The texture of montmorillonites as influenced by the exchangeable inorganic cation and the drying method I: External surface area related to the stacking units of the aggregates

Abstract

Surface areas of smectites vary with ion exchange, preparation method and origin of the clay sample. Moosburg montmorillonite exchanged with univalent or divalent cations and then air dried after centrifugation or sedimentation of freeze dried was investigated. Surface areas were determined with nitrogen adsorption and concomitantly the number of stacked layers was observed by X-ray line broadening. For Moosburg montmorillonite, the surface areas decrease in the order of samples prepared by centrifugation, by freeze drying and by sedimentation if exchanged with monovalent cations but for divalent cations the surface area of the freeze-dried sample is less than that of the sedimented sample in magnitude. For Moosburg montmorillonites, the number of stacked layers is virtually independent of the drying method with the exception of Na +, K + and Sr 2+ but varies with the exchangeable cation as follows: Na +, K +,or Rb +, 5; Cs +, 7;Sr 2+, 4;Ca 2+, 4–5;Mg 2+ or Ba 2+, 6. When different smectites are compared the number of stacked layers decreases in the order Hectorite, Otay, Camp Berteau and Wyoming bentonite. A model for the aggregated clays is proposed which explains an open structure for Moosburg exchanged with univalent cations in comparison with the clay exchanged with divalent cations. Sodium-Wyoming bentonite has a more compact structure than sodium-Moosburg.