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

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.

Figures (12)

• Figure 1: Dependence of the (a) relative polymer collapse free energy, $\Delta\Delta G^{\rm C \to G}$ =$\Delta G^{\rm C \to G}(c_{\rm NaSCN})$ - $\Delta G^{C \to G}(c_{\rm NaSCN}=0)$, and (b) polymer-SCN$^{-}$ and polymer-water KBIs on $c_{\rm NaSCN}$ in pure sodium thiocyanate solutions.
• Figure 2: Dependence of the (a) relative polymer collapse free energy, $\Delta\Delta G^{\rm C \to G}$ =$\Delta G^{\rm C \to G}(c_{\rm Na_{2}SO_{4}})$ - $\Delta G^{C \to G}(c_{\rm Na_{2}SO_{4}}=0)$, and (b) polymer-SO$_{4}^{2-}$ and polymer-water KBIs on $c_{\rm Na_{2}SO_{4}}$ in pure sodium sulfate solutions.
• Figure 3: Dependence of (a) the relative polymer collapse free energy, $\Delta\Delta G^{\rm C \to G}$, and the KBIs for (b) polymer–SCN$^{-}$, (c) polymer–water, and (d) polymer–sulfate on $c_{\rm NaSCN}$ at different background salt concentrations.
• Figure 4: Dependence of (a) the relative polymer collapse free energy, $\Delta\Delta G^{\rm C \to G}$, and (b) the polymer–SCN$^{-}$ and polymer–I$^{-}$ KBIs on $c_{\rm NaSCN}$ and $c_{\rm NaI}$, respectively, at a background salt concentration of 0.5 m.
• Figure S1: (a) Normalized PMF profiles $w(R_g)$ for a mixed salt solution of NaSCN (0.1 M) with $\mathrm{Na_2SO_4}$ (1 M). (b) Normalized PMF profiles $w(R_g)$ for a mixed salt solution of NaSCN (0.1 M) with $\mathrm{Na_2SO_4}$ (1 M) used for calculating $\Delta G^{C\rightarrow G}$.