Combination of Atomistic Simulations and Density Functional Theory Calculations for Probing the Binding of Multi-atomic Counterions to Densely Grafted Polyelectrolyte Brushes

Here we employ a combination of all-atom molecular dynamics simulations and density functional theory (DFT) calculations for capturing the binding of multi-atomic counterions (such as thiocyanate or SCN^- ion and sulphate or SO₄^2- ion) to cationic [poly(2-(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) brushes. While all-atom MD simulations reveal the manner in which the individual atoms of the counterions show variation in their interactions with the tetramethyl ammonium functional group [{N(CH₃)₃}⁺] of the PMETAC brushes, the DFT calculations provide the electron density topology analysis, thereby highlighting the anisotropic (if any) nature (revealed in an electronic scale) of binding of the different atoms of a given counterion with the {N(CH₃)₃}⁺ group. For example, S (N) atom of the SCN^- ion interacts with the {N(CH₃)₃}⁺ group via two (single) points, while such asymmetries are less common for the interaction of the different atoms of the SO₄^2- ion with the {N(CH₃)₃}⁺ group. Overall, this study provides one of the first attempts to capture the ion-PE-brush (or ion-charged-macromolecule) interaction through an unprecedented combination of atomistic simulations and electronic calculations.