DLVO theory: the resultant (total) potential is:
V
T= V
A+ V
Rx
attraction potential
(van der Waals forces)
0 x
DLVO theory
0 x
repulsion potential
(Coulombic repulsion;
aqueous medium, largeε)
0 x
(VT/kT) ≥ 10:
→ “colloid stabiliy”
Derjagin Landau Verwey Overbeek
VR kT VA
kT
VT total kT potential
VT = VA + VR
DLVO theory: conditions for colloid stability
attractive forces overwhelm repulsive forces repulsive forces
overwhelm attractive forces
Coagulation Stability
ccc - critical coagulation concentration of additive / modfier (electrolyte=salt)
DLVO theory: conditions for colloid stability
Colloid stability/flocculation/coagulation/ are controlled by the relative magnitudes of the van der Waals and the Coulombic forces
aqueous As2S3 sol with increasing background electrolyte concentraion
(1:1 electrolyte, mM) aqueous Al2O3 suspension
at different pHs repulsion
total
attraction
potential
particle-particle distance (surface separation) 0
unstable
stable emulsion stable suspension
unstable
(coagulation / flocculation;
sedimentation) (coagulation; creaming and
coalescence)
Colloid stability/flocculation/coagulation/ are controlled by the relative magnitudes of the van der Waals and the Coulombic forces
Schulze-Hardy rule: 6 z
ccc ∝ 1
(
6 6 6)
3 : 1 2 : 1 1
1
For VT = 0, VR = -VA. In this case, the DLVO theory predicts the ccc ratio for 1:2:3 valence of charge of ions to be 1000:16:1.3.
AlCl3 CaCl2 MgCl2
KCl NaCl
electrolyte
1,3 (1,8) 9,3 10-5
16 (13) 6,5 10-4
16 (14) 7,2 10-4
1000 (980) 5,0 10-2
1000 5,1 10-2
Schulze-Hardy-rule ccc (M) (As2S3 dispersion)
stable Fe(OH)3 sol The sol undergoes coagulation upon the addition of Al2(SO4)3 solution
