The dramatic effect of water structure on hydration forces and the electrical double layer Jonathan Hedley 1 , Hélène Berthoumieux 2,3 , Alexei Kornyshev 1,4 1 Department of Chemistry, Imperial College London,UK, 2 Fachbereich Physik, Freie Universität Berlin, Germany, 3 Sorbonne Université, France, 4 Thomas Young Centre for Theory and Simulation of Materials, Imperial College London, UK Overscreening oscillations in water structure play a significant role in water-mediated systems. This is shown in Israelachvili & Pashley’s hydration force experiments 1 alongside recent 3D-AFM experiments 2 . However, these oscillatory force profiles are only observed for atomically flat surfaces. Otherwise, monotonically decaying forces are observed 3 . Whilst oscillations being destroyed by surface roughness is conceptually intuitive, it is not exactly clear how water contributes to the behaviours above. We revisit this hydration force problem with an extended phenomenological Landau-Ginzburg approach describing nonlocal correlations in water, linking them with key features of the wavenumber-dependent dielectric function 4 . This theory predicts the observed oscillatory decay in hydration forces between flat surfaces, but these oscillations disappear with just a tiny surface roughness (corresponding to the size of a water molecule). Importantly, this explanation appears only possible assuming two polarisation modes in water, consistent with the behaviour of the response function. This resolves the “force- oscillation-non-oscillation" paradigm, which is a strong although indirect indication of the existence of these two modes. We also show by considering the same water structural effects that, even in dilute electrolytes, hydrated ions get trapped in potential wells created by the overscreening dielectric response of water to the charged surface. These oscillations are not new findings, having been observed in numerous simulations 5 . However, we establish the precise relation between the water electrostatic potential profile and density profiles of cations and anions - the ion distribution near the polarised interface preferentially follows the potential wells created by ‘resonance’ water- layering. Smearing of the interface results in a familiar Gouy-Chapman-Stern (GCS) picture. This result alone may explain why the GCS model works well for diluted solutions. Finally, we study how interfacial water-layering influences the double-layer capacitance and Parsons-Zobel plot gradients, resolving some recent puzzles 6 . References 1. Israelachvili, J. N., Pashley, R. M., Molecular layering of water at surfaces and origin of repulsive hydration forces. Nature, 306(5940) , 249–250. (1983). 2. Martin-Jimenez, D., Chacon, E., Tarazona, P.& Garcia, R., Atomically resolved three-dimensional structures of electrolyte aqueous solutions near a solid surface. Nature Communications, 7(1) , 1–7. (2016). 3. Pashley, R. M., Hydration forces between mica surfaces in aqueous electrolyte solutions. Journal of Colloid and Interface Science, 80(1) , 153–162. (1981). 4. Hedley, J. G., Berthoumieux, H., Kornyshev, A. A., The dramatic effect of water structure on hydration forces and the electrical double layer. Journal of Physical Chemistry C (in press) , (2023). 5. Kornyshev, A. A., Spohr, E.& Vorotyntsev, M. A., Electrochemical Interfaces: At the Border Line& Schmickler, W., Electrical Double Layers: Theory and Simulations. In Encyclopedia of Electrochemistry (Vol. 1, pp. 133–161). Wiley-VCH. (2002). 6. Ojha, K., Arulmozhi, N., Aranzales, D.& Koper, M. T. M. Double Layer at the Pt(111)–Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy–Chapman Screening. Angewandte Chemie International Edition, 59(2) , 711–715. (2020)
P17
© The Author(s), 2023
Made with FlippingBook Learn more on our blog