Microscopic Kinetics Pathway of Salt Crystallization in Graphene Nanocapillaries

Abstract

The fundamental understanding of crystallization, in terms of microscopic kinetic and thermodynamic details, remains a key challenge in the physical sciences. Here, by using in situ graphene liquid cell transmission electron microscopy, we reveal the atomistic mechanism of NaCl crystallization from solutions confined within graphene cells. We find that rock salt NaCl forms with a peculiar hexagonal morphology. We also see the emergence of a transitory graphitelike phase, which may act as an intermediate in a two-step pathway. With the aid of density functional theory calculations, we propose that these observations result from a delicate balance between the substrate-solute interaction and thermodynamics under confinement. Our results highlight the impact of confinement on both the kinetics and thermodynamics of crystallization, offering new insights into heterogeneous crystallization theory and a potential avenue for materials design.

Reduced Ionic Conductivity but Enhanced Local Ionic Conductivity in Nanochannels

“…Here, we find that σ cross for confined water is smaller than that in bulk water (excepting KCl at high c ), which indicates that the correlation effect is enhanced in confined water, which is consistent with the conclusions from the results of β and ion–pair correlation function . The results show that σ cross in bilayer water is smaller than in bulk for NaCl in all the range of c , while the value for KCl in bulk is larger than bulk when c ≳ 2.0 M, which is reminiscent of the different crystallization kinetics in bulk and confinement found by recent experiment . Here, we also calculate the ion conductivity using other force fields, and the conclusions above are also satisfied (Figures S1–S3).…”

Two-dimensional monolayer salt nanostructures can spontaneously aggregate rather than dissolve in dilute aqueous solutions

“…This was because of the lower free energy barrier to reach the global free-energy minimum (PMF basin) and the deeper global free-energy minimum for the ion dimer state in the case of confined water. We envisaged that the spontaneous formation of 2D monolayer NaCl nanocrystals predicted in this study will simulate future experiments, e.g., focused on dilute NaCl solutions confined between graphene nanocapillaries 35 , to validate the present theoretical findings.…”

“…Jiang and coworkers reported that when the Na + ion was hydrated by three water molecules, its diffusion on the NaCl (001) surface was orders of magnitude faster than that of other hydrated ions 34 . Very recently, based on in situ graphene liquid cell transmission electron microscopy measurements, Wang et al found that when a saturated NaCl solution was confined into graphene nanocapillaries, it formed rock salt NaCl with an intriguing hexagonal morphology 35 . In addition, a metastable hexagonal NaCl phase was detected during the crystallization process.…”

Two-dimensional ionic crystals: The cases of IA-VII alkali halides and IA-IB CsAu

“…Exploring suitable substrates [10,35,36] for the 2D ionic crystals and a phase transition between 2D structures, as an analog of the B1-B2 transition in 3D systems [17,18], is left for future work. calculations were performed within the generalized gradient approximation of Perdew-Burke-Ernzerhof [34].…”