Non-Additive Ion Effects on the Coil-Globule Equilibrium of a Generic Polymer in Aqueous Salt Solutions

Abstract

Mixtures of weakly and strongly hydrated anions induce non-additive changes in the LCST of thermoresponsive polymers such as PNIPAM and PEO. Large-scale atomistic simulations of PNIPAM--NaI--Na$_{2}$SO$_{4}$ mixtures have shown that these effects arise from the interplay between favorable PNIPAM--iodide interactions and 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--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. Simulations are performed at fixed concentrations of the strongly hydrated salt, Na$_{2}$SO$_{4}$, and increasing concentrations of the weakly hydrated salts, NaSCN and NaI. The generic polymer qualitatively reproduces experimentally observed trends in pure NaSCN and Na$_{2}$SO$_{4}$ solutions, as well as the non-additive behavior in mixed salt solutions. In particular, the model captures the mutually reinforcing preferential accumulation of the weakly hydrated SCN$^{-}$ ions and depletion of the strongly hydrated SO$_{4}^{2-}$ ions near the polymer that underlies the non-additive behavior. This mutual enhancement correlates with partitioning of sodium ions from the counterion cloud of SCN$^{-}$ ions to that of SO$_{4}^{2-}$ ions and is consistent with atomistic simulations of PNIPAM solutions. The model also reproduces the effects of background salt concentration and weakly hydrated anion identity on the non-additive behavior. These results demonstrate that non-specific polymer--ion and polymer-water interactions are sufficient to reproduce non-additive salt effects, suggesting a dominant role of bulk ion--ion and ion--water interactions.