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Aggregation in Na-, K- and Ca-montmorillonite dispersions, characterized by impedance spectroscopy

¹ Laboratoire Environnement et Minéralurgie, UMR 7569 CNRS–INPL Pôle de l’Eau, 15, Avenue du Charmois, BP 40, 54501 Vanduvre-les-Nancy Cedex, and ² Laboratoire de Chimie Physique pour l’Environnement, UMR 7592 CNRS, Université H. Poincaré Nancy I, 405, rue de Vanduvre, 54600 Villers-les-Nancy, France

^* E-mail: fthomas{at}ensg.u-nancy.fr

(Received 6 October 1998; revised 17 June 1999)

Montmorillonite-water-cation systems were characterized using high-frequency impedance spectroscopy by studying the influence of the solid concentration and the nature of the exchangeable cation (Na⁺, K⁺, Ca²⁺) on the dielectric characteristics of the dispersions. A new method is proposed to calculate the relaxation frequency (f_r) and the dispersion factor () from a limited number of impedance measurements. By comparison with rheology, microscopy, X-ray diffraction and immersion calorimetry results, it is shown that impedance spectroscopy is a very powerful technique which yields structural information on a complex system. For Na-montmorillonite, two transitions are observed at 2.5% and 3.6% in solids. The cation mobility and the number of connections between particles are described by f_r and , respectively. The two transitions can then be attributed to the formation of the gel and to the reduction of the macroporosity within the gel, respectively. For Ca-montmorillonite, thick layer-stacks form at the lowest concentrations, and connections between these stacks are observed at 9% in solids, in good aggrement with rheological measurements. The K-montmorillonite displays progressive thickening of the tactoids, and no formation of a unique connected network, as revealed by the smooth evolution of f_r and .

KEYWORDS: impedance spectroscopy, montmorillonite, dispersions, XRD

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