Specific ion effects are collective, long-range phenomena

Established in 1888, the Hofmeister series classification describes the specific effects of ions within aqueous solutions and plays a central role in chemistry and biology. Different ions can either stabilize or destabilize biomolecules such as proteins, leading to “salting-in” or “salting-out” behavior. With proteins, ions influence their structural shapes, and in turn, their biological function. But the microscopic origin of these specific ion effects remains unresolved.

Most previous studies have focused either on local ion-solute interactions or on properties of bulk, liquid water. But it is still unclear how ions influence the collective organization of water and solutes over longer distances.

Sappl et al. used individual amino acids — the building blocks of proteins — in solution as well-defined model systems. Amino acids retain key chemical functionalities while avoiding the structural complexity of larger biomolecules, which allowed the researchers to isolate fundamental mechanisms of ion-water-solute interactions.

“We address the research gap by combining broadband dielectric spectroscopy with molecular dynamics simulations to probe how ions modulate long-range dipolar correlations in aqueous amino acid solutions,” said author Vasileios Balos. “The key advance is that we can disentangle and quantify contributions from water, amino acids, and their cross-correlations, allowing us to directly connect macroscopic dielectric response to microscopic structure.”

The study revealed the dielectric response of the aqueous amino acid solutions is governed primarily by ion-induced changes in long-range orientational correlations, rather than by local structural changes alone. In short, specific ion effects are fundamentally collective phenomena.

“Overall, our work bridges the gap between microscopic structure and macroscopic dielectric response, providing a route toward a more unified understanding of ion-specific effects in aqueous systems,” said Balos.

Source: “Specific ion effects on long-range correlations of amino acids in aqueous solution from experimental and computational dielectric spectra,” by Marion Sappl, Christian Fellinger, Amala Elizabeth, Dimitra Kanta, Leon Prädel, Johannes Hunger, Christian Schröder, and Vasileios Balos, Journal of Chemical Physics (2026). The article can be accessed at https://doi.org/10.1063/5.0319199 .

This paper is part of the Computational Spectroscopy Collection, learn more here .