Electrophoretic mobility limiting value

In a bid to calibrate the electrophoresis instrument independently, we carried out iso-electric pH (pi) measurements on gelatin solutions having concentrations 0.001, 0.01 and 0.1 % (w/v). The measured electrophoretic mobility (p) values could be converted to equivalent zeta potential (0 values through the Helmholtz-Smoluchowski relation p = scoC/t/. where the solvent dielectric constant and viscosity are given by b and rj respectively. The permittivity of vacuum is So = 8.85 X 10 C m and the value used for / 10 Pa s. Figure 2 depicts the dependence of C on solution pH. The measured pi = 5.5 0.5 value did not show concentration dependence within the limits of the experimental error. The value provides excellent matching with the nominal pi value cited by the manufacturer ( 5.0 0.2). This established the reproducibility, reliability and robustness of our measured electrophoretic mobility data. 

With these relations, zeta potentials can be calculated for many practical systems. Note that within each set of limiting conditions the electrophoretic mobility is independent of particle size and shape as long as the zeta potential is constant. For intermediate values of Ka another equation, the Henry equation (4.10) can be used other such equations are available in the literature as well [81,253,264],... 

Equation (21.62) shows that as k co, p tends to a nonzero limiting value p°°. This is a characteristic of the electrokinetic behavior of soft particles, in contrast to the case of the electrophoretic mobility of hard particles, which should reduces to zero due to the shielding effects, since the mobility expressions for rigid particles (Chapter 3) do not have p°°. The term p°° can be interpreted as resulting from the balance between the electric force acting on the fixed charges ZeN)E and the frictional force yu, namely. 

Traditional methods of determining the electrophoretic mobility in a d.c. electric field have involved particle concentrations with O g 0.001. This provides the infinite dilution limiting value, and the appropriate theoretical analysis is for an isolated particle in an infinite volume of electrolyte. 

The electrophoretic mobilities derived from the limiting ionic equivalent conductivity differ somewhat from the experimentally measured values that are dependent on the composition of the background electrolyte and its pH. Differences in the ionic mob-... 

Notably, dh calculated using the above simple equation shows a dependence on electrolyte concentration and hence the method cannot be used in a straightforward manner. Cohen-Stuart et al. [43] showed that the measured electrophoretic thickness approaches <5h only at low electrolyte concentrations. Thus, to obtain dh from electrophoretic mobility measurements, results should be obtained at various electrolyte concentrations and 4 should be plotted versus the Debye length (1/k) to obtain the limiting value at high (l//c) (i.e. low electrolyte concentration), which now corresponds to <5h-... 

Drift-limited regime In this regime, the electrophoretic mobility of the polymer dominates over diffusion and barrier contributions. For large values of the applied voltage difference Vm, the steady-state flux is linear with the polymer concentration and Vm and is independent of the chain length... 

The major limitation of PALS is that no mobility distribution information can be obtained and the type of mean is not defined. In addition, the accuracy of the measured electrophoretic mobility depends on the accuracy of the scattering vector K, which can te determined quite accurately based on instrument setup, and the accuracy of A, which is often affected by experimental noise and is difficult to ascertain. Other factors, such as electroosmosis, electronic artifact and the choice of a correct field frequency can also affect the measurement accuracy, as demonstrated in Figure 6.31, in which correct electrophoretic mobihty value using PALS can only be obtained at electric field frequencies between 30-200 Hz. 

The electrophorehc mobility, is related to the zeta potential, which is defined as the electric potenhal at the surface of shear of the particles and is therefore a measure of their total charge. Unfortunately, the electrophoretic mobility of dispersion particles does not depend solely on the zeta potential, but also in a complex way on particle size and on the ionic strength and viscosity of the aqueous phase [21]. It is only at the limits of very high and very low ionic strength that can be directly computed from the measured values (Helmholtz-Smoluchowski or Huckel approximations). 

Relations (25) to (31) which were derived for the limiting situation Cp< Cs are limiting laws with respect to the salt-to-polymer concentration ratio. In practice they apply as long as experimental results are extrapolated to infinite polyelectrolyte dilution and the ionic strength is kept at finite values, w and 1/2 1 electrophoretic mobilities,... 

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