Nucleation begins before supersaturation: glassy solute aggregates dominate solution structure

The molecular-level structure of concentrated solutions is crucial to the understanding of crystal nucleation and polymorph selection, with significant implications for pharmaceuticals, materials science, and geology. Classical models of crystal nucleation, which assume a high surface tension associated with a nascent crystal nucleus, imply that solute molecules in solution exist primarily as monomers. However, numerous recent studies of supersaturated solutions have revealed the presence of pre-nucleation clusters, where an amorphous or liquid-like intermediate forms before crystallization. Here, we demonstrate that significant solute clustering occurs at all concentrations, even in undersaturated solutions, ranging from molecular oligomers to sub-micrometre-scale amorphous aggregates. Using aqueous potassium carbonate solutions as a simple model system, we show for the first time the glassy nature of amorphous aggregates, the filamentous structure of molecular oligomers serving as building blocks for these aggregates, and crystal nucleation occurring within these aggregates. These findings support a non-classical two-step nucleation model, in which amorphous aggregates form through barrierless homogeneous nucleation, followed by crystal nucleation within these aggregates. This has wide-ranging implications not only for the understanding of crystal nucleation and polymorph selection but also for our general understanding of the molecular-level structure and physicochemical properties of concentrated (electrolyte) solutions.