In this study, the influence of the negative charge distribution and the locations of Na⁺ ions on ionic conductivity of Na-micas was investigated. The locations of Na⁺ ions in as-prepared Na-micas and heated Na-micas at 700°C were determined in details using X-ray diffraction analyzer, Fourier transform infrared spectrophotometer, ²³Na magic angle spinning nuclear magnetic resonance and X-ray photoelectron spectroscopy. As the results, four kinds of Na⁺ ions, such as Na⁺ ions induced into vacant sites in the octahedral sheets, hydrated Na⁺ ions in the interlayer, dehydrated Na⁺ ions surrounded by six basal oxygens and Na⁺ ions drawn into the inside of the ditrigonal hole, were found in Na-micas. Ionic conductivities of the heated Na-micas were measured at 400 to 600°C by an alternating current four-probe method. Among the Na-micas, Na-taeniolite showed the maximum ionic conductivity, which was 6.61×10⁻⁴ S/cm at 600°C. Following relationships were obtained from the results. The hydrated Na⁺ ions in the interlayer and the dehydrated Na⁺ ions surrounded by six basal oxygens contributed to the ionic conductivity of Na-mica, while the octahedral Na⁺ ions and the Na⁺ ions drawn into the inside of the ditrigonal hole hardly contribute to the ionic conductivity. In addition, because tetrasilisic type mica had weaker electrostatic bonding force between layer and interlayer cation than trisilisic type mica, the Na⁺ ions of tetrasilisic type mica were migrated more easily than that of trisilisic type mica.

In this report, we demonstrate the direct determination of the layer number of free-standing montmorillonite (MMT) nanosheets using annular dark field (ADF) scanning transmission electron microscopy (STEM). The layer number of the free-standing MMT nanosheets deposited on a holey carbon-coated grid was successfully counted to range from monolayer to four layers by the difference in the ADF contrast. Meanwhile, counting of the layer numbers was difficult using conventional TEM and bright field STEM due to the low contrast. Therefore, ADF–STEM can potentially be applied to further investigate the clay nanosheets and their molecular complexes by the atomic scale imaging for a deeper understanding of clay-nanosheet-based chemistry.

Herein we synthesized Na-type bentonite by hydrothermal ion exchange from Ca-type bentonite. Ca²⁺ ions in the montmorillonite layer in bentonite were exchanged for H⁺ or Na⁺ ions by hydrothermal treatment using ion-exchanged water or NaCl aqueous solution, respectively. The swelling index of the resulting Na-type bentonite, treated with the NaCl aqueous solution, was in the range from 11.2 to 15.0 mL/2 g, close to that of Na-activated bentonite. Likewise, using aqueous Na₂CO₃ solution, Ca²⁺ ions in the montmorillonite layer in bentonite were exchanged for Na⁺ ions by hydrothermal treatment. The swelling index of the sample treated with aqueous Na₂CO₃ solution was in the range from 18.2 to 25.3 mL/2 g, as large as that in natural Na-type bentonite.