Frustrated Interfacial Solvation Enhances Reaction Rates at Acidified Ice Grain Boundaries in Frozen Microdroplets.

Ice is ubiquitous in natural environments, yet the interstitial liquid trapped within polycrystalline grain boundaries remains difficult to access, limiting mechanistic understanding of accelerated reactions in ice. Here, we combine cryogenic stimulated Raman scattering microscopy with multivariate curve resolution to visualize and spectroscopically resolve internal constituents of frozen microdroplets. We identify pronounced structural heterogeneity: the entrapped interstitial liquid displays more strongly hydrogen-bonded water signatures than bulk supercooled water, whereas the ice-water interface shows diminished orientational ordering, whereby water hydrogen atoms are less preferentially aligned toward solutes, consistent with frustrated solvation and partial desolvation. Using sulfur dioxide autoxidation as a model reaction, we find that frozen microdroplets exhibit an approximately 20-fold faster apparent oxidation rate than supercooled liquid microdroplets under otherwise comparable conditions. Kinetic analysis and simulations suggest that interfacial frustrated solvation, together with grain boundary acidification, lowers the effective barrier for electron transfer and radical initiation and the associated enhancement of sulfate formation. These findings provide a microscopic link between buried ice-water interfacial structure and low-temperature reaction kinetics, with implications for atmospheric chemistry in cold and mixed-phase environments.