Non-additive ion effects on the coil-globule equilibrium of a generic polymer in aqueous salt solutions

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-Na2SO4 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, Na2SO4, and increasing concentrations of the weakly hydrated salts, NaSCN and NaI. The generic polymer qualitatively reproduces experimentally observed trends in pure NaSCN and Na2SO4 solutions, as well as the non-additive behavior in mixed salt solutions. In particular, the model captures the mutually reinforcing preferential accumulation of weakly hydrated SCN- ions and the depletion of strongly hydrated SO42- ions near the polymer that underlies the non-additive behavior. This mutual enhancement correlates with the partitioning of sodium ions from the counterion cloud of SCN- ions to that of SO42- 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.