Probing the Surface Hydration of Nonfouling Zwitterionic and Poly(ethylene glycol) Materials with Isotopic Dilution Spectroscopy

Understanding the surface hydration of nonfouling materials such as zwitterionic polymers and poly(ethylene glycol) (PEG) aids in the. design of new and,effective nonfouling materials. Sum frequency generation (SFG) vibrational spectroscopy is a powerful technique used to probe water structures-at solid/liquid interface. However, SFG signals of H2O consisting of symmetric and asymmetric stretches and Fermi resonance Overlap heavily, which complicates the interpretation of the spectra,and leads to controversy. In this work, isotopically diluted water was used instead of H2O to study the surface hydration of three zwitterionic polymers and a PEG coating. Because the water signal contains only an O-H stretch, strongly and weakly hydrogen-bonded water structures were easily distinguished: SFG results showed a majority Of Strongly hydrogen-bonded water molecules at the nonfouling polymer surfaces. For comparison, the water spectra at the surfaces of poly(methyl methacrylate) and poly(ethylene terephthalate) suggested significant amounts of weakly hydrogen-bonded water: The effects of pH on the surface hydration of the nonfouling polymers were also investigated. The materials respond to pH differently because of their different structural formulas. The flip of the water molecules at a carboxybetaine polymer surface in response to pH was also observed.