Water Dynamics from THz Spectroscopy Reveal the Locus of a Liquid–Liquid Binodal Limit in Aqueous CaCO₃ Solutions

Abstract

Many phenomena depend on CaCO₃ nucleation where the role of water remains enigmatic. Changes in THz absorption during the early stages of CaCO₃ nucleation evidence altered coupled motions of hydrated calcium and carbonate ions. The direct link between these changes and the continuous development of the ion activity product reveals the locus of a liquid-liquid binodal limit. The data strongly suggest that proto-structured amorphous CaCO₃ forms through solidification of initially liquid precursors. Furthermore, polycarboxylates, which stabilize liquid precursors of CaCO₃ , significantly enhance the kinetic stability of the metastable liquid-liquid state, but they do not affect the locus of the binodal limit. The importance of water network dynamics in phase separation mechanisms can be understood based on the notions of the pre-nucleation cluster pathway, and is likely to be more general for aqueous systems.