Non-additive Ion Effects on the Coil-Globule Equilibrium of a Generic Uncharged Polymer

TL;DR

This study shows that non-specific polymer-ion interactions can explain non-additive ion effects on polymer coil-globule transitions, matching experimental trends without requiring specific chemical interactions.

Contribution

It demonstrates that generic polymer-ion interaction models can reproduce complex ion effects observed in experiments, emphasizing the importance of bulk ion interactions.

Findings

Generic polymer model reproduces experimental ion effects.

Non-additive behavior arises from ion depletion and accumulation.

Weaker polymer-iodide interactions enhance non-additive effects.

Abstract

Mixtures of weakly and strongly hydrated anions induce non-additive changes in the LCST of thermoresponsive polymers such as Poly(N-isopropylacrylamide) (PNIPAM) and PEO. Large-scale atomistic simulations of PNIPAM-NaI-Na$_{2}$SO$_{4}$ mixtures show that these effects arise from the interplay between favorable PNIPAM-iodide interactions and the depletion of strongly hydrated sulfate ions. Here, we investigate whether chemically specific polymer-anion interactions are necessary to reproduce such behavior. To this end, we study the coil-to-globule transition of a generic uncharged linear polymer with non-specific polymer-water and polymer-ion van der Waals interactions in atomistic aqueous solutions of single and mixed salts. We perform simulations at fixed concentrations of the strongly hydrated salt, Na$_{2}$SO$_{4}$, and increasing concentrations of weakly hydrated salts, NaSCN and NaI. The generic polymer qualitatively reproduces experimental trends in both pure NaSCN and Na$_{2}$SO$_{4}$ solutions, as well as in mixed salt solutions. The model captures the mutual reinforcement between SCN$^{-}$ accumulation near the polymer and SO$_{4}^{2-}$ depletion that gives rise to non-additive behavior, consistent with atomistic simulations in PNIPAM solutions. These features become more pronounced with increasing background salt concentration and are further enhanced upon replacing SCN$^{-}$ with I$^{-}$, owing to weaker polymer-iodide interactions. Our results demonstrate that non-specific polymer-ion interactions are sufficient to reproduce non-additive features, highlighting the dominant role of bulk ion-ion and ion-water interactions.