Coagulation of Na-Montmorillonite by Inorganic Cations at Neutral pH. A Combined Transmission X-ray Microscopy, Small Angle and Wide Angle X-ray Scattering Study

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Abstract
The coagulation of sodium montmorillonite by inorganic salts (NaNO3, Ca(NO3)2 and La(NO3)3) was studied by combining classical turbidity measurements with wide-angle-X-ray scattering (WAXS), small-angle-X-ray scattering (SAXS), and transmission X-ray microscopy (TXM). Using size-selected samples, such a combination, associated with an original quantitative treatment of TXM images, provides a true multiscale investigation of the formed structures in a spatial range extending from a few ångstroms to a few micrometers. We then show that, at neutral pH and starting with fully Na-exchanged samples, coagulation proceeds via the formation of stacks of particles with a slight mismatch between layers. These stacks arrange themselves into larger porous anisotropic particles, the porosity of which depends on the valence of the cation used for coagulation experiments. Face–face coagulation is clearly dominant under those conditions, and no evidence for significant face–edge coagulation was found. These structures appear to arrange as larger clusters, the organization of which should control the mechanical properties of the flocs.

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