Part 1: Nonaqueous electrolytes in two-dimensions
Electrolytes are essential for our daily lives. They regulate our daily functions (e.g., heart beat), allow batteries to function and are used in water treatment.¹ However, the ion structure at a interfaces is still not fully understood. This is important in both 2D interfaces,² and 1D electrolytes, such as those occurring in ion channels,³ which allow (or exclude) ion transport, and desalination of a solvent through a membrane.⁴ Given their broad importance and the predominance of aqueous data, it seems imperative to understand these phenomena more generally, allowing for greater utility and adaptability for in broader purposes. Here, ion solvation in two-dimensions has been investigated, in both aqueous and nonaqueous electrolytes.

Part 2: Modelling the thermodiffusion of lone cations and anions in dilute aqueous solutions
The thermodiffusion of ions in liquids is difficult to predict due to an incomplete understanding of its microscopic origins. It is widely hypothesised that thermodiffusion is driven by changes in solvation and ionic shielding entropy, creating an entropic force.^5,6 Duhr and Braun ⁶ suggest that large negative ionic shielding entropies account for thermophobicity at high temperatures, whilst large positive hydration entropies explain thermophilicity at low temperatures. In the case of aqueous solutions, structural changes in the hydrogen bond network are also thought to contribute to the direction of thermodiffusion.⁵ It is argued that structure formation at low temperatures minimises the free energy, whilst structure breaking due to larger translational entropic gains minimises the free energy at high temperatures.⁷ This free energy change determines the probability of particle diffusion under equilibrium thermodynamics. This study uses alchemical free energy perturbation (FEP) methods to isolate the ion hydration free energy in silico, for individual cations and anions. This will indicate if thermodiffusive behaviour is solely a property of bulk solution. It appears the nature of ion thermophobicity is dependent on the identity of the ion (i.e., ion-specific), correlating with the ion’s radial charge density and size. ⁸

<sub>References
1. Gregory, K. P. et al. Understanding specific ion effects and the Hofmeister series. Phys. Chem. Chem. Phys. 24, 12682–12718 (2022).
2. Costa, M. C. F. et al. 2D Electrolytes: Theory, Modeling, Synthesis, and Characterization. Adv. Mater. 33, 2100442 (2021).
3. Doyle, D. A. et al. The Structure of the Potassium Channel: Molecular Basis of K+ Conduction and Selectivity. Science (80). 280, 69–77 (1998).
4. Corry, B. Designing Carbon Nanotube Membranes for Efficient Water Desalination. J. Phys. Chem. B 112, 1427–1434 (2008).
5. Niether, D. et al. Role of Hydrogen Bonding of Cyclodextrin–Drug Complexes Probed by Thermodiffusion. Langmuir 33, 8483–8492 (2017).
6. Duhr, S. & Braun, D. Why molecules move along a temperature gradient. Proc. Natl. Acad. Sci. 103, 19678–19682 (2006).
7. Wang, Z., Kriegs, H. & Wiegand, S. Thermal Diffusion of Nucleotides. J. Phys. Chem. B 116, 7463–7469 (2012).
8. Gregory, K. P., Wanless, E. J., Webber, G. B., Craig, V. S. J. & Page, A. J. The electrostatic origins of specific ion effects: quantifying the Hofmeister series for anions. Chem. Sci. 12, 15007–15015 (2021).</sub>