, a conclusion experimentally verified by Freundlich and Rona. 

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. 

Instead of employing a cataphoretic method for determining the electrokinetic potentials at air-liquid interfaces, even more valuable information may be obtained by measurement of the E.M.F. of a cell of the type... 

Powis found in the case of oil emulsions, that a relatively large electrokinetic potential, =0 040 volt, did not prevent coagulation, an indication that the surface adsorbed layer is more mobile than for gold. 

On the addition of small quantities of electrolyte all the amicrons are not discharged by ionic adsorption. Only those that are completely discharged or those for which the electrokinetic potential has been reduced below the critical maximum will actually adhere to one another on contact. We should thus with... 

If the diminution in intrinsic potential is accompanied by a corresponding decrease in the electrokinetic potential, the new aggregate will not be at the critical point but will be able to coalesce with another particle which possesses a slight electrokinetic potential. Thus aggregates will be built up which can react more readily than the small ones owing to a decrease in the interfacial potentials. 

Electrogenerative synthesis, 1377 Electrogrowth of metals on electrodes, 1293 see also Electrodeposition Electrokinetic potential, 1069... 

A. M. Grancaric, T. Pusic, I. Soljacic, and V. Ribitsch, Influence of Electrokinetic Potential on Adsorption of Cationic Surfactants," Textile Chemist Colorist 29(12) 33 (1997). 

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