Localized Anionic and Hydrophobic Effects, Multiple Hydration States, and Counterions with Large Mobilities Inside Densely Grafted Cationic Brushes

Understanding the hydration behavior of polyelectrolyte (PE) brushes, resolved across an atomistic scale, can be critical for elucidating the new physics governing the properties and behaviors of these brushes. Here we employ all-atom molecular dynamics (MD) simulations for resolving the localized (of unprecedented atomistic-scale resolution) behavior of the cationic PMETAC [Poly(2-(methacryloyloxy)ethyl trimethylammonium chloride] brushes. Despite the well-known cationic nature of the PMETAC brushes, we identify that around the C=O group of the brushes, water molecules demonstrate dipole orientation that mimics the water behavior around strong anions. Additionally, the {N(CH₃)₃}⁺ group, due to the large steric and phobic effect of three methyl groups, triggers hydrophobic hydration enforcing water molecules to attain clathrate-like structures (as evident by a high value of the water structural order parameter). Such enhanced water ordering around the {N(CH₃)₃}⁺ group prevents a strong attraction between the {N(CH₃)₃}⁺ group and the Cl^- ions, thereby ensuring a very large mobility of the Cl^- ions inside the brush layer. Finally, we employ a combination of data analysis and machine learning algorithm (based on identifying representative clusters) to confirm that the water molecules around the {N(CH₃)₃}⁺ group are in two possible hydration states, namely a weak clathrate-like state and a strong clathrate-like state.