Chasing Crystal Embryos of Benzoic Acid with Classical Molecular Dynamics

Measuring the minimum size a molecular aggregate must reach to form an ordered crystal that is, whether and to what extent subcritical structures with only partially developed translational symmetry might produce recognizable diffraction patterns─remains an unsolved problem. Any ensemble of (partially) translationally ordered molecules should exhibit physicochemical properties distinct from the liquid phase, such as higher density and the ability to scatter X-rays coherently. However, most available experimental approaches are blind to nanometer-sized, intrinsically transient structures that lie at the solid–liquid boundary and are buried within the bulk liquid core. Here, we exploit Molecular Dynamics (MD) simulations to investigate the dissolution kinetics of very small (∼6 nm) crystalline nanoparticles of benzoic acid at large undercooling (∼100 K), embedded in spherical nanodroplets of their own liquid. We identify residual supramolecular aggregates that retain significant translational symmetry for tens of picoseconds, regardless of the initial nanoparticle size and shape, even as complete dissolution is approached. We hypothesize that these persistent clusters may serve as plausible models for crystal embryos, or even small nuclei, on their path across the liquid ⇄ solid transition. We also predict that these aggregates should, in principle, be detectable by diffraction methods, provided their dimensions are larger than 2–3 nm and their average lifetime is not shorter than 100–140 ps. Finally, we show that they share some key packing features with the bulk crystal, despite their fluxional and partially liquid nature. The findings are discussed in the context of modern nucleation theories.