Session 3

A metastable phase of amorphous aggregates governs solution structure

Klaas Wynne
on  Mon, 16:00 for  20min
Zhiyu Liao, Ankita Das, Ben A. Russell, Ikhlas M.M. Ahmed, Immacolata Abate, Ian MacLaren, Rebecca Beveridge, and Klaas Wynne
1. School of Chemistry, University of Glasgow, G12 8QQ, UK
2. Dept. of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, UK
3. School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ UK
klaas.wynne@glasgow.ac.uk

Abstract

The structure of solutions is traditionally assumed to consist of individual solvated solute molecules, occasionally forming transient dimers or small clusters. Here we show that this textbook view is fundamentally incomplete: concentrated aqueous electrolyte solutions contain a previously unrecognised metastable amorphous phase composed of amorphous aggregates (AAs), into which the majority of solute molecules spontaneously assemble, even under undersaturated conditions.1 Using a range of experimental techniques, we reveal a hierarchical population extending from molecular oligomers to micrometre-scale amorphous fractal solids and millimetre-scale assemblies. AAs form by a barrierless diffusion-limited process, rationalised by a modification of classical nucleation theory that accounts for their reduced free-energy barrier and fractal structure. These findings demonstrate that a metastable amorphous aggregate phase is intrinsic to concentrated solutions, revising the classical description of solubility, supersaturation and nucleation kinetics.

Figure from Wynne_1.docx

Figure 1 – Microscopic imaging of the solid forms of amorphous aggregates (AAs) in undersaturated solution. (a) After several days of aging, millimetre-sized fluffy macroscopic objects can be observed by eye in undersaturated solution. (b) Visible light microscopic imaging shows that the fluffy macroscopic aggregates consist of 10–100 μm AA fractals. (c) TEM image of AA fractals on carbon film/copper grid (scale bar, 20 µm). (d) Idem, showing that the AA fractals consist of ~1-μm nodular aggregates of ~100-nm spheres positioned on a lacey carbon film. (e) Annular dark field image calculated from the 4D-STEM imaging dataset showing a typical nodular aggregate (10-nm scan step size). (f) Four exemplar electron-diffraction patterns from the from the selected areas 1-4 shown in (e). (g) Four pair distribution functions (PDFs) extracted from selected areas in (e), showing 3 clear near-neighbour distances.

References

(1) Liao, Z.; Das, A.; Russell, B.; Ahmed, I.; Abate, I.; MacLaren, I.; Beveridge, R.; Wynne, K. A Metastable Phase of Amorphous Aggregates Governs Solution Structure. ChemRxiv 2026. https://doi.org/10.26434/chemrxiv-2025-x4kh4/v2.