Role of hydration effects in driving ion binding to polyelectrolyte brushes and chains: a perspective

Polyelectrolyte (PE) chains and PE brushes are often characterized by the nature of the counterions that bind to them, as such binding regulates different properties of the chains and brushes. However, despite extensive research probing the properties of PE chains and brushes in the presence of a wide variety of counterions, the understanding of what would be the relative strength of binding of a specific type of counterion to a specific type of PE chain/brush remains elusive. In this perspective article, driven by our recent all-atom molecular dynamics (MD) simulations, we propose the following hypothesis that aims to fill this void: more chaotropic (kosmotropic) ions-those that disrupt (preserve) the surrounding water structure-tend to bind more strongly to polyelectrolyte (PE) chains and PE brushes bearing hydrophobic (hydrophilic) functional groups due to solvent mediated interactions. Therefore, our hypothesis accounts for the effect of hydration on ion binding to the PE brushes and chains: ions bind more favorably to PE chains/brushes having functional groups that impart a similar effect towards water. Subsequently, we discuss experimental and ab initio simulation results on counterion binding to PE chains and brushes from a large number of studies and establish the validity of our hypothesis by testing it against the findings of these studies. Finally, we identify the possible applications of our proposed hypothesis (in terms of designing systems that involve PE brushes and chains) and machine learning and density functional theory calculations that can further strengthen our understanding of the PE-counterion binding events.