§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

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

+ + + + + + + + + + + + + + ++++ +

+ + + + + + + + + + + + + + +++

+

+ ++