92168.pdf

186 Charged interfaces

difference between ^d and £ will clearly be most pronounced at high

potentials (£ = 0 when «/fd = 0), and at high electrolyte concentration

(compression of the diffuse part of the double layer will cause more

of the potential drop from «/fd to zero to take place within the shear

plane), The adsorption of non-ionic surfactant would result in the

surface of shear being located at a relatively large distance from the

Stern plane and a zeta potential significantly lower than <^d.

Surface potentials

For an interface such as silver iodide-electrolyte solution the electric

potential difference between the solid interior and the bulk solution

varies according to the Nernst equation:

d<b -2.303 RT

v (= _ 59 mV at 25°C)

d(pAg) F

<f> is made up of two terms, «/» 0 and %• Changes in the x (chi)

potential arise from the adsorption and/or orientation of dipolar (e.g.

solvent) molecules at the surface or from the displacement of

oriented dipolar molecules from the surface. Such effects are difficult

to estimate. It is often assumed that x remains constant during

variations of the surface potential and that an expression such as

-2.303 RT
d(pAg) F
is justified. It is also assumed in this expression that no double layer
occurs within the solid; this may not be so, since the excess Ag+ (or
I~) ions of the silver iodide particle do not necessarily all reside at the
particle surface. The zero point of zeta potential (which is a
measurable quantity, pAg 5.5 at 25°C for Agl in aqueous dispersion
medium) can be identified with fa = 0 if specific adsorption of non-
potential-determining ions is assumed to be absent, fa can, therefore,
be calculated for a given pAg on the basis of the assumptions outlined
here.

Experimentally I —J is found to be about ~40 mV at

25°C for silver iodide^90 (Figure 7.4) and silver bromide^182 hydrosols
prepared by simple mixing, and not -59 mV. Assuming that £ can be