Single-component Water-like Substance Has Two Liquid States

Findings bring closure to long-running debate

liquid states
Thermal behavior of two liquids The black curve represents the behavior of a sample just after cooling (without annealing, liquid 1), and the blue curve represents a sample after being kept (annealed) at -57C (216 K) for 600 min (liquid 2). The yellow dashed curve illustrates the behavior of a sample upon heating after being recooling from a point Trc. © 2016 Hajime Tanaka.

Researchers at the University of Tokyo have found solid experimental evidence corroborating the presence of liquid-liquid transition in triphenyl phosphite, in which the molecular liquid switches back and forth between two liquid states composed of different local structures.

Liquids, like gases, are commonly thought as having a completely random and disordered structure. This implies that there is just one liquid state for a single-component material. However, scientists recently found a second noncrystalline (amorphous) state in water and other molecular liquids, giving rise to a lively discussion among experts on whether or not they had discovered a new liquid state; but such a new amorphous phase in molecular liquids is only formed in a supercooled state below the melting point of crystal, which also produces nanometer-size crystallites. Some scientists claim the new amorphous phase is actually a nanocrystalline state, and so far there has not been any decisive evidence to refute this nanocrystal scenario.

The research group of Professor Hajime Tanaka and Project Assistant Professor Mika Kobayashi at the University of Tokyo Institute of Industrial Science successfully prevented nanocrystal from forming using an ultrafast cooling and heating technique four orders of magnitude faster than the conventional method, and found strong evidence showing that a molecular liquid, triphenyl phosphite, undergoes liquid-liquid transition between two liquid states; the researchers also discovered that this transition—unlike a transformation that cannot be turned back into a previous phase, such as, say, an egg that cannot be returned to its raw state after it has been hard boiled—is reversible, in which the substance can switch back and forth between the two liquid states.

The current finding clearly shows not only that a liquid-liquid transition in molecular liquids actually exists, but that it is also reversible. This experimental method may also be applied to confirm the presence of a liquid-liquid transition in other materials.

“Whether or not a single-component system can have more than two liquid states is both an important and fundamental question for understanding the nature of liquids; but the problem has remained the topic of much heated discussion, which has kept scientists divided on where they stand,” says Tanaka. He continues, “By focusing on the glass-transition behavior unique to liquids, our study revealed that the amorphous phase we found is indeed a second liquid state. We hope that this finding settles the long-running debate and ushers in a new phase to the study of liquid-liquid transitions.”