Rotational Motion of Water Molecules Within the Polyelectrolyte Brush Layer Serving as a Soft and Active Nanoconfinement*

It is well known that the rotational dynamics of water is strongly altered in confinement. Polyelectrolyte (PE) brushes, formed by grafting charge-bearing PE molecules in close proximity, serve as an important class of stimuli-responsive nanomaterial. Molecular Dynamics (MD) simulations have revealed that PE brushes act as a form of lateral nanoconfinement and significantly reduce the translational mobility of the trapped water molecules. We perform an all-atom MD study to probe the rotational dynamics of water molecules inside densely grafted PE brushes. PE brushes represent a special case of soft and active nanoconfinement, where the source of confinement itself forms hydrogen bonds with the water molecules and affects their rotational dynamics. We observe that the rotational diffusivity of water reduces monotonically in all 3 principal directions with increasing grafting density of the brushes (or increasing ionic concentration within the PE layer). This trend is at odds with conventional “water-in-salt” solutions (highly concentrated aqueous electrolytes) and results from the unique topological constraints imposed by the densely grafted PE chains.