Diffuse Double Layer: Concept, Properties and Shortcomings | Soil Colloids

After reading this article you will learn about:- 1. Concept of Diffuse Double Layer 2. Important Properties of Diffuse Double Layer 3. Factors Affecting 4. Shortcomings.

Concept of Diffuse Double Layer:

The exchangeable ions are surrounded by water molecules and may thus be considered as forming a solution which is often a micellar solution or inner solution. The solution containing free electrolytes called outer solution or inter-micellar solution.

The ionic conditions on the outside surface of a clay particle or a pocket of clay particles dispersed in water or an electrolyte solution are controlled by the proportion of the exchangeable cations that disperse into the solution.

A clay surface probably behaves as an effectively uncharged surface if the negative charge on the lattice is neutralized by monovalent cations that are tightly bound to the surface. But if the cations are hydrated a proportion tends to dissociate from the surface and will cause an electrical potential gradient to be set up near the surface.

The system clay lattice-exchangeable cations-solution can be looked upon as forming a complex electrical double layer, known as the Helmholtz double layer, the inner layer being the surface of the lattice carrying the negative charge (Fig. 9.15) and the outer layer being composed of two parts—a positive layer due to the cations bound to the lattice surface known as the fixed layer or stern layer (Fig. 9.15), and a positive layer diffused in the solution close to the lattice surface known as the Gouy diffuse layer (Fig. 9.15 ).

The proportion of the cations that dissociate from the stem into the Gouy layer depends partly on the concentration of salts dissolved in the water, and partly on the tightness with which they are held to the surface. In the stern fixed layer, there is a sharp fall of potential (Fig. 9.16, the vertical broken line, the extent of the fixed part of the double layer).

If the potential of the solid is indicated by A and that of the bulk of the liquid by B, the fall of potential in between may occur in two ways as shown in the above Fig. 9.16, depending on the characteristics of ions present in soil solution which make up the outer portion of the fixed layer.

In each case AC is the sharp fall of potential in the Stern fixed layer and CB the gradual change in the Gouy diffuse layer. The potential involved between the fixed layer and freely mobile portions known as “Zeta potential” (ζ).

The typical ion distribution curve for both the anions and cations outside a negatively charged surface is illustrated here (Fig. 9.17).

Assuming that Helmholtz layer has a uniform negative charge density and this assumption is reasonable when fairly large proportion of the exchangeable cations will be dissociated into the solution and the double layer will thick, but it will become increasingly invalid as the proportion of cations dissociating from the surface drops, so the distance apart between the free negative charges on the surface becomes large.

The charge density of the particle surface is equal to the sum of charges in the inner layer and in the outer layer. With the increase in concentration of ions in the solution, the Stern fixed layer will charge and as a result ion swarm shrinks (thinner the double layer). On the other hand with the decreasing ionic concentration that means diluting the concentration of the solution ion swarm expands (thicker the double layer).

Important Properties of Diffuse Double Layer:

There are four important properties which are described here:

(i) The thickness of the diffuse double layer decreases as the electrolyte concentration in the water increased at equal equivalent concentrations of electrolyte, divalent cations decrease the thickness of the double layer more strongly than that of monovalent cations, and trivalent more strongly than divalent.

If clay particles are suspended in water containing very little electrolyte, the double layer is thicker (wider) if the exchangeable ions are sodium (Na⁺) and potassium (K⁺) than if they are calcium (Ca²⁺) or magnesium (Mg²⁺), and it is more thicker (wider) for sodium than for potassium. That is why soils containing more sodium show high pH resulting from more thicker diffuse double layer.

(ii) The cation concentration gradient in this diffuse double layer sets up an electrical potential difference between the surface of the clay particle and the bulk of the solution, that is, work must be done to transfer a cation from the surface of the particle to the bulk of the solution.

Hence, this double layer hinders the free interchange of cation between these two regions. Hydrogen ions cannot have the same activity throughout these regions, and the thicker the double layer, the smaller will be the contribution of the hydrogen ions that can dissociate from the clay surface to the hydrogen ion concentration in the bulk of the solution.

The more the diffuse double layer is compressed on to the clay surface, the more freely will the hydrogen ions be able to move from the clay surface into the solution, and the lower will be its apparent pH. The activity of the hydrogen ions (H⁺) in the Stern fixed layer may be much higher in some soils than that of outside layer and the activity appeared to be about a hundred fold higher than in the solution.

(iii) The most direct way of estimating the proportion of cations that dissociate into Gouy diffuse layer should be by measuring the activity of the cations around the clay particles using a suitable membrane electrodes.

(iv) The concentration of the anions in the solution is lower in the diffuse double layer than in the bulk of the solution. Hence, if a dilute sodium chloride solution is added to a dry sodium clay, and if the clay does not absorb any chloride (Cl^–), the concentration of chloride (CI^–) in the bulk solution will rise because chloride will be partially expelled from the diffuse double layer.

This phenomenon is known as negative absorption and it creates great difficulties when adsorption of anions by soils is being measured. When the concentration of the adsorbate (substance adsorbed on adsorbent) is less on the surface of the adsorbent (substance on which another substance is adsorbed) than in the bulk, then it is known as “Negative adsorption.”

Factors Affecting Diffuse Double Layer:

The thickness of the diffuse double layer depends upon the following factors:

(i) Concentration of Total Electrolyte:

With an increase in total electrolyte concentration, the thickness of the diffuse double layer decreases. With increasing the electrolyte concentration four times, thickness reduces to half.

(ii) Valence of the Counter Ion:

The higher the valence of the opposite or counter ion, the thinner or smaller of the thickness of the diffuse double layer.

(iii) Dielectric Constant:

With an increase in the dielectric constant of the medium, the thickness of the diffuse double layer increases.

Shortcomings of Diffuse Double Layer:

(i) The main limitation of the structure of diffuse double layer is to state a distinct line between the micellar (inner) and the inter-micellar (outer) solutions.

(ii) The structure and properties of diffuse double layer is applicable only to the large flat or plane surfaces of the clay minerals. The charge densities at the edges and corners and cavities of the clay minerals are considered as assuming the same pattern of ion distribution as for a flat or plane surface of clay minerals.

(iii) For the pure soil organic matter, however, the theories of the diffuse double layer are not valid, partly due to structure of humus and partly due to forces other than coulombic resulting complex bonding’s.

(iv) Due to neglect of the ionic dimensions the Gouy theory is valid only for colloids having a low surface-charge density, as in the plane or flat surfaces of the clay minerals, and at very low concentrations of the solution. The thickness of Gouy’s diffuse double layer decreases with increasing valence and concentration of the electrolyte in solution.