§8.8 Electric properties of colloids

- + 1) Electrokinetic phenomenon of colloids clay sands 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.

Electrokinetic phenomena: Electrophoresis: the motion of colloidal particles under the action of an electric field. 2) Electro-osmosis: the motion of dispersion medium under electric field

Positively charged sols: metallic oxide sol, metallic hydroxide sol and some dyes. Negatively charged sols: metal, metallic sulphide, sulfur, clay, paper, silicic acid. 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.

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-COO NH3+ NH2 OH H+ R-CH-COOH The pH at which protein does not move under electric field is named as isoelectric point.

AgI sol: AgNO3 + KI: Ag+, I, K+, NO3 (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. AgI sol: AgNO3 + KI: Ag+, I, K+, NO3 (AgI)m I- K+ Fajans rule of preferential adsorption Sols preferentially adsorb ions comprising itself, and then the ions with higher charges. Co-ions /similiions; counterions

[(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 Plane of shear +  d E +  d E +  d E    Stern double layer (1924) Holmholtz double layer (1853) Gouy-Chappman layer (1910, 1913) Electrokinetic potential /  (zeta) potential Effect of electrolyte concentration on structure of electric double layer

Compression of diffuse layer (AgI)m I- K+ Co-ions especially with higher charges will decrease zeta potential of the colloidal particle. As the concentration of electrolyte increases, electrokinetic potential decreases.   c=0.01 c=0.001 c=0.004 Isoelectric state Compression of diffuse layer

4) Electrophoresis For electrophoresis with constant velocity Electrophoretic mobility

Apparatus for electrophoresis microscope

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)

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

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