ja073869r_si_001.pdf (126.4 kB)
Specific Ion Effects on Interfacial Water Structure near Macromolecules
journal contribution
posted on 2007-10-10, 00:00 authored by Xin Chen, Tinglu Yang, Sho Kataoka, Paul S. CremerWe investigated specific ion effects on interfacial water structure next to macromolecules with
vibrational sum frequency spectroscopy (VSFS). Poly-(N-isopropylacrylamide) was adsorbed at the air/water interface for this purpose. It was found that the presence of salt in the subphase could induce the
reorganization of water adjacent to the macromolecule and that the changes depended greatly on the
specific identity and concentration of the salt employed. Ranked by their propensity to orient interfacial
water molecules, sodium salts could be placed in the following order: NaSCN > NaClO4 > NaI > NaNO3
≈ NaBr > NaCl > pure water ≈ NaF ≈ Na2SO4. This ordering is a Hofmeister series. On the other hand,
varying the identity of the cation exhibited virtually no effect. We also showed that the oscillator strength
in the OH stretch region was linearly related to changes in the surface potential caused by anion adsorption.
This fact allowed binding isotherms to be abstracted from the VSFS data. Such results offer direct evidence
that interfacial water structure can be predominantly the consequence of macromolecule−ion interactions.