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

Abstract

Interests in subnanofluidic devices have called for molecular dynamics (MD) simulation studies of the thermodynamic behavior of monolayer salt solution within angstrom-scale slits. However, it still remains a grand challenge to accurately describe the Coulombic interactions by incorporating the effects of charge transfer and electronic dielectric screening. Herein, by using the electronic continuum model, where the effective ion charges are fine-tuned with a scaling factor of λ, we present simulation evidence that the effective Coulombic interactions among Na+/Cl– ions can strongly affect the behavior of monolayer ionic aqueous solution. Our microsecond-scale MD simulations show that only the counterions with moderate effective charges (0.3 ≤ λ ≤ 0.8) can dissolve in monolayer water, whereas the high effective charges (λ ≥ 0.85) induce ions to assemble into monolayer nanocrystals, and ions with the low effective charges (λ ≤ 0.2) exhibit gas-like nanobubble. These findings could provide deeper insights into the physical chemistry behind subnanofluidic iontronic devices.

Evidence of Formation of Monolayer Hydrated Salts in Nanopores

“…In the early stages (∼0.001 μs), most Ba 2+ ions and Cl – ions nucleated into ion clusters (Figure A–i), owing to the strong Coulomb attractions between cations and anions, and the very low dielectric constant of the monolayer water in the angstrom-scale slit. , A small population of Ba 2+ ions were bonded with three coordinated Cl – ions (e.g., state 1 in Figure ), but most of Ba 2+ ions were surrounded by one or two Cl – ions together with 4–6 water molecules (e.g., states 2 and 3 in Figure ). The formation of such ion clusters (i.e., partially hydrated states) in monolayer water has been reported in previous studies of NaCl, LiCl, CaCl 2 , and CaSO 4 clusters. ,, …”

“…As a result, the interplay between ion hydration and Coulombic attractions in the monolayer water can lead to the spontaneous formation of monolayer hydrated salts for the alkaline earth metal chlorides. By contrast, the alkali metal chlorides in monolayer water tended to form anhydrous monolayer nanostructures owing to the relatively weak ion hydration that cannot offset the strong Coulombic attraction with the CIPs. , …”

“…Next, we noticed that the electronic continuum model (i.e., the scaled charge ionic model) can be effective to describe the effects of charge transfer and electronic dielectric screening in classical MD simulations of ionic aqueous solutions . Therefore, we carried out additional classical MD simulations with using the scaled charge ionic model.…”

“…The formation of such ion clusters (i.e., partially hydrated states) in monolayer water has been reported in previous studies of NaCl, LiCl, CaCl 2 , and CaSO 4 clusters. 11,33,34 Moreover, fully hydrated Ba 2+ ions with eight water molecules (i.e., state 4) were abundant (Figure 2D-i). The competition between the tendency of hydration and the Coulomb attractions between the cation and anion triggered the transition from partial hydration to full hydration for the Ba 2+ ion at ∼1.0 μs.…”

“…In particular, unconventional Na 2 Cl, Na 3 Cl, and CaCl crystals were observed on the graphene surfaces , owing to the strong hydrated cation−π interactions. Using microsecond-scale classical MD simulations and Born–Oppenheimer MD (BOMD) simulations, we recently predicted that the monovalent ions (e.g., NaCl and LiCl) within an angstrom-scale slit could spontaneously aggregate into monolayer salt nanostructures , rather than being in the dissolution state. Monolayer anhydrous salts of alkaline metal chlorides exhibit either square or zigzag patterns …”