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Abstract The light absorption spectra of Na, K, Ca and Ba‐montmorillonite suspensions were studied in relation to the procedure of sample preparation. These procedures included different sequences of transformations from one homoionic form into another, as well as a drying stage during the preparation. The optical density was usually influenced by the method of preparation, indicating hysteresis effects in the transformation process. The data were interpreted in terms of tactoid formation and size. Transformation of monovalent into divalent clay suspensions resulted in small tactoids. Air drying of the clay before the transformation resulted in larger tactoids in both mono‐ and divalent clays. Negative adsorption of chloride in Ca‐bentonite from two different preparations (which differed in their tactoid size) were in agreement with the light absorption measurements: the higher surface area was found in the suspension with the smaller tactoids. The data available in the literature on negative absorption of chloride on montmorillonite are discussed. It is suggested that the discrepancies in the experimental results of different investigators could be, at least partially, explained by the differences in the preparation procedure of the clay suspensions.
Lahav et al. (Mon,) studied this question.