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Double layer distortion, effect

Overbeek and Booth [284] have extended the Henry model to include the effects of double-layer distortion by the relaxation effect. Since the double-layer charge is opposite to the particle charge, the fluid in the layer tends to move in the direction opposite to the particle. This distorts the symmetry of the flow and concentration profiles around the particle. Diffusion and electrical conductance tend to restore this symmetry however, it takes time for this to occur. This is known as the relaxation effect. The relaxation effect is not significant for zeta-potentials of less than 25 mV i.e., the Overbeek and Booth equations reduce to the Henry equation for zeta-potentials less than 25 mV [284]. For an electrophoretic mobility of approximately 10 X 10 " cm A -sec, the corresponding zeta potential is 20 mV at 25°C. Mobilities of up to 20 X 10 " cmW-s, i.e., zeta-potentials of 40 mV, are not uncommon for proteins at temperatures of 20-30°C, and thus relaxation may be important for some proteins. [Pg.587]

A relatively constant Tafel slope for reactions not involving adsorption, and those involving adsorption with complete charge transfer across the double layer, distorted by second order effects, may also be explained in terms of a non-Franck-Condon process. Since adsorbed intermediates in charge transfer processes also show adsorption energies depending on potential in the same way as the potential energy barrier maxima, these should also follow the same phenomena. [Pg.285]

Calculation of the Zeta Potential. The conversion of electrophoretic mobility to zeta potential is complicated somewhat by the existence of the electrophoretic relaxation effect. Figure 5 shows a schematic diagram of this effect. As we expose an emulsion droplet and its surrounding double layer to an electric field, the double layer distorts to the shape shown in the figure. This distorted double layer now creates its own electric field that... [Pg.56]

Henry s calculations are based on the assumption that the external field can be superimposed on the field due to the particle, and hence it can only be applied for low potentials (f < 25 mV). It also does not take into account the distortion of the field induced by the movement of the particle (relaxation effect). Wiersema, Loeb and Overbeek [19] introduced two corrections for the Henry s treatment, namely the relaxation and retardation (movement of the hquid with the double layer ions) effects. A numerical tabulation of the relationship between mobility and zeta-potential has been provided by Ottewill and Shaw [20]. Such tables are useful for converting u to f at aU practical values of kR. [Pg.137]

When an electric field is applied, localization of ionic distribution takes place and electrical dipoles generate static attractive forces. This electric double layer distortion theory [37-39] is supported by the experimental observation that the ER effect is drastically affected by the addition of water or a surfactant, or the difference in the electric conductivity of the suspended particles and the dispersant. [Pg.758]

The mobility curves have this shape because of double layer distortion. The applied field sweeps the double-layer ions back and forth and this leads to a change in the charge distribution around the particle. For highly charged particles, this has a significant effect on the flow field and the electric forces that act on the particle. [Pg.74]

The double layer distortion is only significant for particles with moderate to high zeta potentials. The effect is most pronounced for Ka values between 1 and 10. In this range it is significant if zeta is more than 50 mV in magnitude. For Ka values outside this range the effect can still be important, but only at higher zeta potentials. [Pg.75]

The electroviscous effect present with solid particles suspended in ionic liquids, to increase the viscosity over that of the bulk liquid. The primary effect caused by the shear field distorting the electrical double layer surrounding the solid particles in suspension. The secondary effect results from the overlap of the electrical double layers of neighboring particles. The tertiary effect arises from changes in size and shape of the particles caused by the shear field. The primary electroviscous effect has been the subject of much study and has been shown to depend on (a) the size of the Debye length of the electrical double layer compared to the size of the suspended particle (b) the potential at the slipping plane between the particle and the bulk fluid (c) the Peclet number, i.e., diffusive to hydrodynamic forces (d) the Hartmarm number, i.e. electrical to hydrodynamic forces and (e) variations in the Stern layer around the particle (Garcia-Salinas et al. 2000). [Pg.103]

The primary electroviscous effect occurs, for a dilute system, when the complex fluid is sheared and the electrical double layers around the particles are distorted by the shear field. The viscosity increases as a result of an extra dissipation of energy, which is taken into account as a correction factor pi" to the Einstein equation ... [Pg.103]

The slopes of the different curves correspond to the fuU electrohydrodynamic effect, ( ) + ( ) pj, where the first term expresses the hydrodynamic effect, and the second is the consequence of the distortion of the electrical double layer that surrounds the particles. To determine this second term and, more exactly, the primary electroviscous coefficient, pi. [Pg.104]

A corrected and more general analysis of the primary electroviscous effect for weak flows, i.e., for low Pe numbers (for small distortions of the diffuse double layer), and for small zeta potentials, i.e., f < 25 mV, was carried out by Booth in 1950. The result of the analysis leads to the following result for the intrinsic viscosity [rj] for charged particles in a 1 1 electrolyte ... [Pg.178]

Although Booth s result already indicates the influence of the charges on the particles and the electrolyte in the dispersion on the viscosity of the dispersion, one sees more complex behavior only when the effect of larger distortions of the double layer is included in the analysis. An extension for larger distortions (represented by the Peclet number Pe) of the double layer is available (Russel 1978a) and can be written as... [Pg.178]

Because the charged particle and its ion atmosphere move in opposite directions, the center of positive charge and the center of negative charge do not coincide. If the external field is removed, this asymmetry disappears over a period of time known as the relaxation time. Therefore, in addition to the fact that the colloid and its atmosphere move countercurrent with respect to one another (which is called the retardation effect), there is a second inhibiting effect on the migration that arises from the tug exerted on the particle by its distorted atmosphere. Retardation and relaxation both originate with the double layer, then, but describe two different consequences of the ion atmosphere. The theories we have discussed until now have all correctly incorporated retardation, but relaxation effects have not been included in any of the models considered so far. [Pg.549]

As can be seen in this figure, the combined effect of ohmic drop and double-layer capacitance is much more serious in the case of CV. The increase of the scan rate (and therefore of the current) causes a shift of the peak potentials which is 50 mV for the direct peak in the case of the CV with v = 100 V s 1 with respect to a situation with Ru = 0 (this shift can be erroneously attributed to a non-reversible character of the charge transfer process see Sect. 5.3.1). Under the same conditions the shift in the peak potential observed in SCV is 25 mV. Concerning the increase of the current observed, in the case of CV the peak current has a value 26 % higher than that in the absence of the charging current for v = 100 Vs 1, whereas in SCV this increase is 11 %. In view of these results, it is evident that these undesirable effects in the current are much less severe in the case of multipulse techniques, due to the discrete nature of the recorded current. The CV response can be greatly distorted by the charging and double-layer contributions (see the CV response for v = 500 V s-1) and their minimization is advisable where possible. [Pg.347]

Electrophoretic problems can be greatly simplified when the distortion of double layer due to the external field and particle movement is negligible. A boundary layer analysis for the ion cloud [6,7] shows that the effect of ion cloud distortion remains insignificant provided that... [Pg.588]

O Brien and White [25] investigated the effect of the distortion of the ion cloud on electrophoresis of a sphere with radius a and constant zeta potential ( when a constant external electric field is applied. The electrical field is assumed to be sufficiently weak so that the deformation of the double layer can be treated as a small perturbation. Hence the electrical potential and the ion concentrations can be expressed by Eq. (27) and... [Pg.593]

The numerical results show that the polarization effect of the double layer impedes particle s migration because an opposite electric field is induced in the distorted ion cloud, which acts against the motion of the particle. For a given ica, the electrophoretic mobility increases first, reaches a maximum value and then decreases as the absolute zeta potential is increased. This maximum mobility arises because the electrophoretic retarding forces increase at a faster rate with zeta potential than does the driving force. [Pg.593]

Analytical expression for the electrophoretic velocity of a sphere can be obtained for a thin but distorted double layer. Dukhin [6] first examined the effect of distortion of thin ion cloud on the electrophoresis of a sphere in a symmetric two-species electrolyte. Dukhin s approach was later simplified and extended by O Brien [7] for the case of a general electrolyte and a particle of arbitrary shape. Since 0(k 1) double layer thickness is much smaller than the characteristic particle size L, the ion cloud can be approximated as a structure composed of a charged plane interface and an adjacent diffuse cloud of ions. Within the double layer, the length scales for variation of quantities along the normal and tangential directions are k ] and L, respectively. [Pg.594]

Charged particles in weak electrolytes have associated with them an electrical double layer. When these particles settle under gravity the double layer is distorted with the result that an electrical field is set up that opposes motion. This effect was first noted by Dorn [74] and was studied extensively by Elton et. al. [75-78] and later by Booth [79,80]. [Pg.335]

Normally large electrodes are used for the common sensors. In our case we use 25 pm diameter microelectrode (see Figure IB) for two reasons The first reason is that we can directly combine the complete electrochemical setup, namely working, reference and counter electrode on top of a small tip. Because of the low currents (some nA) at microelectrode, counter and reference electrode can be combined and the potential of the counter/reference electrode is nearly stable and is not distorted by the small currents. The second positive effect of the use of microelectrodes is that the ratio between Faradayic and double layer current is increasing with decreasing active surface. The reason for that is the different diffusion mechanism compared with Targe electrodes. At microelectrodes the diffusion takes place in a spherical way like to the surface of a drop [2]. [Pg.150]

As discussed in Section 8.3.4, the presence of a finite double-layer capacity results in a charging current contribution proportional to dEldt (equation 8.3.11) and causes /f to differ from the total applied current, /. This effect, which is largest immediately after application of the current and near the transition (where dE/dt is relatively large), affects the overall shape of the E-t curve and makes measurement of r difficult and inaccurate. A number of authors have examined this problem and have proposed techniques for measuring T from distorted E-t curves or for correcting values obtained in the presence of significant double-layer effects. [Pg.314]

The effect of double-layer charging is clearly most important at small r values (see equation 8.3.20). Problems with distorted E-t curves and the difficulty of obtaining corrected T values have discouraged the use of controlled-current methods as opposed to con-trolled-potential ones. [Pg.316]


See other pages where Double layer distortion, effect is mentioned: [Pg.28]    [Pg.25]    [Pg.228]    [Pg.75]    [Pg.104]    [Pg.188]    [Pg.149]    [Pg.351]    [Pg.81]    [Pg.63]    [Pg.173]    [Pg.549]    [Pg.137]    [Pg.203]    [Pg.204]    [Pg.112]    [Pg.88]    [Pg.57]    [Pg.594]    [Pg.148]    [Pg.246]    [Pg.273]    [Pg.485]    [Pg.584]    [Pg.226]    [Pg.136]    [Pg.366]   


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