Ionic Behavior in Highly Concentrated Aqueous Solutions Nanoconfined between Discretely Charged Silicon Surfaces

TL;DR

This study uses molecular dynamics simulations to explore how ionic behaviors and electric double layer structures in highly concentrated NaCl solutions are affected by nanoconfinement and discretely charged silicon surfaces, revealing charge inversion and layering phenomena.

Contribution

It provides new insights into ionic distributions, charge inversion, and layering effects in nanoconfined, highly concentrated aqueous solutions with discretely charged surfaces.

Findings

Charge inversion depends on surface charge density and concentration.

Ions form layered distributions that can solidify at high concentrations.

Confinement alters ionic hydration and reduces Na+ coordination number.

Abstract

Through molecular dynamics simulations considering thermal vibration of surface atoms, ionic behaviors in concentrated NaCl solutions confined between discretely charged silicon surfaces have been investigated. The electric double layer structure was found sensitive to the density and distribution of surface charges. Due to the surface charge discreteness, slight charge inversion appeared which depended on the surface charge density, bulk concentration and confinement. In the nanoconfined NaCl solutions differently concentrated from 0.2 M to 4.0 M, the locations of accumulation layers for Na+ and Cl- ions kept stable, but their peak values increased. The higher the concentration was, the more obvious charge inversion appeared. In 4.0 M NaCl solution, Na+ and Cl- ions show obvious alternating layered distributions which may be corresponding to the solidification found in experiments. By changing surface separation, the confinement had a large effect on ionic distributions. As both surfaces approached each other, many ions and water molecules were squeezed out of the confined space. Two adjacent layers in ion or water distribution profiles can be forced to closer to each other and merge together. From ionic hydration analysis, the coordination number of Na+ ions in highly-confined space was much lower than that in the bulk.