Zeta Potential: Meaning, Factors and Flocculation | Soil Colloids

After reading this article you will learn about:- 1. Meaning of Zeta Potential 2. Factors of Zeta Potential 3. Dispersion and Flocculation.

Meaning of Zeta Potential:

The potential difference (electro-osmosis and allied phenomena) between the fixed part and freely mobile portions of double layer known as Zeta potential (ζ).

Mathematically it can be simplified as follows:

Assuming that the diffuse double layer is equivalent to an electrical condenser with parallel plates d cm apart, each carrying a charge e per sq cm; the difference of potential between the plates is equal to potential called zeta potential (ζ). If D is the dielectric constant of the medium (water).

ζ = 4 Ï€ ed/D …(1)

The rate of flow of ions will be determined by two forces namely the force of electro- osmosis and the frictional force between the moving liquid layer and the wall of this capillary pores.

If u is the uniform velocity thus attained, then u/d may be taken as the velocity gradient in the double layer, since the velocity at one side, i.e. the wall, is zero. So the frictional force, f is equal to

f= Æžu/d

where Æž = coefficient of viscosity of the liquid.

Again, the force of electro-osmosis may be written as,

Electrical force = Ee

where, E = is the potential gradient

e = is the charge per sq cm at the boundary of movement.

Hence at equilibrium, both the forces are equal

Ee= Æžu/d

d = Æžu/Ee

Putting the value of d in equation No. 1, we get,

ζ = 4 πed/D

ζ = 4 πe ƞu/Ee/D

ζ = 4 πƞu/DE…(2)

If q is the area of cross-section of all the pores, the volume v of liquid transported per second is equal to qu, so that,

v = qu

u = v/q

Putting the value of u in equation 2 we get,

ζ = 4 πƞu/DE

ζ = 4 πƞv/qDE … (3)

In the case of electro-osmosis through a single capillary tube, q is of course equal to Ï€r², where r is the radius of the tube, so that

ζ = 4Ï€v/r²DE [Substituting the value q in equation 3]

Factors of Zeta Potential:

There are various factors by which zeta potential can be changed.

(i) Due to decrease in pH the zeta potential can be reduced resulting from the reduction of negative charge on the clay micelle and also from the replacement of sodium or other monovalent ions with hydrogen (H⁺) ion.

(ii) Divalent (Ca²⁺, Mg²⁺) or trivalent (Al³⁺) can be introduced to exchange with the monovalent cations and reduces the zeta potential and thereby accelerates the phenomenon of flocculation.

(iii) By the addition of simple salts the concentration of cations will increase around the clay particles and thereby causes flocculation by reducing the zeta potential.

Dispersion and Flocculation of Zeta Potential:

When the dilute colloidal particles (suspended in water) tend to repel each other, allowing each particle to act completely independent of the others. This is known as the dispersion and this dispersion condition is encouraged by the fineness of the colloidal particles. Dispersion is encouraged by higher pH values, where the electro-negativity of clay particles or the zeta potential is at a maximum.

Highly hydrated monovalent cations like Na⁺ stabilize the dispersed colloids. The rate of movement of the silicate clay particles toward the positive and is directly proportional to the zeta potential. So the conditions favourable for causing dispersion are high hydration, low electrolyte content, a pH far from isoelectric point and absence of opposite charge colloids.

The sodium-saturated colloidal clay hydrolyzes to form NaOH and Na₂CO₃ and is highly dispersed, highly hydrated. The sodium-saturated colloidal clay particles will able to move easily in an electrical field than that of Ca-saturated clay.

Flocculation process is the opposite of dispersion that means colloidal particles tend to attract each other and coagulate like a bigger particle caused by a decrease in zeta potential (ζ) and hydration. The condition of flocculation is generally beneficial in the formation of good soil structure.

The ability of common cation to flocculate soil colloids is in the order of Al>Ca and H > Mg > K > Na. Conditions favourable for flocculation are dehydration, a high electrolyte content, a pH at the isoelectric point, and the presence of an oppositely charged colloid.

When the zeta potential (double layer potential) is decreased below the critical value, the repulsion between the approaching colloidal particles is reduced to such an extent that those colliding with a certain velocity forming floes. This is the simple mechanism in the formation of floes resulting from the decrease of charge density or thickness of the double layer or both.