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.

Economy

Understanding the mesoscopic structure of liquids, liquid-mixtures, and solutions is of enormous relevance to chemistry, chemical technology, and the pharmaceutical industry. Preparation of drugs often involves the mixing of multiple liquids and solutions leading phase separation and crystal nucleation. As there are effectively innumerable combinations of liquid mixtures, a fundamental understanding of nanoscale segregation and phase separation is critical. The room temperature ionic liquids used in some of the work proposed here have remarkable and tuneable physicochemical properties, which means that they have distinct performance advantages over conventional solvents. They have been studied extensively because of a number of useful properties such as low volatility (making them a 'green' substitutes for volatile organic solvents), a large electrochemical window, and good conductivity.

Knowledge

The knowledge generated by the proposed work will be communicated through standard routes: papers, conferences, etc. The PI has a good track record of getting his results published in high impact factor journals such as PRL and JACS including a paper in 2009 in the top 1% of all papers in chemistry. Other channels of knowledge transfer include the organisation of conferences. The PI has a track record in this area and plans are in place for meetings in 2011 and 2013.

People

The work proposed here will contribute to the development of career advancing skills amongst the members of the team. The PI has a history of successfully mentoring undergraduate and postgraduate students and postdocs. It is hoped that this good track record can be continued through the appropriate mentoring of PDRAs and PGRSs. Postgraduates are catered for by a number of postgraduate courses in the School of Chemistry and through WestCHEM/ScotCHEM, as well as through the Scottish Universities Physics Alliance (SUPA). We endeavour to provide PDRAs with similar training and career-development support.

Society

It is unlikely that the research proposed here will influence policy decisions or will be of huge interest to the public at large. Nevertheless, it will be attempted to communicate a flavour of the results to the public. The PI has a track record in the area of communicating to a wider audience. He is in charge and the designer of various websites. The PI also contributes to a blog with a relatively broad (but academic) target audience. The plan for the future in this area is to explore video as a medium for dissemination while first steps in this direction have been made with three videos posted on YouTube.