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

Condensed Matter > Soft Condensed Matter [Submitted on 27 Mar 2026 (v1), last revised 1 Jun 2026 (this version, v2)] Title:Non-Additive Ion Effects on the Coil-Globule Equilibrium of a Generic Polymer in Aqueous Salt Solutions View PDFAbstract: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. Submission history From: Swaminath Bharadwaj [view email][v1] Fri, 27 Mar 2026 16:14:14 UTC (10,335 KB) [v2] Mon, 1 Jun 2026 08:17:46 UTC (19,411 KB) Current browse context: cond-mat.soft Change to browse by: References & Citations Loading... Bibliographic and Citation Tools Bibliographic Explorer (What is the Explorer?) Connected Papers (What is Connected Papers?) Litmaps (What is Litmaps?) scite Smart Citations (What are Smart Citations?) Code, Data and Media Associated with this Article alphaXiv (What is alphaXiv?) CatalyzeX Code Finder for Papers (What is CatalyzeX?) DagsHub (What is DagsHub?) Gotit.pub (What is GotitPub?) Hugging Face (What is Huggingface?) ScienceCast (What is ScienceCast?) Demos Recommenders and Search Tools Influence Flower (What are Influence Flowers?) CORE Recommender (What is CORE?) IArxiv Recommender (What is IArxiv?) arXivLabs: experimental projects with community collaborators arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them. Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs.