Water refuses to behave like a normal liquid—it expands when cooled, becomes less viscous under pressure, and defies simple explanation—and this strange behaviour has puzzled scientists for 236 years. The problem is that water’s oddities may stem from a hidden transition between two distinct liquid forms, one low-density and one high-density. This so-called liquid-liquid transition is thought to occur in a “no man’s land” of extreme cold where water freezes instantly, making it nearly impossible to study directly. The researchers have now found a way around this bottleneck: they have identified simple organic liquids that undergo the same transition above their melting point, where the liquid flows freely and can be cycled repeatedly through the transition. This is fundamental science, not applied engineering. If the team can map the transition in detail using spectroscopic imaging and simulations, they will test a theory that could explain water’s anomalies and reveal whether liquid-liquid transitions are common across many materials. Past fundamental work on water’s structure led to unexpected advances in cryopreservation and desalination. A clear understanding of this transition could eventually guide the design of new liquids with tailored properties for manufacturing, energy storage, or drug delivery—but that lies years ahead.
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The peculiar behaviour of liquid and supercooled water has been baffling science for at least 236 years and is still seen as a major challenge facing chemistry today (Whitesides & Deutch, Nature 469, 21 (2011)). In the 1970s and 1990s, it was suggested that such strange behaviour might be caused by thermodynamic transitions, possibly even a second critical point. This second critical point would terminate a coexistence line between low- and high-density amorphous phases of water. Unfortunately, this second critical point (if it exists) and the associated polyamorphic liquid-liquid transition is difficult to study as it is thought to lie below the homogeneous nucleation temperature in a region known as "no man's land" (Angell, Science 319, 582 (2008)). Recent work, notably by Hajime Tanaka of the University of Tokyo (see, for example, Nature Communications 1, 16 (2010)) has suggested that such a second critical point and the associated liquid-liquid transition might be much more common. In fact, liquid-liquid transitions have now been observed in a range of highly interacting atomic liquids such as phosphorus, gallium, silicon, germanium, and bismuth, and even in supercooled Y2O3-Al2O3. Theoretical considerations (independently by the groups of Hajime Tanaka and Eugene Stanley) suggest that liquid-liquid transitions should be very common even in molecular liquids. However, the only molecular liquid in which such a transition is now well established is triphenyl phosphite, which has been studied by Tanaka. Unfortunately, the effect is only observed when the liquid is deeply supercooled and extremely viscous. This precludes a detailed and definitive study of the phenomenon and has led to considerable controversy. In preliminary work, we have discovered the presence of a liquid-liquid transition in a few simple organic liquids above their melting point where the viscosity is low. This will allow a unique series of comprehensive studies of the liquid-liquid transition involving repeated thermal cycling through the transition. A number of experimental techniques will be used providing access to dynamics from femtosecond to kiloseconds and structure from molecular to macroscopic scales. Spectroscopic imaging techniques will be used to provide insight into molecular interactions and molecular-scale environments and their association with the liquid-liquid transition. This will allow detailed comparison with theoretical models and will give unprecedented insight into the physical origin of these phenomena and allow manipulation and control in future applications. To ensure maximum impact of the experimental work, it is critical to have strong ties with experts in the theory and simulation of LLTs and we have secured the collaboration of H. Eugene Stanley.
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