Cation Exchange, Specific Surface and Absorbed H2O Molecules

There is a direct relation between the cation exchange capacity of soil particles and their specific surfaces. With the exception of strongly weathered soils like terra rossas, latosols, etc., with large amounts of inactive sesquihydroxides, the average surface occupied by one cation is about 70 A2. Hygroscopic water absorbed by the soil particles is composed of H2O molecules bonded the surface as a monolayer and of a varying number of H2O molecules bonded to the cations, the kind of cation determining their number. Heat of wetting and specific surface are directly related. For Na+ saturated montmorillonite samples 150 ergs/cm2 is found. The H2O molecules of the monolayer on the surface of the soil particles have an OH stretching vibrationband at 3220 cm−1. This corresponds with an OH … O distance of 2.77 A which is almost the same as that of H2O molecules in ice = 2.76 A (H2O molecules in water = 2.86 A).