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* After Norrish 26

Osmotic swelling occurs because the concentration of cations between the layers is greater than that in the bulk solution. Consequently, water is drawn between the layers, thereby increasing the c-spacing and permitting the development of the diffuse double layers that are discussed in the next section Although no semi-permeable membrane is involved, the mechanism is essentially osmotic, because it is governed by a difference in electrolyte concentration.

Osmotic swelling causes much larger increases in bulk volume than does crystalline swelling. For example, sodium montmorillonite adsorbs about 0.5 g water per g of dry clay, doubling the volume, in the crystalline swelling region, but about 10 g water per g dry clay, increasing the volume twenty fold, in the osmotic region. On the other hand, the repulsive forces between (he layers are much less in the osmotic region than in the crystalline region.

I f hydrated aggregates of sodium montmorillonite are left on the bottom of a beaker full of distilled water, they will, in time, sub-divide, and auto-disperse throughout the body of the liquid. The size of dispersed montmorillonite particles, and the number of unit layers per particle, are discussed in the section on smectites, earjier in this chapter.

The Electrostatic Double Layer

At the beginning of this chapter, we said that particles in colloidal suspension carried a surface charge. This charge attracts ions of the opposite sign, which are called counter ions, and the combination is called the electrostatic double layer. Some counter ions are not tightly held to the surface and tend to drift away, forming a diffuse ionic atmosphere around the particle, in addition to attracting ions of the opposite sign, the surface charge

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