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Spectroscopic evidence for a gold-coloured metallic water solution

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Abstract

Insulating materials can in principle be made metallic by applying pressure. In the case of pure water, this is estimated1 to require a pressure of 48 megabar, which is beyond current experimental capabilities and may only exist in the interior of large planets or stars2,3,4. Indeed, recent estimates and experiments indicate that water at pressures accessible in the laboratory will at best be superionic with high protonic conductivity5, but not metallic with conductive electrons1. Here we show that a metallic water solution can be prepared by massive doping with electrons upon reacting water with alkali metals. Although analogous metallic solutions of liquid ammonia with high concentrations of solvated electrons have long been known and characterized6,7,8,9, the explosive interaction between alkali metals and water10,11 has so far only permitted the preparation of aqueous solutions with low, submetallic electron concentrations12,13,14. We found that the explosive behaviour of the water–alkali metal reaction can be suppressed by adsorbing water vapour at a low pressure of about 10−4 millibar onto liquid sodium–potassium alloy drops ejected into a vacuum chamber. This set-up leads to the formation of a transient gold-coloured layer of a metallic water solution covering the metal alloy drops. The metallic character of this layer, doped with around 5 × 1021 electrons per cubic centimetre, is confirmed using optical reflection and synchrotron X-ray photoelectron spectroscopies.

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Fig. 1: A pure NaK drop in vacuum and the time evolution of a NaK drop exposed to water vapour.
Fig. 2: Schematic demonstrating the formation of a thin gold-coloured metallic water layer by water vapour adsorption on a NaK drop.
Fig. 3: Spectroscopic signatures of the metallic water solution from optical and X-ray photoelectron spectroscopy.
Fig. 4: Fits to the experimental data employing a free electron gas model.

Data availability

The datasets generated during the current study are available as Source Data or are available from the corresponding author upon reasonable request. Source data are provided with this paper.

Code availability

Data-processing and fitting results can be generated using numerical methods described in Methods and Supplementary Information and developed computer codes that are available from the corresponding author upon reasonable request.

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Acknowledgements

P.J. acknowledges support from the European Regional Development Fund (project ChemBioDrug number CZ.02.1.01/0.0/0.0/16_019/0000729). D.M.N. and C.L. acknowledge support from the Director of the Office of Basic Energy Science, Chemical Sciences Division of the US Department of Energy under contract number DE-AC02-05CH11231. D.M.N., C.L. and P.J. thank the Alexander von Humboldt Foundation for support. S.E.B., T.B. and R.S.M. are supported by the US National Science Foundation (CHE-1665532). P.E.M. acknowledges support from the viewers of his YouTube popular science channel. M.V. acknowledges support from the Charles University in Prague and from the International Max Planck Research School in Dresden. S.T. acknowledges support from JSPS KAKENHI grant number JP18K14178. R.S. acknowledges the German Research Foundation (DFG) for an Emmy-Noether Young-Investigator stipend (DFG, project SE 2253/3-1). F.T. and B.W. acknowledge support from the MaxWater initiative of the Max-Planck-Gesellschaft. All authors thank the staff of the Helmholtz-Zentrum Berlin for their support during the beamtime at BESSY II.

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P.E.M., H.C.S., T.B., B.W., S.E.B. and P.J. designed the experiments. P.E.M., H.C.S., T.B., B.W., H.A., V.K., M.V., F.T., C.L., D.M.N., R.S. and P.J. performed the experiments, and P.E.M., H.C.S., T.B., B.W., S.E.B., R.S.M., C.L., R.S., S.T. and P.J. analysed the obtained data. P.J. wrote the main paper, and H.C.S. and P.J. wrote the Supplementary Information, both with critical feedback from all co-authors. P.E.M. produced Supplementary Video 1.

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Correspondence to Pavel Jungwirth.

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The authors declare no competing interests.

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Peer review information Nature thanks Christoph Salzmann and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Supplementary information

Supplementary Information

This Supplementary Information file contains the following sections: (1) Sample preparation and employment in the experimental setup; (2) Photoelectron spectroscopy measurements; (3) Photoelectron spectra of pure NaK jets versus drops; (4) Photoelectron spectra of water vapour adsorbing on the NaK drop surface; (5) Data-analysis of photoelectron spectra acquired in fixed-mode operation; (6) Data-fitting of photoelectron spectra; (7) Optical reflection spectroscopy; and (8) Supplementary References

Supplementary Video 1

A detailed description of the experimental setup and conduction of the experiments.

Peer Review File

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Mason, P.E., Schewe, H.C., Buttersack, T. et al. Spectroscopic evidence for a gold-coloured metallic water solution. Nature 595, 673–676 (2021). https://doi.org/10.1038/s41586-021-03646-5

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