§8.8 electric properties of colloids
DESCRIPTION
§8.8 Electric properties of colloids. -. -. +. +. sands. clay. clay. 1) Electrokinetic phenomenon of colloids. The experiments done by PeNcc in 1809 demonstrated that both colloidal particles and dispersion medium are charged and can move under electric fields. - PowerPoint PPT PresentationTRANSCRIPT
§8.8 Electric properties of colloids
1) Electrokinetic phenomenon of colloids
The experiments done by PeNcc in 1809 demonstrated that both colloidal particles and dispersion medium are charged and can move under electric fields.
The colloidal particles of clay is negatively charged. A colloidal particle may has hundreds of charge.
+ -
clay clay
sands
+ -
Electrokinetic phenomena:
1) Electrophoresis:
the motion of colloidal particles under the action of an electric field.
2) Electro-osmosis:
the motion of dispersion medium under electric field
Some sol, such as AgI sol, can be either positively charged or negatively charged.
Lyophilic sols (protein solution): can be positively, negatively charged or neutral depending on the pH and the colloids.
Positively charged sols:
metallic oxide sol, metallic hydroxide sol and some dyes.
Negatively charged sols:
metal, metallic sulphide, sulfur, clay, paper, silicic acid.
2) Origination of charge
(1) Ionization and unequal dissolution:
Silica sol: H2SiO3 = 2H+ + SiO32-
clay, glass, soap, biological macromolecules
AgI sol: dissolution of Ag+ is more readily than that of I-
proteins
R-CH-COOH
The pH at which protein does not move under electric field is named as isoelectric point.
R-CH-COO
NH3+
R-CH-COO
NH2NH3
+
OHH+
(2) Adsorption:
AgI, when prepared by adding KI into dilute AgNO3 solution,
positively charged AgI sol can be prepared. While by adding
AgNO3 into KI solution, negatively charged AgI sol was obtained.
Fajans rule of preferential adsorption
AgI sol: AgNO3 + KI: Ag+, I, K+, NO3
Sols preferentially adsorb ions
comprising itself, and then the ions
with higher charges.
Co-ions /similiions; counterions
(AgI)m
I- I -
I -I
-I
-I-I-
I-I-
I-
K+
K+
K+
K+
K+
K+ K+
K +
K+
K+
[(AgI)m · n I– · (n-x)K+ ]x x K+
Colloidal core Surface charge Compact layer Diffusion layer
Colloidal particle
Colloid
(3) Substitution of crystal lattice:
Caolin:
{[m(Al3.34Mg0.66)(Si8O20)(OH)4]0.66m-(0.66-x)Na+}x- xNa+
(4) Dielectric difference Water droplet in petroleum is negatively charged.
3) Electric double layer and electrokinetic potential
Holmholtz double layer (1853)
Gouy-Chappman layer (1910, 1913)
Stern double layer (1924)
Electrokinetic potential / (zeta) potential
++++++++
0
d
E
++++++++
0
d
E
Plane of shear
++++++++
0
d
E
Effect of electrolyte concentration on structure of electric double layer
Co-ions especially with higher charges will decrease zeta potential of the colloidal particle.
As the concentration of electrolyte increases,
electrokinetic potential decreases.
Isoelectric state
(AgI)m
I- I -
I -I
-I
-I-I-
I-I-
I-
K+
K+
K+
K+
K+
K+ K+
K +
K+
K+
c=0.01c=0.001
c=0.004
Compression of diffuse layer
4) Electrophoresis
qEfes f 6f rv
For electrophoresis with constant velocityr
qEv
6
r
q
0r4
3
2
6
4 0r0r E
r
Erv
Electrophoretic mobility
3
2 0rU0r2
3
U
microscope
Apparatus for electrophoresis
solution Starch gel Paper
Electrophoresis can be used for separation and detection of macromolecules.
Electrophoretogram:
protein: globulin(血红蛋白 ), albumin (血清蛋白 ), ribose (核糖 )
DNA gel electrophoresisThe indicated proteins are present in different concentrations in the two samples.
5) Electroosmosis Glass capillary
3
2 Ev
3
2 EAvAV
Sedimentation potential
Streaming potential
+ + + + + + + + + + + + + + ++++ +
+ + + + + + + + + + + + + + +++
+
+ ++