publications

2026

1. Speciation and hydration forces in sodium carbonate/bicarbonate aqueous solutions nanoconfined between mica sheets

   Daria Turculet, Shurui Miao, Kieran Agg, and 1 more author

   Faraday Discuss., 2026

   Abs

   The equilibrium between hydrated and hydrolysed forms of CO2 in water is central to a multitude of processes in geology, oceanography and biology. Chemistry of the carbonate system is well...

2025

1. Specific ion effects enhance local structure in zwitterionic osmolyte solutions

   Kieran J. Agg, Timothy S. Groves, Shurui Miao, and 10 more authors

   Chemical Science, Apr 2025

   Abs DOI

   Zwitterionic osmolytes are widely known to have a protein-protective effect against high salt concentration, but a mechanistic picture of osmolyte function remains elusive. Here total scattering is used to determine the room temperature liquid structure of two model cytosol solutions containing trimethylglycine (TMG) with either sodium or potassium chloride. H/D isotopic substitution is used to obtain differential neutron scattering cross sections at multiple contrasts in addition to an X-ray structure factor, and an Empirical Potential Structure Refinement (EPSR) simulation is fitted to the experimental data. We reveal the nature of the interaction between TMG molecules and ions in solution, observing binding between cations and the TMG carboxylate group. We observe three key specific ion effects: first, that sodium ions are more tightly localised at the carboxylate group; second, that sodium localisation in turn promotes head-to-head bridging between carboxylate groups when compared to potassium or no added ions, resulting in strong oxygen–oxygen correlations; and third, that sodium ions promote TMG clusters with greater orientational order, more fully shielding the ion but also in turn limiting access to the carboxylate groups for other molecules. These observations have implications for the bioavailability and protein-stabilising effect of osmolytes under changing extracellular salt conditions.

2. The influence of proline on surface interactions in aqueous solutions

   Kieran J. Agg, James E. Hallett, and Susan Perkin

   Biophysical Journal, Oct 2025

   Abs DOI

   The amino acid proline is accumulated in a variety of plant species in response to environmental stresses, such as high salinity and extreme temperatures. Although the colligative role of proline as an osmoprotectant is well known, its influence on molecular interactions within the cell has received less attention. Here, we investigate the effects of proline on interaction free energies in aqueous environments, and we find that the presence of proline significantly enhances the repulsive force between charged surfaces relative to pure water. At elevated concentrations, proline alters the short-range, structural interaction, forming layers at the surfaces. In the presence of proline and salt, the near-surface hydration structure is disrupted compared to salt solutions without proline. Overall, we observe that the far-field component of the interaction is relatively insensitive to proline concentration above a low threshold, and the results show that proline contributes to maintaining repulsive colloidal interactions while allowing for tuning of osmotic pressure over a wide spectrum of osmolarity.

3. Lithium solvation and anion-dominated domain structure in water-in-salt electrolytes

   Timothy S. Groves, Kieran J. Agg, Shurui Miao, and 5 more authors

   EES Batteries, Dec 2025

   Abs DOI

   Water-in-Salt (WiS) electrolytes are an emerging class of high concentration aqueous electrolytes with large electrochemical stability windows, making them attractive as green alternatives in next-generation electrochemical energy storage devices. Recent work has highlighted the existence of water-rich and anion-rich domains in WiS electrolytes, but the extent, morphology and importance of these domains are still disputed. Here, we present neutron total scattering measurements of the archetypal WiS, lithium bis(trifluoromethanesulfonyl)imide, and use empirical potential structure refinement to match the structure of a simulated system to the experimental data for two technologically relevant concentrations, revealing ion solvation, geometric isomerism and long-range structures in unprecedented detail. Our analysis of the modelled WiS electrolyte suggests that water domains are small and isolated and points to a system dominated by percolating, anion-rich domains that assemble through the association of hydrophobic regions, extending throughout the entire system. This structural insight places restrictions on feasible transport mechanisms in WiSs and, more generally, will aid in the understanding of the structure and behaviour of WiS electrolytes, with implications for the design and manufacture of WiS-containing devices.

2023

1. Zwitterions fine-tune interactions in electrolyte solutions

   James E. Hallett, Kieran J. Agg, and Susan Perkin

   Proceedings of the National Academy of Sciences of the United States of America, Feb 2023

   Abs DOI

   Cellular organisms regulate electrolyte composition in the cytosol to optimize intracellular molecular interactions at the same time as balancing external osmotic pressure. While osmotic pressure can be tuned using multiple ionic, zwitterionic, and nonionic solutes, interactions between proteins and other macromolecules are sensitive to the precise composition of the medium. Nonetheless, the roles of individual ions and nonionic solutes in mediating cellular interactions remain relatively unexplored, and standard buffer solutions used in laboratory studies often contain only a few simple salts. Here, we report on model experiments investigating the combined effect of ionic and zwitterionic solutes on interaction forces across electrolytes, revealing a clear role for zwitterions in modifying interactions compared to simple salt solutions. First, we find that zwitterions act to disrupt water layering at interfaces, leading to smoothed interaction potentials. Second, we find that zwitterions strengthen electrostatic repulsions by enhancing effective surface charge. Third, zwitterions enhance the effective dielectric permittivity of the solution, and this “dielectricizer” effect extends the range of electrostatic repulsions compared to solutions without zwitterion present. The latter two effects are likely important in stabilizing proteins and other macromolecules when external osmotic and mechanical pressure are very high and simple ionic solutes alone would lead to collapse.