Coulomb explosion during the early stages of the reaction of alkali metals with water

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Abstract

Alkali metals can react explosively with water and it is textbook knowledge that this vigorous behaviour results from heat release, steam formation and ignition of the hydrogen gas that is produced. Here we suggest that the initial process enabling the alkali metal explosion in water is, however, of a completely different nature. High-speed camera imaging of liquid drops of a sodium/potassium alloy in water reveals submillisecond formation of metal spikes that protrude from the surface of the drop. Molecular dynamics simulations demonstrate that on immersion in water there is an almost immediate release of electrons from the metal surface. The system thus quickly reaches the Rayleigh instability limit, which leads to a ‘coulomb explosion’ of the alkali metal drop. Consequently, a new metal surface in contact with water is formed, which explains why the reaction does not become self-quenched by its products, but can rather lead to explosive behaviour.

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Figure 1: High-speed camera images of a Na/K alloy drop versus a water drop impacting on water.
Figure 2: High-speed camera images of a Na/K alloy drop impacting on liquid ammonia.
Figure 3: AIMD simulations of the time evolution of the radius of gyration (green) and the average charge on a sodium atom (blue) of a medium-sized sodium cluster in water.
Figure 4: Time evolution of the radius of gyration of a large sodium cluster surrounded by water for different values of the initial separation of the positive and negative charges.

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Acknowledgements

We thank J. Jiráček for boldly making his chemical laboratory available to us for the initial experiments in liquid ammonia. P.J. acknowledges the Czech Science Foundation (Grant P208/12/G016) for support and thanks the Academy of Sciences for the Praemium Academiae award. S.B. acknowledges support from the Deutsche Forschungsgemeinschaft (Grants BA 2176/3–2 and BA 2176/4–1). P.E.M. acknowledges support from the viewers of his YouTube popular science channel.

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P.E.M., S.B. and P.J. designed and analysed the experiments. P.E.M., V.V., T.B. and S.B. performed the experiments. F.U. and P.J. designed and analysed the simulations and F.U. executed the calculations. P.J. wrote the paper with critical feedback from all the co-authors.

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Correspondence to Pavel Jungwirth.

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The authors declare no competing financial interests.

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Mason, P., Uhlig, F., Vaněk, V. et al. Coulomb explosion during the early stages of the reaction of alkali metals with water. Nature Chem 7, 250–254 (2015). https://doi.org/10.1038/nchem.2161

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