Atomistic investigation on the effect of different counterions and associated bridging mechanism in polyelectrolyte brush.*

The structural behavior of a polyelectrolyte (PE) brush layer is highly sensitive to the valence of the screening counterions. Multivalent screening counterions are known to reduce the brush height and collapse the brush layer. This collapse is hypothesized to be the effect of multivalent ion-induced "bridging". Such multivalent-counterion-induced bridging has been studied using coarse-grained molecular dynamics (MD) simulation that fails to incorporate appropriate atomistic understanding. In this study, we have performed all-atom MD simulations of polyelectrolyte brush grafted systems with different counterions and salt. We observe that the bridging phenomenon, which was previously thought to occur only in presence of multivalent counterions, occurs even in the presence of monovalent counterions. We also discover that the size of a counterion plays a vital role in bridging: larger counterions cause more interchain bridging. In this study, therefore, we have provided an atomistic-level exploration of the physical factors dictating the counterion- induced bridging. Our findings will be helpful for designing PE grafted surfaces for biomedical and sensing applications.