Controlled Spherulitic Crystal Growth from Salt Mixtures: A Universal Mechanism for Complex Crystal Self-Assembly

TL;DR

This study uncovers the conditions and mechanisms behind controlled spherulitic crystal growth of sodium sulfate, highlighting the role of divalent ions and supersaturation in complex crystal self-assembly.

Contribution

It demonstrates how divalent metal ions induce spherulitic growth in sodium sulfate and quantifies the supersaturation and kinetics involved, providing new insights into nonclassical nucleation.

Findings

Divalent ions trigger spherulitic growth at high supersaturation.

Supersaturation at growth onset is approximately 111 Pa·s.

Spherulites can evolve into other shapes as supersaturation decreases.

Abstract

Spherulites are complex polycrystalline structures that form through the self-assembly of small aggregated nanocrystals starting from a central point and growing radially outward. Despite their wide prevalence and relevance to fields ranging from geology to medicine, the dynamics of spherulitic crystallization and the conditions required for such growth remain ill-understood. Here, we report on the conditions to induce controlled spherulitic growth of sodium sulfate from evaporating aqueous solutions of sulfate salt mixtures at room temperature. We reveal that introducing divalent metal ions in the solution cause spherulitic growth of sodium sulfate. For the first time, we quantify the supersaturation at the onset of spherulitic growth from salt mixtures and determine the growth kinetics. Our results show that the nonclassical nucleation process induces the growth of sodium sulfate spherulites at high supersaturation in highly viscous solutions. The latter reaches approximately 111 Pa$\cdot$s, triggered by the divalent ions, at the onset of spherulite precipitation leading to a diffusion limited growth. We also show that spherulites, which are metastable structures formed under out-of-equilibrium conditions, can evolve into other shapes when supersaturation decreases as growth continues at different evaporation rates. These findings shed light on the conditions under which spherulites form and offer practical strategies for tuning their morphology.