CATION EXCHANGE AND ABSORBED WATER
Cations that neutralize the net negative charge on the surface of soil particles in water are readily exchangeable with other cations. The exchange reaction depends mainly on the relative concentrations of cations in the water and also on the electrovalence of the cations. Cation exchange capacity, measured in miliequivalents of cations per gram of soil particles, is a measure of the net negative charge on the soil particles, resulting from isomorphous substitution and broken bonds at the boundaries. The values of the cation exchange capacity for the principal clay minerals are indicated in Table 1. Montmorillonite has a relatively large exchange capacity because its particles may consist of single unit sheets.Very fine particles of other minerals such as mica and quartz also carry a net negative charge in water as a result of broken bonds at the boundaries. However, even in the range of small particle sizes in which nonclay minerals occurs in soils, the exchange capacity is relatively small.
Table 1: Cation Exchange Capacity of Principal Clay Mineral
The highly polar water molecule has the ability to form strong bonds with the surface of soil particles, as well as with the exchangeable cations that surround it. The strong short-range adsorption forces hold one to four molecular layers of water at the surface of the soil particles. This water is said to be adsorbed. It is of most importance if the particles are very small,such as films of sodium montmorillonite 1 nm thick, and is insignificant if they are large, such as 200-µm grains of quartz sand.
If a soil particle is surrounded by water, the exchangeable cations are not attached to it. Its negative electrical charge tends to attract thee cations, but the cations diffuse toward the lower cation concentration away from the particle. Therefore, the soil particle is surrounded by a domain know as an electric double layer (van Olphen 1977, Mitchell 1976). The inner layer of the double layer is the negative charge on the surface of the soil particle. The outer layer is the excess of cations and deficiency of anions with respect to the concentration in the free water not influenced by the force field of the particle. The cation concentration has a finite value near the surface of the particle and decreases exponentially with distance to the concentration of the cations in the free pore water. The thickness of double-layer water, which is determined by the valence of the exchangeable cations and by the electrolyte concentration in free pore water, can exceed 50 nm. Thick double-layer water develops with exchangeable cations of low electrovalence such as Na+, and in free pore water of low electrolyte concentration, as in freshwater rivers and lakes. On the other hand, exchangeable cations of high valence such as Ca2+ and high electrolyte concentration as in the marine environment tend to depress the thickness of the double-layer water. Thus, in general, a soil particle is covered by a 1-nm layer of absorbed water, surrounded by to 1 to > 50 nm of double-layer water, enveloped in turn by free water. Double-layer water is most significant in sodium montmorillonite because of its very small and filmy particles.