Chemical and physical equilibria shape dual ice-nucleation pathways in an organic crystal

Organic crystals critically influence ice formation in natural environments, yet the molecular mechanisms of their ice nucleation activity remain poorly understood. Here we reveal that the exceptional freezing efficiency of phloroglucinol (PGL), a simple polyhydroxylated aromatic compound, arises from a dynamic interplay of chemical and physical equilibria that generate two concurrent nucleation pathways. One pathway originates from crystalline PGL surfaces that act as potent ice templates, whereas the second occurs in solution, where dissolved molecules assemble into nanoscale aggregates capable of nucleating ice even below the solubility limit. We find that alkaline pH eliminates both pathways by inducing tautomeric shifts that alter hydrogen bonding motifs and suppress molecular assembly. Aging in solution diminishes ice-nucleating activity, likely through oxidative or polymerization processes, while freeze-thaw cycling partially restores activity by generating fresh PGL crystals. These results identify molecular structure, solubility, pH-dependent tautomerism, and phase behavior as key determinants which collectively control ice nucleation, offering a generalizable framework for understanding and predicting the activity of phenolic and other organic ice nucleators in complex environmental settings encountered in atmospheric and cryobiological systems.