Nitrogen sorption isotherms and the microstructure of vertisols

Abstract

The microstructure of 12 Queensland cracking clay soils has been examined using low temperature nitrogen sorption. The adsorption isotherms are satisfactorily described, below about P/Po = 0.8, by the general BET equation with n values in the range 2-4; this is consistent with the presence of pores of width 1.4-2.8 nm. Increased values of n, obtained after critical point drying or addition of kaolinite to the clay fraction of one soil, support the physical significance of this parameter as a measure of pore size. The desorption isotherms all exhibit pronounced steps at about P/Po = 0.5, which is near the widely accepted lower limit of applicability of the Kelvin equation, but which is also consistent with the presence of pores with widths of 2.7 nm. The consistency of these two sorption branches is such that the desorption isotherms have been interpreted as arising from the drainage of independent assemblies of slits and wedge-shaped voids, with widths concentrated in the range 0-3 nm, probably generated by the extended interleaving of smectite lamellae. The validity of this interpretation of the data, and the origin and significance of this microstructure in relation to clay particle shape and size, are discussed.