Study of self-assembly for mechanochemically-milled saponite nanoparticles

Abstract

The rheological mechanism of long-term self-assembly triggered by H 2 O molecules was studied for unmilled and mechanochemically-milled saponite nanoparticles by means of thermogravimetry and differential thermal analysis (TG-DTA), dilatometry (DLT), and positronium (Ps) lifetime spectroscopy. For unmilled saponite, the adsorption of H 2 O molecules due to hydration caused volume expansion arising from an increase in the basal spacing as well as weight gain with a time scale of ∼10 h. Ps lifetime spectroscopy revealed two kinds of voids with sizes of ∼0.3 nm and ∼0.9 nm for unmilled saponite before hydration. The intensity of the larger void component in the annihilation spectra decreased from ∼9 % to ∼5 % with increasing time up to ∼100 h and correspondingly the intensity of the smaller void component increased from ∼5 % to ∼9 % due to long-term rheological self-assembly. Both the weight gain and volume expansion were largely suppressed for milled saponite, indicating that the adsorption of H 2 O molecules is reduced. Furthermore, the larger void disappeared and a single void component, corresponding to a void size slightly larger than the original smaller void, was formed for milled saponite. The intensity of this void, created as a result of destruction, decreased with increasing time up to ∼100 h.