Rheological mechanism of long-term self-assembly in saponite nanoparticles

A rheological mechanism of long-term self-assembly induced by H ₂O molecules is highlighted for layered saponite nanoparticles based on the results of thermogravimetry and differential thermal analysis, time-resolved length change measurement with high-resolution dilatometry, solid-state nuclear magnetic resonance, positronium annihilation spectroscopy, and molecular dynamics simulation. Prior to self-assembly, saponite nanoparticles exhibit two kinds of local molecular structures, where one and two nanosheets are inserted into interlayer spaces forming open spaces with their sizes of ∼0.3 and ∼0.9 nm, respectively. H ₂O molecules adsorbed at Na ⁺ cations in the interlayer spaces immediately trigger off the onset of the rheological motion of nanosheets in parallel to the layer direction. One of two nanosheets inserted into the interlayer spaces is thus gradually released away, with the local molecular structure with smaller open spaces getting to dominant for the self-assembled saponite nanoparticles.