Effect of interfacial water structure on the adsorption of weakly hydrated anions

Weakly hydrated anions tend to partition to macromolecular surfaces, leading to salting-in at low salt concentrations. Recent studies have linked this partitioning to disruption of the interfacial water network. Here, we investigate the role of interfacial-water-network disruption as an indirect driving force for the adsorption of weakly hydrated anions and its interplay with direct solute-anion and anion-cation interactions using computer simulations. We study the adsorption of SCN$^{-}$ and I$^{-}$ ions on a model extended solute that interacts with water and ions only via van der Waals interactions. To probe the indirect contribution, we increase the effective solute size while simultaneously decreasing the attractive solute-water and solute-ion interactions, resulting in progressively greater disorder in the interfacial water network in pure aqueous solutions. Our results show that iodide adsorption increases despite weaker direct solute-iodide interactions and correlates with greater disruption of the interfacial water network, although iodide remains depleted overall. In contrast, preferential adsorption of SCN$^{-}$ decreases, but the rate of decrease becomes smaller at higher levels of interfacial-water-network disorder. This suggests that the indirect contribution associated with interfacial-water-network disruption partly compensates for the decrease in SCN$^{-}$ adsorption caused by weaker direct solute-SCN$^{-}$ interactions. Our results highlight that the indirect contribution associated with interfacial-water-network disruption promotes the adsorption of weakly hydrated anions on macromolecular surfaces, while the extent of adsorption also depends on direct solute-ion interactions and anion-cation pairing. This interplay may be particularly important in mixed-salt solutions, where the presence of strongly hydrated anions can also disrupt the interfacial water network.

Source: arXiv:2610.27964v1 - http://arxiv.org/abs/2610.27964v1 PDF: https://arxiv.org/pdf/2610.27964v1 Original Link: http://arxiv.org/abs/2610.27964v1