Reversible structural transformations in supercooled water from 135 to 245 K

TL;DR

This study reveals that supercooled water undergoes reversible structural transformations between two distinct liquid states across a wide temperature range, providing insights into its anomalous properties and supporting the liquid-liquid coexistence hypothesis.

Contribution

The paper provides direct experimental evidence that supercooled water can be described as a mixture of two interconvertible structures over 135-245 K, with reversible transformations prior to crystallization.

Findings

Water's structure relaxes to a steady state before crystallization.

Steady-state structure is a linear combination of two temperature-independent forms.

Structural changes are reversible across the studied temperature range.

Abstract

Water has many anomalous properties compared to "simple" liquids, and these anomalies are typically enhanced in supercooled water. While numerous models have been proposed, including the liquid-liquid critical point, the singularity-free scenario, and the stability limit conjecture, a molecular-level understanding remains elusive.The main difficulty in determining which, if any, of these models is correct is the limited amount of data in the relevant temperature and pressure ranges. For water at ambient pressures, which is the focus of this work, data is largely missing from 160 - 232 K due to rapid crystallization. Whether rapid crystallization is just an experimental obstacle, or a fundamental problem signaling the inability of water to thermally equilibrate prior to crystallization is also a major unanswered question. Here, we investigate the structural transformations of transiently-heated, supercooled water with nanosecond time resolution using infrared vibrational spectroscopy. The experiments demonstrate three key results. First, water's structure relaxes from its initial configuration to a "steady-state" configuration prior to the onset of crystallization over a wide temperature range. Second, water's steady-state structure can be reproduced by a linear combination of two, temperature-independent structures that correspond to a "high-temperature liquid" and a "low-temperature liquid." Third, the observed structural changes are reversible over the full temperature range. Taken together, these results show that supercooled water can equilibrate prior to crystallization for temperatures from the homogeneous nucleation temperature down to the glass transition temperature. Second, the results provide support for the hypothesis that supercooled water can be described as a mixture of two, structurally-distinct, interconvertible liquids from 135 K to 245 K.