Abstract
We use molecular dynamics simulations of the primitive model of electrolytes to study the ionic structure in aqueous monovalent electrolyte solutions confined by charged planar interfaces over a wide range of electrolyte concentrations, interfacial separations, surface charge densities, and ion sizes. The investigations are inspired by recent experiments that have directly measured the increase in the decay length for highly concentrated electrolytes with an increase in concentration. The behavior of ions in the nanoconfinement created by the interfaces is probed by evaluating the ionic density profiles, net charge densities, integrated charges, and decay lengths associated with the screening of the charged interface. The results show the presence of two distinct regimes of screening behavior as the concentration is changed from 0.1M to 2.5M for a wide range of electrolyte systems generated by tuning the interfacial separation, surface charge density, and ionic size. For low concentrations, the integrated charge exhibits a monotonic decay to 0 with a decay length that decreases sharply with increasing concentration. For high concentrations (≳1M), the integrated charge has a non-monotonic behavior signaling charge inversion and formation of structured layers of ions near the interfaces. The decay length under these conditions rises with increasing concentration. To complement the simulation results, a variational approach is developed that produces charge densities with characteristics consistent with those observed in simulations. The results demonstrate the relation between the rise in the strength of steric correlations and the changes in the screening behavior.
Highlights
The behavior of electrolyte ions in liquids confined between charged macromolecules regulates many processes in soft materials such as colloids, emulsions, polymeric membranes, and proteins.1,2 The local ionic environment can modulate the effective interaction between the charged macromolecules, changing their assembly behavior.3,4 Many energy storage applications5–9 and separation process technologies10–12 based on these materials rely on the complex organization and transport of ions near macromolecular surfaces
The ionic structure is quantified by evaluating the ion number densities, net charge densities, integrated charges,72,75,77,78 and characteristic decay lengths associated with the screening of the charged interface
Using molecular dynamics simulations of the primitive model of electrolytes, we performed a systematic study of the ionic structure of aqueous monovalent electrolyte solutions confined by two planar interfaces over a wide range of electrolyte concentrations c ∈ (0.1, 2.5)M, interfacial separations h ∈ (5, 8) nm, surface charge densities σs ∈ (−0.005, −0.02) C/m2, and counterion sizes d+ ∈ (0.2–0.63) nm
Summary
The behavior of electrolyte ions in liquids confined between charged macromolecules regulates many processes in soft materials such as colloids, emulsions, polymeric membranes, and proteins. The local ionic environment can modulate the effective interaction between the charged macromolecules, changing their assembly behavior. Many energy storage applications and separation process technologies based on these materials rely on the complex organization and transport of ions near macromolecular surfaces. We use MD simulations to perform a systematic study of the ionic structure of aqueous monovalent electrolyte solutions confined by two planar interfaces over a wide range of concentrations c ∈ (0.1, 2.5)M, interfacial separations h ∈ (5, 8) nm, surface charge densities σs ∈ (−0.005, −0.02) C/m2, and counterion sizes d+ ∈ (0.2–0.63) nm. For high concentrations (c ≳ 1M), the integrated charge exhibits a non-monotonic, oscillatory decay to 0 as the distance from the interface is increased, with a decay length that rises with increasing c These changes in the screening behavior are attributed to the rise in the strength of the steric ion–ion correlations with increasing concentration, which produce dramatic changes in the ionic structure including the enhanced accumulation of counterions and co-ions near the interface and the non-monotonic behavior of the net charge density.
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