Anomalous Phase Behaviors of Monolayer NaCl Aqueous Solutions Induced by Effective Coulombic Interactions within Angstrom-Scale Slits

Atomic scale insights into NaCl nucleation in nanoconfined environments

In this work we examine the nucleation from NaCl aqueous solutions within nano-confined environments, employing enhanced sampling molecular dynamics simulations integrated with machine learning-derived reaction coordinates. Through our simulations, we successfully induce phase transitions between solid, liquid, and a hydrated phase, typically observed at lower temperatures in bulk environments. Interestingly, while generally speaking nano-confinement serves to stabilize the solid phase and elevate melting points, there are subtle variations in the thermodynamics of competing phases with the precise extent of confinement. Our simulations explain these findings by underscoring the significant role of water, alongside ion aggregation and subtle, anisotropic dielectric behavior, in driving nucleation within nano-confined environments. This report thus provides a framework for sampling, analyzing and understanding nucleation processes under nano-confinement.

Formation of compounds with diverse polyelectrolyte morphologies and nonlinear ion conductance in a two-dimensional nanofluidic channel

Aqueous electrolytes subjected to angstrom-scale confinement have recently attracted increasing interest because of their distinctive structural and transport properties, as well as their promising applicability in bioinspired nanofluidic iontronics and ion batteries. Here, we performed microsecond-scale molecular dynamics simulations, which provided evidence of nonlinear ionic conductance under an external lateral electric field due to the self-assembly of cations and anions with diverse polyelectrolyte morphologies (e.g., extremely large ion clusters) in aqueous solutions within angstrom-scale slits. Specifically, we found that the cations and anions of Li2SO4 and CaSO4 formed chain-like polyelectrolyte structures, whereas those of Na2SO4 and MgSO4 predominantly formed a monolayer of hydrated salt. Additionally, the cations and anions of K2SO4 assembled into a hexagonal anhydrous ionic crystal. These ion-dependent diverse polyelectrolyte morphologies stemmed from the enhanced Coulomb interactions, weakened hydration and steric constraints within the angstrom-scale slits. More importantly, once the monolayer hydrated salt or ionic crystal structure was formed, the field-induced ion current exhibited an intriguing gating effect at a low field strength. This abnormal ion transport was attributed to the concerted movement of cations and anions within the solid polyelectrolytes, leading to the suppression of ion currents. When the electric field exceeded a critical strength, however, the ion current surged rapidly due to the dissolution of many cations and anions within a few nanoseconds in the aqueous solution.

Evidence of Formation of Monolayer Hydrated Salts in Nanopores

Despite much effort being devoted to the study of ionic aqueous solutions at the nanoscale, our fundamental understanding of the microscopic kinetic and thermodynamic behaviors in these systems remains largely incomplete. Herein, we reported the first 10 μs molecular dynamics simulation, providing evidence of the spontaneous formation of monolayer hexagonal honeycomb hydrated salts of XCl₂·6H₂O (X = Ba, Sr, Ca, and Mg) from electrolyte aqueous solutions confined in an angstrom-scale slit under ambient conditions. By using both the classical molecular dynamics simulations and the first-principles Born-Oppenheimer molecular dynamics simulations, we further demonstrated that the hydrated salts were stable not only at ambient temperature but also at elevated temperatures. This phenomenon of formation of hydrated salt in water is contrary to the conventional view. The free energy calculations and dehydration analyses indicated that the spontaneous formation of hydrated salts can be attributed to the interplay between ion hydration and Coulombic attractions in the highly confined water. In addition to providing molecular-level insights into the novel behavior of ionic aqueous solutions at the nanoscale, our findings may have implications for the future exploration of potential existence of water molecules in the saline deposits on hot planets.