Double diffuse layer in clay

Clay particles carry a net negative charge on their surface because of broken bonds at the edges and isomorphous substitution. When the clay is in water, this charge attracts positive ions (cations) and the dipolar water molecules.

• Near the surface, the cations are held tightly together with water molecules (the adsorbed or fixed layer).
• Further away, the attraction weakens, so the cations are free to move and their concentration falls with distance until it equals the concentration in the free water. This region is the diffuse layer.

The negative charge of the particle surface and the diffuse layer of positive ions together form the double diffuse layer (diffuse double layer).

 - + - + - |
 - + -  +  |   conc. of cations
 - +  +    |   falls with distance
 particle   |-- adsorbed -- diffuse -->

Thickness increases with a lower electrolyte concentration, lower cation valence, higher dielectric constant of the pore fluid and (in theory) higher temperature. A thicker layer gives more repulsion, a dispersed structure, higher plasticity and more swelling (as in montmorillonite).