From miscibility to mesoscale heterogeneity: tannic-acid-mediated assembly of pyridine in aqueous liquids

Small molecules that are fully miscible with water are often treated as molecularly dispersed solutes, yet the organization of these molecules can change in the presence of multivalent associating species. This study shows that tannic acid, a polyphenol containing multiple galloyl and phenolic groups, converts pyridine-water mixtures from homogeneous liquids into clustered, condensed, and compositionally heterogeneous soft-matter states. Density functional theory calculations using gallic acid as a proxy for tannic-acid phenolic units yielded favorable electronic association energies for selected hydrated clusters, supporting the plausibility of local pyridine-polyphenol association without establishing its solution-phase binding free energy. Gallic acid did not induce demixing, whereas tannic acid produced non-monotonic phase behavior, including flocculation at low pyridine concentration, droplet formation at intermediate composition, and re-entrant solubilization at higher concentration. Synchrotron micro-FTIR qualitatively showed co-localized spectral contributions attributable to pyridine and tannic acid within separated flocs and droplets, while small-angle X-ray scattering showed that optically transparent mixtures could contain nanoscale and mesoscale heterogeneity. Temperature-dependent scattering indicated that these heterogeneities were thermally sensitive, consistent with hydrated hydrogen-bonded association. Tannic-acid-free freeze-thaw experiments showed that a nominally miscible pyridine-water liquid was redistributed into pyridine-enriched fractions during freezing and partial thawing. X-ray absorption spectroscopy, microscopy, micro-IR, and complementary calculations provided evidence that tannic-acid/pyridine-containing domains associate with solid interfaces, with mechanistic support for phenolic anchoring and co-association at hydroxylated silica. Together, these results show that multivalent polyphenols can transform a water-miscible aromatic heterocycle from a molecularly dispersed solute into clustered, phase-separated, re-entrant, and surface-associated soft-matter states.