Clay Swelling: New Insights from Neutron-Based Techniques

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Clayey materials are complex hierarchical and deformable porous media whose structure and organization vary at different spatial scales depending on external conditions, in particular water activity. It is therefore important, on the one hand, to follow all the structural changes that are associated with the adsorption of water molecules in the interlamellar spaces (at the scale of the particles) and, on the other hand, to describe the textural modifications induced at larger scales as a result of the swelling of individual particles. Neutron-based techniques are important to achieving this multiscale description, thanks to some special features of neutrons [e.g., specific interaction with hydrogen atoms, with in addition differential interaction with isotopes (H and D), and high penetration length of neutron beams, which allows easy preparation of versatile sample-cells container]. Finally, water dynamics in the interlayers can be investigated because of the unique interaction of neutrons with hydrogen. After a brief discussion of the crystal chemistry of swelling clays, some examples of the application of neutron techniques to the study of clays will be given, including application of neutron diffraction to the study of the structure evolution of various expandable clays upon hydration and investigation of interlayer water dynamics by quasielastic neutron scattering (QENS) experiments, illustrating water molecule mobility as a function of hydration states. The difficulties in applying these techniques to materials with such complex crystal chemistry and anisotropic shape will be pointed out. Clay hydration and swelling induce modification of aggregates at larger spatial scale than the nanometric one investigated by diffraction. Clay fabric and particles organization at the sub-micronic scale can be investigated by small-angle neutron scattering (SANS) experiments. Examples of SANS measurements will be given in the so-called crystalline swelling domain, for water activity below 1. In the clay–water system, as the solid/liquid ratio decreases, gels formation is observed in which individual clay layers are now separated by distances larger than a few nanometers. Experimental studies performed on clay suspensions or gels will be presented, demonstrating that clay layers are equilibrated in gels via electrostatic repulsions.