Liquid-liquid transitions (LLTs) between amorphous phases of a single (chemically unchanged) liquid were predicted to occur in most molecular liquids but have only been observed in triphenyl phosphite (TPP) and n-butanol, and even these examples have been dismissed as "aborted crystallization". One of the foremost reasons that LLTs remain so controversial is the lack of an obvious order parameter, that is, a physical parameter characterizing the phase transition. Here, using the technique of fluorescence lifetime imaging, we show for the first time that the LLT in TPP is characterized by a change in polarity linked to changes in molecular ordering associated with crystal polymorphs. We conclude that the LLT in TPP is a phase transition associated with frustrated molecular clusters, explaining the paucity of examples of LLTs seen in nature.

Frustration of crystallisation by locally favoured structures is critically important in linking the phenomena of supercooling, glass formation, and liquid-liquid transitions. Here we show that the putative liquid-liquid transition in n-butanol is in fact caused by geometric frustration associated with an isotropic to rippled lamellar liquid-crystal transition. Liquid-crystal phases are generally regarded as being “in between” the liquid and the crystalline state. In contrast, the liquid-crystal phase in supercooled n-butanol is found to inhibit transformation to the crystal. The observed frustrated phase is a template for similar ordering in other liquids and likely to play an important role in supercooling and liquid-liquid transitions in many other molecular liquids.

Controlled induction of crystal nucleation 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, always lead to irregular and uncontrollable crystal growth. Here, we show that under highly nonequilibrium conditions of spinodal decomposition, water crystals grow as thin wires in a template-less formation of "Haareis". This suggests that such nonequilibrium conditions may be employed more widely as a mechanism for crystal growth control.The physical chemistry of crystal nucleation is of great fundamental and practical importance but is yet poorly understood. It is therefore one of the grand challenges on the border between physics, chemistry, and chemical engineering. Crystal nucleation in melt or solution is typically described by Gibbs's classical nucleation theory based on the competition between the free energy of solidification and the free energy due to the presence of the interface. 1, 2 The latter results in a barrier to crystallization and hence super-saturation and a metastable nonequilibrium state. Thermodynamic fluctuations then lead to pre-nucleation sites, the majority of which will redissolve. 3 Occasionally, a nucleus will grow big enough to overcome the barrier (a critical nucleus) and continue to grow. Only at considerable super-saturation will the energy barrier disappear, at which point homogeneous nucleation will occur.As a result, crystal nucleation is generally a rare process that is difficult to study either experimentally or even through computer simulation. In addition, Ostwald's rule of stages suggests that there are intermediate metastable states critical to the understanding of the path and thermodynamics of nucleation. Such metastable states are typically too rare or short-lived to be observed.However, recent work by Gebauer and others has shown that in some cases (such as the nucleation of carbonates from aqueous solution 4-7 ) solute clusters may form that aggregate into amorphous clusters, which then transform into crystal nuclei. 4, 7-9 Such non-classical nucleation theories do not require a "critical nucleus". These theories appear to, but may not necessarily, 10 be counter to thermodynamic theory. Interestingly, a number of light scattering studies of solutions have shown anomalous clustering in solution suggesting that the effect might be more general. 11,12 In the 1990s, Frenkel introduced the concept of the enhancement of crystal nucleation due to the presence of liquid-liquid critical points. 13 Such a critical point would induce concentration fluctuations that would give rise to droplets of so-called "dense fluid" in which the nucleation probability would be greatly enhanced. 7,[14][15][16] Thus, in this scheme the nucleation mechanism is not changed (it could be classical or non-classical) other than to provide an environment with an increased concentration. Although Frenkel's theory was developed for protein crystallization, it is now widely used in chemical-engineering d...