Observation of the Structure and Dynamics of Water in Aqueous Alkaline Chloride Solutions under Gigapascal Pressure...

Water in aqueous salt solutions under gigapascal pressure plays a pivotal role in geoscience, protein technology, and planetary science. However, little direct experimental observation of the water structure and dynamics in alkaline chloride solutions at gigapascal pressures is available. Here, we show, using high-pressure neutron diffraction, that applying gigapascal pressure transforms the tetrahedral hydrogen-bonded network structure of water into a dense random packing, due to an increase in the number of interstitial water molecules, regardless of the ion species. Contrarily, dissolved ions disturb the second shell but preserve the tetrahedral-like network structure of water. High-pressure quasielastic neutron scattering experiments reveal that the anomalous trend in water diffusion from Li⁺ to Cs⁺ at ambient pressure disappears at gigapascal pressures, owing to the breakdown of the water tetrahedral network. The present observation of water structure and dynamics will provide insights into the underlying mechanisms in various high-pressure processes.