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The usual liquid state of water that we all know corresponds to liquid water at normal temperatures. However, a new study published in the journal Science shows that water at low temperatures exists in two different liquid states, a low-density liquid at low pressures and a high-density liquid at high pressures; these two liquids have remarkably different properties and differ by 20% in density; the results imply that, under appropriate conditions, water should exist as two immiscible liquids separated by a thin interface similar to the coexistence of oil and water.
Conceptual representation showing a glass containing two water liquids separated by a thin interface; the lower liquid is thicker than the upper liquid and thus shows that it is composed of tightly packed water molecules. Image credit: Jerker Lokrantz / Anders Nilsson.
Theoretical simulations suggest that deeply supercooled water undergoes a transition between high and low density forms, but this transition is difficult to study experimentally because it occurs under conditions where ice crystallization is extremely rapid.
“The possibility that water could exist in two different liquid states was proposed about 30 years ago, based on the results obtained from computer simulations,” said co-author Professor Nicolas Giovambattista, a researcher in the Department of Physics at Brooklyn College. and the Graduate Center. of the City University of New York.
“This counterintuitive hypothesis has been one of the most important questions in the chemistry and physics of water and a controversial scenario since its inception.”
“This is because the experiments that can access the two liquid states in water have been very challenging due to the apparently unavoidable ice formation under the conditions in which the two liquids should exist.”
Professor Giovambattista and colleagues combined X-ray lasers for rapid structure determination with infrared pulses for rapid heating of amorphous ice layers formed at approximately 200 K.
The heating process created high-density liquid water at higher pressures.
As the layer expanded and decompressed, a low-density liquid appeared that grew on time scales ranging from 20 nanoseconds to 3 microseconds.
The dynamics of these processes provide support for a liquid-liquid transition in super-cooled bulk water.
“It remains an open question how the presence of two liquids can affect the behavior of aqueous solutions in general, and in particular, how the two liquids can affect biomolecules in aqueous environments,” said Professor Giovambattista.
“This motivates further studies in the search for potential applications.”
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Kyung Hwan Kim et al. 2020. Experimental observation of liquid-liquid transition in bulk supercooled water under pressure. Science 370 (6519): 978-982; doi: 10.1126 / science.abb9385
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