Hydration breaking and chemical ordering in a levitated NaCl solution droplet beyond metastable zone width limit: Evidence for early stage of the two-step nucleation

Over two decades, NaCl nucleation from a supersaturated aqueous solution has been predicted to occur via two-step nucleation (TSN) mechanism, i.e., two sequential events, formation of locally dense liquid regions followed by structural ordering. However, the formation of dense liquid region in the very early stage of TSN has never been experimentally observed. By using a state-of-the-art technique, a combination of electrostatic levitation (ESL) and in-situ synchrotron X-ray and Raman scatterings, we find experimental evidence that indicates the formation of the dense liquid regions in the NaCl bulk solution at an unprecedentedly high level of supersaturation (S = 2.31). As supersaturation increases, evolution of the ion clusters leads to chemical ordering, but no topological ordering, which is a precursor for forming the dense disordered regions of ion clusters in early stage of TSN. Moreover, as the ion clusters proceed to evolve under highly supersaturated conditions, we observe the breakage of water hydration structure indicating the stability limit of the dense liquid regions, and thus leading to nucleation. The evolution of solute clusters and breakage of hydration in highly supersaturated NaCl bulk solution will provide new insights for the detailed mechanism of the TSN on many other aqueous solutions.