4. Schultz-Hardy rule
♦ critical concentration of electrolyte required to flocculate the colloid
: the valence of the opposite-charge ion plays
the principal role on colloid stability
- the flocculation concentration for monovalent counter ion
is 100 times higher than divalent ion,
and is 1000 times higher than trivalent ion.
⇒ The effect on the screening of the surface charge is related to
the counter-ion valence
M ∝ Z-6
M : critical flocculation concentration (CFC)
Z ; valence of the counter ion
26= 64, 36=729
(ex) the use of Al or Fe ion to cause the flocculation the particles
with negative zeta potential in water treatment plants
Advanced Electronic Ceramics I (2004)
Critical Flocculation Concentration
6.9X10-4 M of divalent cation is needed to flocculate the negative As2S3 sols.
6.3X10-4 M of divalent anion is required to flocculate the positive Al2O3 sols
Advanced Electronic Ceramics I (2004)
5. Critical Flocculation Concentration
(Motivation)
Does every monovalent ion give the same effect upon the flocculation?
(ex.)
1. In order to flocculate the Ag2S3 solution (negative colloid)
- 0.058 M Li+ ion or 0.051 M Na+ concentrations is required.
2. In order to flocculate the AgI solution (negative colloid)
- 0.165 M Li+ ion or 0.140 M Na+ concentrations is required.
⇒ Na+ is more effective than Li+ in inducing flocculation
The effectiveness in inducing flocculation
For monovalent cations
Cs+ > Rb+ > NH4+ > K+ > Na+ > Li+
Advanced Electronic Ceramics I (2004)
Stern adsorption
♦ Stern correction for EDL model
1. The adsorption of potential-determining ion at surface
⇒ determine ψ0
ψ0 = (2.303RT/F) ln (C/Czp)
2. Diffuse double layer in Debye-Hückel and Gouy-Chapmann
approximation indifferent ions as point charge (with no volume)
⇒ acceptable for the bulk region of dilute solution
⇒ but situation becomes different at high concentration. Saturation
would occur at the inner edge of the diffuse part of double layer.
⇒ the model considering specific adsorption effect is required.
Advanced Electronic Ceramics I (2004)
6. Stern layer
♦ Stern layer : the layer of specific adsorption
♦ In general specific adsorption of an
ion is enhanced by large size
(large polarizability)
(Problem) introduce the parameters
which is hard to measure
δ : the thickness of Stern layer,
K : constant related with surface
adsorption
Advanced Electronic Ceramics I (2004)
