8.9 Stability and coagulation of colloids
8.9.1 Stability of colloids
Colloids, a dispersion system with high specific area and
thus high interfacial specific energy, is thermodynamically
unstable. Collision between colloidal particles frequently
occur and aggregation is always a possibility.
8.9 Stability and coagulation of colloids
Stabilizing factors:
(1) Dynamic stabilization: Brownian motion and diffusion
prevents colloidal particles from sedimentation.
(2) Electric stabilization: All the colloid particles in a
particular system have the same charge tends to repulse each
other and keep the colloid in suspension.
(3) Solvation shell: The interaction between particle and solvent
also helps to prevent aggregation of colloidal particles on
collision.
8.9.1 Stability of colloids
8.9 Stability and coagulation of colloids
Electrolytes As2S3 () Electrolytes Al(OH)3(+)
LiCl 58 NaCl 43.5
NaCl 51 KCl 46
KCl 49.5 KNO3 60
CaCl2 0.65 K2SO4 0.30
MgCl2 0.72 K2Cr2O7 0.63
MgSO4 0.81 (KOOC)2 0.69
AlCl3 0.093 K3[Fe(CN)6] 0.08
Al(NO3)3 0.095
Precipitating value:
The lowest concentration of electrolyte ( in mmol dm-3) at
which precipitation of colloid can be easily observed.
8.9.2 Precipitation of colloids by electrolytes
8.9 Stability and coagulation of colloids
8.9.2 Precipitation of colloids by electrolytes
8.9 Stability and coagulation of colloids
Precipitating value and precipitating efficiency / power
The ion which is effective in causing precipitation of a sol is the
one whose charge is opposite to that of the colloidal particles, i.e.,
counterions
(2) Hardy-Schulze rules.
(1) Valence:
the higher the valence, the lower the precipitating value.
6 6 6
I II III 1 1 1M : M : M : : 100 :1.6 : 0.3
1 2 3
is only valid without specific adsorption.
The precipitating efficiency of morphia (I) chloride is larger than
Mg (II) and Ca (II)
8.9.2 Precipitation of colloids by electrolytes
8.9 Stability and coagulation of colloids
(2) radius
Hofmeister / lyotropic series
H+ > Cs+ > Rb+ > NH4+ > K+ > Na+ > Li+
F- > Cl- > Br- > NO3- > I-
(3) co-ions
When counterion is the same, the higher the valence of the
co-ions, the higher the precipitating value.
Electrolytes KNO3 ½ K2SO4
Precipitating values 50 65.5
Precipitating values for As2S3 colloids
Fe(OH)3 sol + HClNonregular aggregation
8.9.2 Precipitation of colloids by electrolytes
8.9 Stability and coagulation of colloids
Aggregation of sols by electrolytes
1) Preparation of soy-bean curd.
2) Detoxification of heavy metal ions
8.9 Stability and coagulation of colloids
8.9.2 Precipitation of colloids by electrolytes
3) Formation of delta and alluvion
When the river water containing colloidal clay flows into the sea, the
brine induces coagulation. This is a major cause of silting in estuaries.
Aggregation of sols by electrolytes
8.9 Stability and coagulation of colloids
8.9.2 Precipitation of colloids by electrolytes
8.9 Stability and coagulation of colloids
8.9.2 Precipitation of colloids by electrolytes
8.9 Stability and coagulation of colloids
8.9.2 Precipitation of colloids by electrolytes
Now the seashore of Yancheng City is 15 kilometers away.
8.9.3 mutual precipitation of colloids
Generally, mixing of colloids with the same charge does not
lead to precipitation, while mixing of colloids with different
charge will result in mutual precipitation.
Fe(OH)3(3.04 g dm-3) 9 8 7 5 3 2 1 0.2
As2S3(2.07 g dm-3) 1 2 3 5 7 8 9 9.8
charge + + + + 0 - - -
phenomenon --- Turbid ---
Purification of water using alum
8.9 Stability and coagulation of colloids
8.9.4 Coagulation – precipitation by polymeric electrolyte
Coagulation is the process of adding chemicals to water to
make dissolved and suspended particles bind together
(coagulate) and form larger particles (flocculant) that settle
out of the water.
Aluminum sulphate, ferric sulphate, ferric chloride, and
forms of aluminum or iron salts called polymers are all
suitable coagulation chemicals approved for water treatment.
8.9 Stability and coagulation of colloids
(1) effect of macromolecules on colloids
1) Stabilization effect:
When lyophilic sol, such as gelatin, albumin, agar, casein, gum
arabic, glue, starch, etc. is added to a sol, the latter may be
prevented from precipitation by electrolytes. Such
macromolecules are named as stabilizing agent or stabilizers.
The macromolecules adsorbed on the colloidal particle form a
tough shell which helps to keep the particle apart.
8.9.4 Coagulation – precipitation by polymeric electrolyte
8.9 Stability and coagulation of colloids
Gold number:
The number of milligrams of the protective colloid that just
prevents the change of color when 1 cm3 of the standard salt
solution (10 % NaCl) is added to 10 cm3 of the standard gold sol
(0.006 %). (By Zsigmondy)
Protective colloids Gold number / mg
Gelatin (明胶) 0.005-0.01
Albumin (白蛋白) 0.1-0.2
Gum arabic 0.15-0.5
Dextrin (糊精) 6.0-20.0
Potato starch 25
Red number: Congo red
8.9.4 Coagulation – precipitation by polymeric electrolyte
8.9 Stability and coagulation of colloids
2) Sensitization effect:
When small amount of some lyophilic sols was added into a
lyophobic sol, the sol can be precipitated by less amount of an
electrolyte. In other words, the addition of the lyophilic sols
decrease the precipitating value of the sol. This phenomenon is
named as sensitization.
The recoil of the macromolecules helps to draw several
particles together.
LaMer: Bridging effect
8.9.4 Coagulation – precipitation by polymeric electrolyte
8.9 Stability and coagulation of colloids
Flocculation of colloids:
flocculants
Chemicals by adding which
suspended solids in the
wastewater are sedimented
and clearized water is gotten.
Kinds of flocculants:
1. inorganic: polyalminum chloride
2. polymers: polyacrylic acid, polyacrylamide derivatives
3. naturally occurring flocculants: chitosan, sodium arginate
8.9.4 Coagulation – precipitation by polymeric electrolyte
8.9 Stability and coagulation of colloids
Polymeric electrolyte – polyacrylamide (PAM)
M > 106
-[- CH2 – CH-]m-
C=O
NH2
Differences in flocculation and aggregation
flocculants are widely used in water treatment plant, pulp
industries ,the food industries and so on.,
8.9.4 Coagulation – precipitation by polymeric electrolyte
8.9 Stability and coagulation of colloids
The electrolyte causes a compression of the diffusion layer
of the double layer and decreases electrokinetic potential,
which allows two particles to make a closer approach to each
other.
8.9.5 DLVO theory
6 6 6
I II III 1 1 1M : M : M : : 100 :1.6 : 0.3
1 2 3
8.9 Stability and coagulation of colloids
Around 1942, Deijaguin-Landao-Verwey-Overbeek
1) The inter-particle attraction: long-range dispersion forces
x
aHVa
12
222
03
4nhH Hamaker constant
2) The inter-particle repulsion:
x
r eKTn
V
22064 Debye-Hückel
constantkT
en
2
08
xa
r erKV 00
8.9.5 DLVO theory
8.9 Stability and coagulation of colloids
The inter-particle potential
x
aHerKx
xa
12)( 00
Interparticle potential curve Potential curves for colloidal systems
with different .
x
E
0
x
E
0
62
453
cZH
rkTC
)(
8.9.5 DLVO theory
8.9 Stability and coagulation of colloids
As the concentration of electrolyte
increases, of colloid decreases.
When = 0.03 V, colloid begins to settle.
When = 0, precipitation rate attains
maximum.
/V
0.03 0.00
r
c/mmol/L
8.9.5 DLVO theory
8.9 Stability and coagulation of colloids