In situ formation of surface-functionalized ionic calcium carbonate nanoparticles with liquid-like behaviours and their electrical properties

This paper reports a new route to synthesize calcium carbonate (CaCO₃)-based nanoscale ionic materials (NIMs) via an in situ formation method to form the CaCO₃ nanoparticles with a polysiloxane quaternary ammonium salt (PQAC) corona (PQAC-CaCO₃ nanoparticles), followed by an ionic exchange reaction to fabricate a poly(ethylene glycol)-tailed sulfonate anion (NPEP) canopy. The chemical compositions and structures of the CaCO₃-based NIMs synthesized in this work were confirmed by Fourier-transform infrared spectroscopy and solid-state ¹³C NMR spectroscopy. Transmission electron microscopic observation indicated that the CaCO₃-based NIMs presented a rhombohedral shape with a well-defined core-shell structure, and they also obtained an NPEP canopy with a thickness of 4-6 nm. X-ray powder diffraction investigation confirmed that the CaCO₃ inner core had a calcite crystalline structure, whereas the NPEP canopy was amorphous. The NPEP canopy was found to show a characteristic crystallization-melting behaviour in the presence of the ion bonding with PQAC-CaCO₃ nanoparticles according to the characterization of differential scanning calorimetry. Thermogravimetric analysis indicated that the CaCO₃-based NIMs achieved a high content of NPEP canopy as well as an improvement in thermal stability owing to the ion-bonding effect. Most of all, the CaCO₃-based NIMs demonstrated a liquid-like behaviour above the critical temperature in the absence of solvent. Moreover, the CaCO₃-based NIMs also showed a relatively high electrical conductivity with a temperature dependency due to the ionic conductive effect. This work will provide a more feasible and energy-saving methodology for the preparation of CaCO₃-based NIMs to promote their industrialization and extensive applications.