Local Hydration Properties of Zwitterionic Functional Groups from Molecular Dynamics Simulations

Polymer membranes enable efficient water purification; however, they face persistent challenges associated with membrane fouling. Zwitterionic materials (ZIs), due to their strong hydration shells, have shown potential in mitigating membrane fouling. Nonetheless, most studies have focused on a limited set of ZI chemistries, leaving the broader effects of ZI molecular structure largely unexplored. Recent experimental work has applied ODNP techniques to investigate local water dynamics around ZI functional groups in synthesized oligo-peptoids. These studies reveal that water dynamics can be sensitive to both the extent of cationic charge delocalization in ZIs and the surrounding salt concentration.



In this study, we provide an understanding of how ZI chemistry influences hydration properties using all-atom molecular dynamics (MD) simulations of (1) small-molecule ZIs and (2) ZI-tethered oligo-peptoids in the aqueous solution. We interpret experimentally measured water dynamics via molecular-level descriptors for dynamical behavior per hydration layer. Furthermore, we correlate water dynamics with equilibrium enthalpic and entropic driving forces, including water structure around the ZI functional group and solvation free energy. Our study spans the parameter space of varying salt concentration, chemistry, and environment of the ZI functional group to establish broadly accessible design principles in membrane surface modification for antifouling applications.