Relationships between Water’s Structure and Solute Affinity at Polypeptoid Brush Surfaces

Abstract

Excellent antifouling surfaces are generally thought to create a tightly bound layer of water that resists solute adsorption, and highly hydrophilic surfaces such as those with zwitterionic functionalities are of significant current interest as antifoulant strategies. However, despite significant proofs-ofconcept, we still lack a fundamental understanding of how the nanoscopic structure of this hydration layer translates to reduced fouling, how surface chemistry can be tuned to achieve antifouling through hydration water, and why, in particular, zwitterionic surfaces seem so promising. Here, we use molecular dynamics simulations and free energy calculations to investigate the molecular relationships among surface chemistry, hydration water structure, and surface−solute affinity across a variety of surfacedecorated chemistries. Specifically, we consider polypeptoid-decorated surfaces that display well-known experimental antifouling capabilities and that can be synthesized sequence specifically, with precise backbone positioning of, e.g., charged groups. Through simulations, we calculate the affinities of a range of small solutes to polypeptoid brush surfaces of varied side-chain chemistries. We then demonstrate that measures of the structure of surface hydration water in response to a particular surface chemistry signal solute−surface affinity; specifically, we find that zwitterionic chemistries produce solute−surface repulsion through highly coordinated hydration water while suppressing tetrahedral structuring around the solute, in contrast to uncharged surfaces that show solute−surface affinity. Based on the relationship of this structural perturbation to the affinity of small-molecule solutes, we propose a molecular mechanism by which zwitterionic surface chemistries enhance solute repulsion, with broader implications for the design of antifouling surfaces.

Salt-Screened Transition toward Bulk-Like Water Dynamics near Polymeric Zwitterions

“…A growing body of literature attempts to understand the hydration behaviors of zwitterions but primarily probes only the structure of the closely bound surface water. Molecular dynamics simulations of zwitterions offer insights into the role of molecular design on water coordination number, water orientation, solvation correlation times, and hydrogen bonding character. − Experimental sum frequency generation (SFG) vibrational spectroscopy experiments − support these simulations, where enhanced SFG signal from zwitterionic surfaces indicates a net increase in the number of oriented water molecules at the interface, suggesting stronger hydration. Differential scanning calorimetry and low-field nuclear magnetic resonance further reveal that these surface waters are more tightly bound to zwitterionic polymers than uncharged hydrophilic polymers.…”

“…This may therefore suggest differences in hydration structure around the zwitterionic versus ether side chains which facilitate water diffusion. Indeed, recent molecular dynamics simulations provide additional nuances to zwitterion-water interactions, indicating an increase in icosahedral hydration structure relative to bulk water that is counter to changes in local water structure around uncharged, polar solutes . Such increases in icosahedral structure (and reduction in tetrahedral water structure) are linked to increased water diffusivity caused by disruption of the hydrogen bond network relative to the uncharged functional groups .…”