Adsorbed water; diffuse layer formation

Adsorbed water

Adsorbed water is the water held around a clay particle by the electrical attraction of the particle surface. Near the surface it is very strongly attracted, has a density higher than ordinary water and behaves almost like a solid. It is only partly removed by oven drying at 105 °C, and its influence decreases with distance from the surface. It gives clay its plasticity and cohesion.

Formation of double diffuse layer

1. A clay particle has a negative charge on its surface because of isomorphous substitution and broken bonds at the edges.
2. In water, the negative charge attracts cations (Na$^+$, Ca$^{2+}$, K$^+$) and dipolar water molecules. The cations are also hydrated.
3. The cations nearest the surface are held strongly, forming the adsorbed (Stern) layer.
4. Because of thermal energy, the cations further out are free to diffuse. Their concentration falls gradually with distance until it equals that in the free pore water. This is the diffuse layer.
5. The negative surface charge and the diffuse cloud of positive ions together form the double diffuse layer.

 particle  adsorbed   diffuse layer
 -----|+ + +|+  +   +    +      (free)
 - - -|+ + +| +    +      +
 conc of cations decreases ->

Thickness increases as the salt concentration and cation valence decrease, and as the dielectric constant of the water and the temperature increase. Thick layers give more repulsion, dispersion, swelling and higher plasticity.