Diffuse layer; most swelling mineral

Double diffuse layer

Clay particles have a negative surface charge. In water they attract cations and polar water molecules. Close to the surface the cations are held tightly (adsorbed layer), and further out their concentration decreases gradually (the diffuse layer). The particle surface and the diffuse cloud of cations make up the double diffuse layer. A thicker layer produces greater repulsion between particles and more water held in the clay.

 Surface (-)  | + + +  +   +    +
              |  adsorbed | diffuse
              |<-- ion concentration falls -->

Mineral that swells most: montmorillonite

Montmorillonite swells the most, followed by illite, with kaolinite swelling the least. Reasons:

1. Structure: it is a 2:1 mineral; the layers are held together only by weak van der Waals forces between two silica sheets, so water and cations enter easily between the layers.
2. Large isomorphous substitution (Mg for Al), giving a high negative charge and thick double layers.
3. Very high specific surface (up to 800 m²/g, compared with about 15 m²/g for kaolinite), which adsorbs a large amount of water.
4. Kaolinite layers are bonded by strong hydrogen bonds, so water cannot enter; illite layers are held by K$^+$ ions, a fairly strong bond, giving only moderate swelling.

Hence montmorillonite (bentonite, black cotton soil) shows the highest swelling, shrinkage and plasticity (activity 1 to 7).