The violation of the Stokes–Einstein relation in supercooled water

Abstract

By confining water in nanopores, so narrow that the liquid cannot freeze, it is possible to explore its properties well below its homogeneous nucleation temperature T H Ϸ 235 K. In particular, the dynamical parameters of water can be measured down to 180 K, approaching the suggested glass transition temperature T g Ϸ 165 K. Here we present experimental evidence, obtained from Nuclear Magnetic Resonance and Quasi-Elastic Neutron Scattering spectroscopies, of a well defined decoupling of transport properties (the self-diffusion coefficient and the average translational relaxation time), which implies the breakdown of the Stokes-Einstein relation. We further show that such a non-monotonic decoupling reflects the characteristics of the recently observed dynamic crossover, at Ϸ225 K, between the two dynamical behaviors known as fragile and strong, which is a consequence of a change in the hydrogen bond structure of liquid water.decoupling of transport properties ͉ dynamic crossover ͉ MCM-41 D espite its fundamental importance in science and technology, the physical properties of water are far from completely understood. The liquid state of water is unusual, especially at low temperatures (1-3). For example, contrary to other liquids, water behaves as if there exists a singular temperature toward which its thermodynamical properties, such as compressibility, thermal expansion coefficient, and specific heat, diverge (1). The efforts of scientists from many disciplines to seek a coherent explanation for this unusual behavior, in combination with its wide range of impacts, make water one of the most important open questions in science today. On the other hand, the nature of the glass transition (GT) of water represents another challenging subject for current research (4). Dynamical measurements in glassforming liquids have shown a dramatic slowdown of both macroscopic (viscosity and self-diffusion coefficient D) and microscopic (average translational correlation time ) observables, as temperature is lowered toward the GT temperature T g . Accordingly, a comprehension of the GT has been sought through the study of the dynamics at the molecular level, which, despite all efforts, has not yet been completely understood (5-8). Keeping in mind the ''complexities'' of both low-temperature water and its GT, we present here direct measurements of two dynamical parameters of water: the self-diffusion coefficient and the average translational relaxation time, in the temperature range from 280 to 190 K, obtained by NMR and quasi-elastic neutron scattering (QENS) experiments, respectively.Bulk water can be supercooled below its melting temperature (T M ) down to Ϸ235 K, below which it inevitably crystallizes; it is just in such supercooled metastable state that the anomalies in its thermodynamical properties are most pronounced, showing a power law divergence toward a singular temperature T S ϭ 228 K. At ambient pressure, water can exist in a glassy form below 135 K. Depending on T and P, glassy water has two amorphous phases with di...

The puzzling unsolved mysteries of liquid water: Some recent progress

“…In this work we concentrated on reviewing the evidence for changes in dynamic transport properties, such as diffusion constant and relaxation time. Additional examples include: (1) a breakdown of the Stokes-Einstein relation for ToT W ðPÞ [74][75][76][77][78][79], (2) systematic changes in the static structure factor SðqÞ and the corresponding pair correlation function gðrÞ revealing that for ToT W ðPÞ the system more resembles the structure of LDL than HDL, (3) appearance for ToT W ðPÞ of a shoulder in the dynamic structure factor Sðq; oÞ at a frequency o % 60 cm À1 % 2 THz [51], (4) rapid increase in hydrogen bonding degree for ToT W ðPÞ [80,81], (5) a minimum in the density at low temperature [82], and (6) a scaled equation of state near the critical point [83]. It is important to know how general a given phenomenon is, such as crossing the Widom line which by definition is present whenever there is a critical point.…”

“…This work, supported by the NSF Chemistry Division, has been carried out with many collaborators and has been heavily influenced by a number of experimentalists including S.-H. Chen [44,50,77,82], L. Liu [39,40], F. Mallamace [85,86], O. Mishima [13,87,88], J. Teixeira [89][90][91], M.-C. Bellissent [92][93][94][95], and H. Reichert [96].…”

Self-Assembly and Water-like Anomalies in Janus Nanoparticles

“…In addition, silicon 33 and others material, as silica [34][35][36] shows thermodynamic anomalies 31 , while silicon 44 and silica 33,35,36,45 show a maximum in the diffusion coefficient at constant temperature. These systems and their density and diffusion anomalies can be described in an effective way by a two length scale core-softened potential.…”

Possible relation of water structural relaxation to water anomalies

“…Many experiments have been carried out on water confined in nanopores (9,13,14). These studies show that when T is decreased to a certain point the water HB lifetime increases by approximately six orders of magnitude, indicating the location of the Widom line (9), and signs of LDL and HDL inside the supercooled region are observed (14).…”

“…The Widom line reflects water's response functions because they are related to the correlation length. This line and the hypothesized associated polymorphic transition is difficult to study as it lies inside the no-man's land, but the crystallization inside this region can be retarded by confining water within nanoporous structures so narrow that the liquid cannot freeze (9), or within its own ice phase (10), or by using electrolytic solutions (11,12).Many experiments have been carried out on water confined in nanopores (9,13,14). These studies show that when T is decreased to a certain point the water HB lifetime increases by approximately six orders of magnitude, indicating the location of the Widom line (9), and signs of LDL and HDL inside the supercooled region are observed (14).…”