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

Polyelectrolyte (PE) brushes find use in a plethora of applications like current rectification, ion sensing, biosensing, nanoscale energy conversion etc. We carry out all-atom Molecular Dynamics (MD) simulations of fully ionized PE brushes for various degrees of polymerization and grafting densities. Our brush height results show an excellent match with the existing scaling laws of the non-linear osmotic brush regime. We observe a most remarkable ultraconfinement effect created by the brushes, quantified by orders of magnitude reduction in the mean squared displacement (MSD) of counterions and water molecules. Moreover, we observe that within the PE brushes, the counterion – PE brush complex supersedes the water molecules by both mass and volume above a critical grafting density. This gives rise to a hitherto unknown “water-in-salt” like behavior inside the brushes, with the counterions acting as the cations and the PE brush repeating units acting as the anions. Our calculations reveal a significant lowering of the dielectric constant of water within the brushes due to strong electrostatic binding with the negatively charged PE’s.