The structure of montmorillonite gels revealed by sequential cryo-XHR-SEM imaging.

Imaging by extreme high resolution-scanning electron microscopy (XHR-SEM) with a monochromated and decelerated beam was applied on 5% (wt/wt) Na and Ca-montmorillonite gels frozen by high pressure freezing (HPF). In order to visualize the three-dimensional structure and the contacts between clay platelets, a new approach was developed. It consists in recording a sequence of micrographs on a region of interest during controlled sublimation. This simple method allows to rewind and to relate the instantaneous configuration between several particles to their original position in the hydrated state. Consequently, aggregates of parallel platelets (i.e. curved tactoids) were present in the Ca-sample and the instantaneous position of these aggregates in the course of sedimentation was revealed. The Na-sample consisted of a continuous network of flexible platelets sharing mostly face-to-face (FF) contacts caused by jamming at the relatively high concentration of the suspension (5% (wt/wt)), which was above the gel transition. Yet individual platelets belonging to the smallest size fraction were observed to be fully dispersed within the entangled structure, which confirmed the repulsive character of the gel. Substructures consisting of several platelets connected by FF-associations were also evidenced. The origin and potential impact of such substructures on the occurrence of the sol-gel transition and birefringence are discussed.