Weakly hydrated anions help to solubilize hydrophobic macromolecules in aqueous solutions, but small molecules comprising the same chemical constituents precipitate out when exposed to these ions. Here, this apparent contradiction is resolved by systematically investigating the interactions of NaSCN with polyethylene oxide oligomers and polymers of varying molecular weight. A combination of spectroscopic and computational results reveals that SCN− accumulates near the surface of polymers, but is excluded from monomers. This occurs because SCN− preferentially binds to the centre of macromolecular chains, where the local water hydrogen-bonding network is disrupted. These findings suggest a link between ion-specific effects and theories addressing how hydrophobic hydration is modulated by the size and shape of a hydrophobic entity.
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The datasets generated during and/or analysed during the current study are available via the following DOIs: Fig. 2, https://doi.org/10.5281/zenodo.5123016; Fig. 3, https://doi.org/10.5281/zenodo.5123100; Fig. 4, https://doi.org/10.5281/zenodo.5123104. Source data are provided with this paper. Source data for the Supplementary figures are available at https://doi.org/10.5281/zenodo.5123295.
The codes and algorithms generated during the current study are available from the corresponding author on reasonable request.
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We thank C. Chen, Z. Tian and H. Allcock for poly-N,N-diethylacrylamide (PDEA) synthesis (The Pennsylvania State University), T. Mal and C. Pacheco for NMR assistance, as well as D. Ben-Amotz and W. Noid for insightful discussions. P.S.C. thanks the National Science Foundation (CHE-2004050) for funding support. J.H. acknowledges support from the Czech Science Foundation (grant no. 20–24155 S) and the Ministry of Education, Youth and Sports of the Czech Republic through the e-INFRA CZ (ID: 90140).
The authors declare no competing interests.
Peer review information Nature Chemistry thanks Amish Patel and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Salt-induced chemical shifts, free energies of adsorption and preferential interaction coefficients as a function of position along the PEO chains.
Hydration shell spectra for the PEO monomer and polymer, chain length dependence of the OH stretch areas, probability distributions of the tetrahedral order parameter and probability of observing a disordered tetrahedral water structure as a function of position along the PEO chains.
Correlations of the experimental and computational parameters for ion binding and water structure.
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Rogers, B.A., Okur, H.I., Yan, C. et al. Weakly hydrated anions bind to polymers but not monomers in aqueous solutions. Nat. Chem. 14, 40–45 (2022). https://doi.org/10.1038/s41557-021-00805-z
Nature Chemistry (2022)
Journal of Solution Chemistry (2021)