In situ Raman and X-ray scattering of a single supersaturated aqueous Mg(NO₃)₂ droplet ultrasonically levitated

A single liquid droplet in the air generated by ultrasonic levitation provides such analytical advantages as a small sample volume (~ μL) for expensive proteins, container-free condition for deeply supercooling and supersaturation, time-dependent observation, and homogeneous rapid mixing. The investigation of the properties and structure of a droplet at a molecular level is highly needed for understanding the physicochemical behaviors of a droplet and an underlying mechanism of processes in the droplet. We develop in situ Raman and synchrotron X-ray scattering methods of a single liquid droplet of ~ 1 mm size ultrasonically levitated. The composition of a supersaturated Mg(NO₃)₂ droplet and speciation in the droplet are determined by analyzing the nitrate N-O and the water O-H stretching vibrational Raman bands. The X-ray interference function of an supersaturated Mg(NO₃)₂ droplet is subjected to an empirical potential structure refinement modeling to reveal the ion solvation, association, and solvent water structure. Furthermore, crystallization of Mg(NO₃)₂⋅nH₂O from a saturated droplet is observed and identified.