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In this work, we use large-scale molecular dynamics simulations coupled tofree energy calculations to identify for the first time a limit of stability(spinodal) and a change in the nucleation mechanism in aqueous NaCl solutions.This is a system of considerable atmospheric, geological and technicalsignificance. We find that the supersaturated metastable NaCl solution reachesits limit of stability at sufficiently high salt concentrations, as indicatedby the composition dependence of the salt chemical potential, indicating thetransition to a phase separation by spinodal decomposition. However, themetastability limit of the NaCl solution does not correspond to spinodaldecomposition with respect to crystallization. We find that beyond thisspinodal, a liquid/amorphous separation occurs in the aqueous solution, wherebythe ions first form disordered clusters. We term these clusters as "amorphoussalt". We also identify a transition from one- to two-step crystallizationmechanism driven by a spinodal. In particular, crystallization from aqueousNaCl solution beyond the spinodal is a two-step process, in which the ionsfirst phase-separate into disordered amorphous salt clusters, followed by thecrystallization of ions in the amorphous salt phase. In contrast, in theaqueous NaCl solution at concentrations lower than the spinodal,crystallization occurs via a one-step process, as the ions aggregate directlyinto crystalline nuclei. The change of mechanism with increasingsupersaturation underscores the importance of an accurate determination of thedriving force for phase separation. The study has broader implications on themechanism for nucleation of crystals from solutions at high supersaturations.

Nucleation in aqueous NaCl solutions shifts from 1-step to 2-step mechanism on crossing the spinodal