ELECTROKINETIC PROPERTIES OF COLLOIDS

Particles in a liquid medium may become charged. Because of this charge on the surface of the particles, electro kinetic properties arise.

There are several ways in which a colloidal particle may acquire a charge:

·         Selective adsorption of a particular ion species present in a dispersion or a solution. The ion may have been added to the solution or in the case of water, it may be the hydronium (H⁺) or hydroxyl (OH^-). Thus particles in water are negatively charged due to preferential adsorption of (OH^-) ions. Ionic surfactants are also adsorbed and impart corresponding charge.

·         Particles also acquire charge through ionization of groups or moieties at the surface. In proteins, one end of the polypeptide has carboxylic acid group (COOH) which ionize to give a negative charge. The other end group has amino group (NH₂) and ionize to give a positive charge. The predominating charge depends on the pH of the medium. Thus the particles may be negatively or positively charged or neutral. Similarly Aluminium hydroxide gel (Al(OH)₃) form Al³⁺ in acid and [Al(OH)]⁴⁺ in alkalis.

·         Non-ionizing particles may also acquire a charge due to a difference in dielectric constant between the disperse phase and the dispersion medium. The phase with a greater e acquires a negative charge.

Electric Double Layer.

Due to a charge on the particles in a colloidal sol, ions of opposite charge are attracted firmly to the surface of the charged particle to form a layer.  The ions of opposite charge further distribute themselves diffusely in  the vicinity of the particle to form a second layer. The two layers constitute a phenomenon known as the electrical double layer. Although the particle is charges, the system is electrically neutral, because the two layers described above neutralize each other.

In order to understand the electrical double layer better, let us follow an example of silver iodide sol. Silver iodide may be obtained in the following reaction:

------>AgNO₃ + NaI                         Ag I   + NaNO₃                            

If the reaction is carried out with excess AgNO₃ there will be excess Ag+ than I ^– ions on the surface of the AgI precipitate, and the particle will be positively charged. The NO₃^- ions will surround the particles and will form the first layer. Na⁺ and other NO₃^- ions will distribute diffusely to form the second layer of the electrical double layer. If on the other hand there were excess of NaI in the reactants, the surface would contain negative charge.

In the bulk of the particle, the two ions (Ag⁺ and I^-)are exactly I equimolar proportions. In the medium (water) where the particles are suspended, Na⁺ and NO₃^- ions dominate, and some I ^– , H⁺, OH^-  ions are also apparent.

The negative charge on the surface of the particle attracts positive ions from the solution, and repels negative ions. These I ^– ions adsorbed at surface are known as potential determining ions. They attract oppositely charged ions from the medium to the surface. Thus, Na⁺ are concentrated in the immediate vicinity of the surface and tend to stick to the surface, approaching it as closely as their hydration spheres permit. Na⁺ Ions in this respect are called counterions or gegenions. They form a compact layer called Stern layer.

NO₃^- ions are repelled, and together with some Na⁺, I ^–, H⁺, OH^- and Ag+ , due to thermal agitation of water, form a second diffuse layer called the Gouy-Chapman layer. The combination of the two layers of oppositely charged ions constitutes an Electric double layer.

The electric potential at the surface of the particle is equal to the work against electrostatic forces required to bring a unit electric charge from the bulk of the solution (infinity) to the surface of the particle, and is known as surface potential or Electrothermodynamic potential or Nernst Potential g₀. On moving away from the surface towards the bulk, the potential drops rapidly across the Stern layer (Na ions screen further removal of other Na ions in the diffuse part of the double layer).In the Gouy-Chapman layer, the drop of potential is more gradual, tending to zero asymptotically as the composition of cations and anions approach equilibrium. The thickness of the double layer has been arbitrarily assigned the value of 0.37g_o.

(0.37 = 1/e). It usually ranges from 10 – 1000 Å and decreases as the concentration of electrolytes in solution increases, more rapidly for counter ions of high valence.

In aqueous dispersions, particles are surrounded by a layer of water of hydration attached to them by ion-dipole and dipole-dipole interaction. When a particle moves, this shell of bound water and all ions located inside it move along with the particle. If on the other hand water in a solution flows through a fixed bed of these solid particles, the hydration layer surrounding each particle remains stationary attached to it. The electric potential at the plane of shear separating the bound water from the free water is the zeta potential, ζ. It is less than the Stern potential. Zeta-potential is of practical importance when compared to other potentials because it can be measured experimentally. It is important in the stabilization of dispersed systems since it governs the degree of repulsion between adjacent dispersed particles. If the zeta potential is reduced below a certain value, the attractive forces exceed the repulsive forces and the particles come together or undergo flocculation.