Nanoscale chemomechanical variations of montmorillonite induced by the specificity of counterions—An in situ XRD and AFM study

The variable structural characteristics of montmorillonite (Mnt) in salt solutions depend not only on the quantity and distribution of the Mnt layer charge, but also on the specificity of counterions, including their radius, valence, concentration, and hydratability. In order to determine the nanoscale chemomechanical processes in a clay mineral–water–salt system, in situ X-ray diffraction (XRD) and atomic force microscopy (AFM) methods were used to investigate the structural variations of Mnt caused by different counterion species (e.g., Li + , Na + , K + , Rb + , Cs + , Mg 2+ , Ca 2+ , and Sr 2+ ) at different concentrations (0.2–3.0 M). With an increasing concentration of counterions, separation of water molecules from the interlayer space of more weakly hydrated cations (Cs + , Rb + , and K + ) is faster than for the more strongly hydrated cations (Li + , Na + , Mg 2+ , Ca 2+ , and Sr 2+ ), water molecules are lost more slowly from the hydration shell, which remains stable even at high counterion concentrations. The sequential ion exchange in a monolayer Mnt observed by in situ AFM is consistent with the d (001) results from the real-time XRD analysis. Moreover, various defects on Mnt surfaces were visible in the high-resolution AFM images, which restrict the spatial arrangement of adsorbed cations. The water molecule partitioning between the interlayers in Mnt is controlled by the specificity of the counterions, and the cations can spontaneously form ordered structures on the surface of Mnt. These nonclassical interfacial phenomena occurred in the electric double layer (EDL) is controlled by the hydration of counterions, the surface structure and the charge distribution of Mnt.