WATER RETENTION BY OSMOTIC SWELLING OF CERTAIN COLLOIDAL CLAYS WITH VARYING IONIC COMPOSITION

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Abstract
Summary: Experimental data and theoretical calculations of water retention by osmotic swelling of colloidal homoionic Na‐ and Ca‐montmorillonites, vermiculites, and kaolinites were carried out at several levels of electrolyte concentration, in order to evaluate their respective contributions to the physical response of a soil system to changes in its chemical environment. Similar experiments were conducted for mixed‐ion Na‐Ca‐montmorillonite systems to establish whether a threshold value exists for cation proportions in the exchange positions, at which the physical properties of the system show a sudden change. The results are projected as relationships between the chemical treatment and moisture contents at three equilibrium pressures.Theoretical calculations, based on the diffuse double‐layer model, predict that montmorillonite systems with a certain form of cation saturation and salt content should have greater swelling pressure or higher capability for water retention than vermiculites, which have greater values than kaolinites under similar chemical conditions. This was confirmed by the experimental data, although measured values were consistently higher than those predicted by theory.The studies on mixed‐ion montmorillonites show that mutual existence of the two cation species in the clay‐water system produces a nearlinear relationship between exchangeable Na percentage and the moisture retention at a particular equilibrium pressure.