Hydronium Ions Are Less Excluded from Hydrophobic Polymer-Water Interfaces than Hydroxide Ions

The cloud point temperatures of aqueous poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene) oxide (PEO) solutions were measured from pH 1.0 to pH 13.0 at a constant ionic strength of 100 mM. This ionic strength was reached by mixing the appropriate concentration of NaCl with either HCl or NaOH. The phase transition temperature of both polymers was nearly constant between pH 2.0 and 12.0. However, the introduction of 100 mM HCl (pH 1.0) led to an increase in the cloud point temperature, although this value was still lower than the cloud point temperature in the absence of salt. By contrast, the introduction of 100 mM NaOH (pH 13.0) caused a decrease in the cloud point temperature, both relative to adding 100 mM NaCl and adding no salt. Nuclear magnetic resonance (NMR) studies of these systems were performed below the cloud point temperature, and the chemical shifts closely tracked the corresponding changes in the phase transition temperature. Specifically, the introduction of 100 mM HCl caused the ¹H chemical shift to move downfield for the CH resonances from both PNIPAM and PEO, while 100 mM NaOH caused the same resonances to move upfield. Virtually no change in the chemical shift was seen between pH 2.0 and 12.0. These results are consistent with the idea that a sufficient concentration of H₃O⁺ led to polymer swelling compared to Na⁺, while substituting Cl^- with OH^- reduced swelling. Finally, classical all-atom molecular dynamics (MD) simulations were performed with a monomer and 5-mer corresponding to PNIPAM. The results correlated closely with the thermodynamic and spectroscopic data. The simulation showed that H₃O⁺ ions more readily accumulated around the amide oxygen moiety on PNIPAM compared with Na⁺. On the other hand, OH^- was more excluded from the polymer surface than Cl^-. Taken together, the thermodynamic, spectroscopic, and MD simulation data revealed that H₃O⁺ was less depleted from hydrophobic polymer/water interfaces than any of the monovalent Hofmeister metal cations or even Ca²⁺ and Mg²⁺. As such, it should be placed on the far-right side of the cationic Hofmeister series. On the other hand, OH^- was excluded from the interface and could be positioned in the anionic Hofmeister series between H₂PO₄^- and SO₄^2-.