Liquid-liquid transitions in molecular liquids: from supramolecular structure to phase separation

"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."

Controlled birth of new crystals is a highly desirable but elusive goal. Attempts to speed up crystallization, such as high super saturation or working near a liquid-liquid critical point, has usually led to uncontrollable nucleation and irregular crystal growth. We discovered that under highly nonequilibrium conditions of spinodal decomposition, water crystals can grow as thin wires without any molecular templates. This suggests that such nonequilibrium conditions may be employed more widely as mechanisms for crystal nucleation and growth control.