1. §8.8 Electric properties of colloids

2. 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 sands + -

3. 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

4. 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.

5. 2) Origination of charge (1) Ionization and unequal dissolution: Silica sol: H 2 SiO 3 = 2H + + SiO 3 2- 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  NH 3 + R-CH-COO  NH 2 NH 3 + OH  H+H+

6. (2) Adsorption: AgI, when prepared by adding KI into dilute AgNO 3 solution, positively charged AgI sol can be prepared. While by adding AgNO 3 into KI solution, negatively charged AgI sol was obtained. Fajans rule of preferential adsorption AgI sol: AgNO 3 + KI: Ag +, I , K +, NO 3  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- I-I- I-I- I-I- I-I- I-I- K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+

7. [(AgI) m · n I – · (n-x)K + ] x  x K + Colloidal core Surface charge Compact layerDiffusion layer Colloidal particle Colloid (3) Substitution of crystal lattice: Caolin: {[m(Al 3.34 Mg 0.66 )(Si 8 O 20 )(OH) 4 ] 0.66m- (0.66-x)Na + } x- xNa + (4) Dielectric difference Water droplet in petroleum is negatively charged.

8. 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  

9. 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- I-I- I-I- I-I- I-I- I-I- K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+   c=0.01 c=0.001 c=0.004 Compression of diffuse layer

10. 4) Electrophoresis For electrophoresis with constant velocity Electrophoretic mobility

11. microscope Apparatus for electrophoresis

12. solutionStarch gelPaper 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 )

13. DNA gel electrophoresis The indicated proteins are present in different concentrations in the two samples.

14. 5) Electroosmosis Glass capillary Sedimentation potential Streaming potential + ++ + ++ + ++ + ++ + ++ + ++ + + ++ + + ++ ++ + ++ + ++ + + + + + +