8.9 Stability and coagulation of colloids
description
Transcript of 8.9 Stability and coagulation of colloids
8.9 Stability and coagulation of colloids
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.
Stabilizing factors:
Dynamic stabilization: Brownian motion and diffusion
prevents colloidal particles from sedimentation.
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.
Solvation shell: The interaction between particle and solvent
also helps to prevent aggregation of colloidal particles on
collision.
2) Precipitation of colloids by electrolytes
Precipitating value and precipitating efficiency / power
Precipitating value:
The lowest concentration of electrolyte (usually in mmol
dm-3) at which precipitation of colloid can be easily
observed.
A lyophobic colloid is very sensitive to the addition of a
small amount of an electrolyte.
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 of different electrolytes towards the same colloids
Hardy-Schulze rules.
The ion which is effective in causing precipitation of a sol is the
one whose charge is of opposite sign to that of the colloidal
particles, i.e., counterions
(1) Valence:
the higher the valence, the lower the precipitating value.6 6 6I II III 1 1 1
M : M : M : : 100 :1.6 : 0.31 2 3
is only valid without specific adsorption.
The precipitating efficiency of morphia (I) chloride is larger than
Mg (II) and Ca (II)
(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
Aggregation of sols by electrolytes
1) Preparation of soy-bean curd.
2) Detoxification of heavy metal ions
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
4) 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
Coagulation
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.
5) 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.
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
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
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
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.,
Coagulation treatment improved the water quality by
1. reducing the dissolved organic carbon (DOC) concentration
by about 60%;
2. reducing color by over 80%;
3. reducing the dissolved phosphate concentrations by as
much as 99%;
4. reducing nutrient availability;
5. reducing the alkalinity and pH of the water.
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.
6) DLVO theory
6 6 6I II III 1 1 1
M : M : M : : 100 :1.6 : 0.31 2 3
Around 1942, Deijaguin-Landao-Verwey-Overbeek
1) The inter-particle attraction: long-range dispersion forces
x
aHVa 12
22203
4nhH Hamaker constant
2) The inter-particle repulsion:
xr e
KTnV
22064 Debye-Hückel
constantkT
en
208
xar erKV 00
The inter-particle potential
x
aHerKx xa
12)( 00
x
aHerKx xa
12)( 00
Interparticle potential curve Potential curves for colloidal systems with different .
x
E
0
x
E
0
62
453
c ZH
rkTC
)(
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