Densely Grafted Polyelectrolyte Brushes Trigger “Water-in-Salt”-like Scenarios and Ultraconfinement Effect

Abstract

Summary This paper presents an all-atom molecular dynamics (MD) simulation of highly charged and densely grafted polyelectrolyte (PE) brushes on a flat surface. Simulation findings for the variation of brush height with polymer size and grafting density are explained by the scaling laws for brushes in the non-linear osmotic regime. More importantly, this study establishes the triggering of an ultraconfinement effect by the densely grafted brushes. This effect leads to significant changes in the distribution, structure, and properties as well as a massive mobility reduction of both the counterions and water. Furthermore, the interplay of the ultraconfinement effect and the large counterion-PE electrostatic interactions trigger a “water-in-salt”-like scenario (witnessed in highly concentrated aqueous electrolyte solutions), which is characterized by the counterion-PE-functional group serving as the “salt” with the “salt” overwhelming the water in both mass and volume and affecting the solvation structure of the counterions by replacing the solvation water with the PE-functional group.