In-situ liquid SAXS studies on the early stage of calcium carbonate formation

TL;DR

This study uses in-situ liquid SAXS to investigate the early formation stages of calcium carbonate from supersaturated solutions, providing new insights into nucleation mechanisms before amorphous calcium carbonate forms.

Contribution

It introduces contactless SAXS and WAXS methods to analyze early calcium carbonate nucleation, revealing size distributions of initial entities in supersaturated solutions.

Findings

Identified size distribution of early entities before ACC nucleation

Demonstrated the effectiveness of in-situ SAXS for real-time analysis

Provided theoretical insights into initial solid formation pathways

Abstract

Calcium carbonate is a model system to investigate the mechanism of solid formation by precipitation from solutions, and it is often considered in the debated classical and non-classical nucleation mechanism. Despite the great scientific relevance of calcium carbonate in different areas of science, little is known about the early stage of its formation. We, therefore, designed contactless devices capable to provide informative investigations on the early stages of the precipitation pathway of calcium carbonate in supersaturated solutions using classical scattering methods such as Wide-Angle X-ray Scattering (WAXS) and Small-Angle X-ray Scattering (SAXS) techniques. In particular, SAXS was exploited for investigating the size of entities formed from supersaturated solutions before the critical conditions for amorphous calcium carbonate (ACC) nucleation are attained. The saturation level was controlled by mixing four diluted solutions (i.e., NaOH, CaCl2, NaHCO3, H2O) at constant T and pH. The scattering data were collected on a liquid jet generated about 75 sec after the mixing point. The data were modeled using parametric statistical models providing insight about the size distribution of denser matter in the liquid jet. Theoretical implications on the early stage of solid formation pathway are inferred.