ELECTRICAL PROPERTIES OF COLLOIDS
Colloidal particles always carry some charge which is responsible for the stability of the sols. Certain colloidal solutions such as ferric hydroxide hydrosol are positively charged, whereas the particles of arsenious sulphide hydrosol negatively charged. The charge on the colloidal particles may be due to the following factors.
(a)THE PRESENCE OF ACIDIC AND BASIC GROUPS
The charge on the sole like proteins, amino acids. polypeptides etc. can be explained due to the presence of acidic (- COOH ) and basic (-NH2 ) groups’ in the molecule. In acidic solution, the molecules will have positive charge due to the protonation of the basic group.
It is clear that the charge in such cases depends on the pH of the medium. The pH at which the net charge on the molecule is zero, is called the isoelectric point. The molecules at the isoelectric point exist as Zwitter ions.
(b) DUE TO THE DISSOCIATION OF THE SURFACE -MOLECULES
Colloidal solutions of soaps such as sodium palmitate (C15H31COONa) dissociate in solution forming ions. The negative ions (palmitate ions) have greater affinity for one another thus they are drawn together and form aggregates colloidal size. Thus the particles in the colloidal soap solution have negative charge.
(c) DUE TO PREFERENTIAL ADSORPTION OF IONS
The charge on the colloidal particles in some cases results from the medium in which they are dispersed. For example; if silver nitrate solution is added to potassium iodide solution, the colloidal particles of silver iodide (precipitate) formed will adsorb iodide ions from the dispersion medium and thus negatively charged colloidal solution is formed. However if silver iodide formed by adding potassium iodide solution to silver nitrate solution, a positively charged colloidal solution is formed due to the preferential adsorption of silver ions (Ag+) from the dispersion medium.
(d) ELECTRICAL DOUBLE LAYER AND ZETA POTENTIAL
The surface of colloidal particles acquires a positive or negative charge by selective adsorption of a layer of positive and negative ions around it. This layer attracts counter ions from the medium which form a second layer of opposite charges. The combination of two layers of positive and negative charges around the sol particles is called double layer. This layer was observed by Helmholtz, hence it is also called Helmholtz double layer. According to Helmholtz, the charges next to the particle surface were fixed while the compensating charges along the medium were mobile. Recent considerations have shown that the double layer is made up of (a) A compact layer of positive and negative charges which are fixed firmly onto the particle surface.(b) A diffused layer of counter ions containing opposite ions. The combination of the compact and diffused layer is called the stern double layer. Because of the distribution of the charge around the particle, there is a difference in potential between the compact layer and the bulk of the solution across the diffused layer. This is called Electro kinetic or Zeta potential. The electrical properties of colloids such as electrophoresis and electro-osmosis can be explained with the help of Zeta potential.
(e) ELECTROPHORESIS OR CATAPHORESIS
The movement of electrically charged sol particles under the influence of an applied electric field is called electrophoresis. The velocity of the colloidal particles under a fall of potential one volt per cm. is known as the electrophoretic mobility. It has been found that the range of 2 to 4×10-4 cm per sec. Electrophoresis consists of a U-tube fitted with a stopcock at the bottom for drainage. A colloidal solution like ferric hydroxide sol added into the tube. Now water (dispersion medium) is added on to the sot in both the limbs of U—tube. The electrodes are dipped in water one in each limb and connected to a high potential source. On applying the voltage the boundary between the sol and water begins to move towards the cathode. The level of water falls gradually on the side of the positive electrode and rises on the side of the negative electrode. On the other hand, if the sol particles are negatively charged, the level gradually falls on the side of the negative electrode and rises on the side of the positive electrode. Thus by noting the direction of movement of sol particles the nature of their charged can be known.
The movement of the dispersion medium through a porous material under the influence of an applied electric field is called Electro-osmosis. The sol particles are kept stationary by some suitable means. Electro-osmosis is the inverse of electrophoresis. In electrophoresis, the sol particles move and the dispersion medium remains stationary. Whereas in electro-osmosis the dispersion medium moves and the sot particles remain stationary. The apparatus used for demonstrating the phenomenon of electro-osmosis is shown in the figure 3.10.
The two diaphragms (D) and D‘) divide the apparatus into three compartments. The central compartment is filled with sot while the two-side compartments are filled with water (solvent). The electrode is also fitted in the side compartments. The diaphragms are made of porous clay. When an electric field is applied across the two electrodes, water (dispersion medium) is observed to move. The direction of flow of water depends on the charge of the particles present in the sal. The applications of electro-osmosis are as follow;
(1) Electra-osmosis is used to remove water from peat.
(2) The process is used for the removed of water moist clay.