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Five-molecule water clusters show liquid-like properties

Five-molecule water clusters show liquid-like properties

13 June 2024

With theory-guided rotational spectroscopy measurements, researchers can tell whether a hydrogen chloride molecule in water is dissolved or not.

Water droplets on a leaf.
Droplets of water come in various sizes. But how small can they be before they stop acting like liquids? Credit: Aathavan jaffna, Wikimedia Commons/CC BY-SA 3.0

How big is a drop of water? That’s not an unanswerable question, like “How long is a piece of string?” A single isolated water molecule isn’t a cluster—it’s a gas-phase molecule. How many molecules need to cluster together before they start displaying behaviors, such as the ability to dissolve other substances, that are associated with liquid water?

At the German Electron Synchrotron (DESY), Melanie Schnell and her postdocs Fan Xie and Denis Tikhonov are shedding some light on that question. They’ve focused their investigation on a specific system, the microhydration of hydrogen chloride. In the gas phase, HCl is a covalently bonded molecule. Only when dissolved in water does it split into H+ and Cl ions and take on its identity as hydrochloric acid. And as Schnell and colleagues found, the dissolution can happen in clusters of several water molecules.

Reaching that conclusion was a complicated feat. There’s no way to snap a photograph of a single H2O–HCl cluster to see if the HCl molecule is dissociated or not. It’s not even possible to reliably produce clusters with uniform composition. The DESY researchers produced a jet of clusters, each with a random number of H2O and HCl molecules. To decipher the clusters’ compositions and structures, the researchers combined rapid-fire rotational spectroscopy measurements with a comprehensive theoretical search for all the forms the clusters might take.

Some computed structures, including isomer 1 below, were fully dissociated: The Cl ion (green) was completely separated from its H+ partner, which attached itself to an H2O molecule on the opposite side of the cluster. Others, including isomer 2, were what the researchers called contact ion pairs: The H+ ion was joined to a water molecule but still adjacent to the Cl ion. In others, including isomer 3, the HCl molecule was intact. As the theory-guided experiments revealed, in clusters of four or fewer water molecules, the HCl molecule was intact. With five or more water molecules, the H–Cl bond spontaneously dissociated to form a contact ion pair. The researchers didn’t observe any fully dissociated clusters, but that was because of the unnaturally cold conditions of their jet. Under ambient conditions, such structures should be plentiful.

Isomers of water and hydrogen chloride molecules.
In a cluster of water molecules, a hydrogen chloride molecule can be fully dissociated (left), separated into a contact ion pair (center), or fully intact (right). In four-water clusters, such as the ones shown here, only the intact-HCl structures are observed. But with five or more water molecules, the H–Cl bond can break. Credit: Adapted from F. Xie, D. S. Tikhonov, M. Schnell, Science (2024), doi:10.1126/science.ado7049

Beyond philosophical curiosity, the question of how many water molecules make a drop is relevant to the chemistry of such environments as Earth’s atmosphere and interstellar space, where molecules are typically found in isolation or in small clusters. It’s also relevant to computational chemistry: Simulating molecules takes a lot of computing power, so modelers want to know how many molecules they really need to reproduce liquid water’s properties. Although “five” isn’t a universal answer—different amounts of water are needed under different conditions—it’s a step toward demarcating the boundary between the molecular and condensed-phase worlds. (F. Xie, D. S. Tikhonov, M. Schnell, Science, 2024, doi:10.1126/science.ado7049.)

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