General nature of liquid-liquid transition in aqueous organic solutions.

The presence or absence of a liquid-liquid transition in water is one of the hot topics in liquid science, and while a liquid-liquid transition in water/glycerol mixtures is known, its generality in aqueous solutions has remained elusive. Here we reveal that 14 aqueous solutions of sugar and polyol molecules, which have an ability to form hydrogen bonding with water molecules, exhibit liquid-liquid transitions. We find evidence that both melting of ice and liquid-liquid transitions in all these aqueous solutions are controlled solely by water activity, which is related to the difference in the chemical potential between an aqueous solution and pure water at the same temperature and pressure. Our theory shows that water activity is determined by the degree of local tetrahedral ordering, indicating that both phenomena are driven by structural ordering towards ice-like local structures. This has a significant implication on our understanding of the low-temperature behaviour of water.