Hydronium Ions\nAre Less Excluded from Hydrophobic\nPolymer–Water Interfaces than Hydroxide Ions

The cloud point temperatures of aqueous poly(<i>N</i>-isopropylacrylamide) (PNIPAM) and poly(ethylene) oxide\n(PEO) solutions\nwere measured from pH 1.0 to pH 13.0 at a constant ionic strength\nof 100 mM. This ionic strength was reached by mixing the appropriate\nconcentration of NaCl with either HCl or NaOH. The phase transition\ntemperature of both polymers was nearly constant between pH 2.0 and\n12.0. However, the introduction of 100 mM HCl (pH 1.0) led to an increase\nin the cloud point temperature, although this value was still lower\nthan the cloud point temperature in the absence of salt. By contrast,\nthe introduction of 100 mM NaOH (pH 13.0) caused a decrease in the\ncloud point temperature, both relative to adding 100 mM NaCl and adding\nno salt. Nuclear magnetic resonance (NMR) studies of these systems\nwere performed below the cloud point temperature, and the chemical\nshifts closely tracked the corresponding changes in the phase transition\ntemperature. Specifically, the introduction of 100 mM HCl caused the <sup>1</sup>H chemical shift to move downfield for the CH resonances from\nboth PNIPAM and PEO, while 100 mM NaOH caused the same resonances\nto move upfield. Virtually no change in the chemical shift was seen\nbetween pH 2.0 and 12.0. These results are consistent with the idea\nthat a sufficient concentration of H<sub>3</sub>O<sup>+</sup> led\nto polymer swelling compared to Na<sup>+</sup>, while substituting\nCl<sup>–</sup> with OH<sup>–</sup> reduced swelling.\nFinally, classical all-atom molecular dynamics (MD) simulations were\nperformed with a monomer and 5-mer corresponding to PNIPAM. The results\ncorrelated closely with the thermodynamic and spectroscopic data.\nThe simulation showed that H<sub>3</sub>O<sup>+</sup> ions more readily\naccumulated around the amide oxygen moiety on PNIPAM compared with\nNa<sup>+</sup>. On the other hand, OH<sup>–</sup> was more\nexcluded from the polymer surface than Cl<sup>–</sup>. Taken\ntogether, the thermodynamic, spectroscopic, and MD simulation data\nrevealed that H<sub>3</sub>O<sup>+</sup> was less depleted from hydrophobic\npolymer/water interfaces than any of the monovalent Hofmeister metal\ncations or even Ca<sup>2+</sup> and Mg<sup>2+</sup>. As such, it should\nbe placed on the far-right side of the cationic Hofmeister series.\nOn the other hand, OH<sup>–</sup> was excluded from the interface\nand could be positioned in the anionic Hofmeister series between H<sub>2</sub>PO<sub>4</sub><sup>–</sup> and SO<sub>4</sub><sup>2–</sup>.