Microscopic identification of the order parameter governing liquid–liquid transition in a molecular liquid

Abstract

A liquid-liquid transition (LLT) in a single-component substance is an unconventional phase transition from one liquid to another. LLT has recently attracted considerable attention because of its fundamental importance in our understanding of the liquid state. To access the order parameter governing LLT from a microscopic viewpoint, here we follow the structural evolution during the LLT of an organic molecular liquid, triphenyl phosphite (TPP), by timeresolved small-and wide-angle X-ray scattering measurements. We find that locally favored clusters, whose characteristic size is a few nanometers, are spontaneously formed and their number density monotonically increases during LLT. This strongly suggests that the order parameter of LLT is the number density of locally favored structures and of nonconserved nature. We also show that the locally favored structures are distinct from the crystal structure and these two types of orderings compete with each other. Thus, our study not only experimentally identifies the structural order parameter governing LLT, but also may settle a longstanding debate on the nature of the transition in TPP, i.e., whether the transition is LLT or merely microcrystal formation.liquid-liquid transition | order parameter | X-ray scattering | locally favored structure | transition kinetics L iquid-liquid transition (LLT) is an intriguing phenomenon in which a liquid transforms into another one via a first-order transition. This means that there can be more than two liquid states for a single-component substance. Despite its counterintuitive nature, there have recently been many pieces of experimental and numerical evidence for the existence of LLT, for various liquids such as water (1-5), aqueous solutions (6-8), triphenyl phosphite (9-12), l-butanol (13), phosphorus (14), silicon (15, 16), germanium (17), and Y 2 O 3 -Al 2 O 3 (18,19). This suggests that the LLT may be rather universally observed for various types of liquids. However, none of the LLTs reported so far is free from criticisms (20, 21), mainly because these LLTs take place under experimentally difficult conditions [e.g., at high temperature and pressure (14,15,(17)(18)(19)] or in a supercooled state below the melting point (1-3, 5-7, 9, 10), where the transition is inevitably contaminated by microcrystal formation. The latter is not limited to experiments but arises in numerical simulations, often causing many controversies vs. crystallization (26-28)]. For ST2 water, however, this issue has recently been settled by an extensive simulation study by Palmer et al. (4).One of the hottest and long-standing debates is on the nature of the transition found in a molecular liquid, triphenyl phosphite (TPP), by Kivelson and his coworkers (29). The transition is very easy to access experimentally, because it takes place at ambient pressure and at a temperature range between 230 and 210 K and the transformation speed is slow enough to follow the kinetics. Since the finding of this transition (29, 30), many researchers thus have been inte...

Time-Resolved Light Scattering Study on the Kinetics of the Liquid–Liquid Transition in Triphenyl Phosphite

“…Thus, further study is necessary to address this problem. Nevertheless, combining other pieces of experimental evidence, 17,46,47 we conclude that the phenomenon is most reasonably explained by the LLT scenario.…”

“…[31][32][33][34] There is no doubt that the newly formed states formed in TPP and n-butanol are not a homogeneous amorphous state, but contains nano-crystallites. 13,15,47 There is no controversy on this point. Thus the key question is whether nano-crystallites are embedded in the glassy state of a normal liquid (liquid I) or that of another liquid (liquid II).…”

“…Together with other signatures of LLT, thus, we interpreted the phenomena as LLT (see Refs. 16,17,47 for the details). We also mention that further evidence supporting LLT scenario was reported by Wynne and his coworkers, 46 using the technique of fluorescence lifetime imaging.…”

“…Furthermore, we have recently identified that the order parameter of LLT is the number density of locally favored structures, S, whose size is a few nm, by simultaneous time-resolved measurements of small and wide-angle X-ray scattering during LLT of TPP. 47 Although LLT is a counter-intuitive phenomenon, phenomenological mixture models were proposed to explain LLT. [48][49][50] Following these pioneering works, we proposed that LLT can be naturally explained if we assume an extra structural order parameter besides density, and argued that such an extra scalar order parameter is necessary for the general description of a liquid state, in addition to density ρ.…”

The reversibility and first-order nature of liquid–liquid transition in a molecular liquid

“…X-ray diffraction data of the glacial phase formed at a very low temperature show only broad amorphous peaks and no sharp Bragg peaks, indicating the absence of distinct translational order in the glacial phase2426353637. This feature cannot be explained by the plastic-crystal scenario.…”