92168.pdf

206 Charged interfaces

Figure 7.13 Electrophoretic mobility and zeta potential for spherical colloidal

particles in electrolyte solutions containing polyvalent ions (A.+/z+ = A_/z_ = 70 ft~!

cm^2 mol""'). Electrolyte type is numbered with counter-ion charge number first:

eg/kT — 2 in each case

go = (2e^2 NAcz^2 /EkT)V2a

where c is the electrolyte concentration and z is the counter-ion charge number [After

P,H. Wiersema, A.L. Loeb and J.Th.G. Overbeek, /. Colloid Interface ScL, 22, 78

(1966]

Permittivity and viscosity

Further difficulty in the calculation and interpretation of zeta
potentials will arise if the electric field strength (d^d*) close to the
shear plane is high enough to significantly decrease e and/or increase
17 by dipole orientation. Lyklema and Overbeek^192 examined this
problem and concluded that the effect of dift/dx on e is insignificant,
but that its effect on 17 may be significant, especially at high potential
and high electrolyte concentration. A significant (and positive)
viscoelectric effect would result in the effective location of the shear
plane moving farther away from the particle surface with increasing £
and/or increasing K; in other words, the physical meaning of the term
'zeta potential' would vary. More recently investigations by Stigter