Weakly hydrated anions, such as SCN-, I-, and ClO4-, adsorb to interfaces in aqueous solutions. The propensity of these anions to partially shed their hydration shells and bind directly with surfaces underlies their ability to denature proteins, dissolve micelles, and even disorder lipid membranes. Herein, we explore the interactions of weakly hydrated anions with polyethylene oxide and polyacrylamide macromolecules. These thermoresponsive polymers are soluble and extended at cold temperatures, but undergo hydrophobic collapse and aggregation above a lower critical solution temperature. Weakly hydrated anions inhibit this hydrophobic self-assembly, consistent with their tendency to impede protein folding. We model the lower critical solution temperature measurements based on the idea that direct binding hinders hydrophobic collapse and quantify a dissociation constant for anion-polymer interactions. Our binding model is corroborated by nuclear magnetic resonance spectroscopy experiments that probe the accumulation of anions at hydrophobic patches on the surface of the polymer chains. Moreover, we examine the structure of water in the polymer hydration shells using vibrational hydration shell spectroscopy. Collectively, our results demonstrate a correlation between interfacial water structure and anion-polymer affinity, which advances the current understanding of ion-specific chemistry in aqueous solutions.

Ion-specific effects on the hydrophobic collapse of thermoresponsive polymers: Role of interfacial water structure

2019 Fall National Meeting

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