Anomalously large isotope effect in the glass transition of water

Abstract

We present the discovery of an unusually large isotope effect in the structural relaxation and the glass transition temperature T g of water. Dielectric relaxation spectroscopy of low-density as well as of vapor-deposited amorphous water reveal T g differences of 10 ± 2 K between H 2 O and D 2 O, sharply contrasting with other hydrogen-bonded liquids for which H/D exchange increases T g by typically less than 1 K. We show that the large isotope effect and the unusual variation of relaxation times in water at low temperatures can be explained in terms of quantum effects. Thus, our findings shed new light on water's peculiar low-temperature dynamics and the possible role of quantum effects in its structural relaxation, and possibly in dynamics of other low-molecularweight liquids.dynamics of water | isotope effect | quantum effects | glass transition | amorphous ice A lthough water is arguably the most important liquid for life, many of its properties remain puzzling (1, 2). In particular, its behavior in the "no-man's land" between 240 K and 150 K, its lowtemperature structural relaxation, and even its glass transition temperature (T g ) continue to be topics of active discussion (3-7).The unusually weak temperature dependence of viscosity near T g ∼136 K, estimated indirectly from crystallization rates, has long been known as one of water's startling features (8). In glassforming liquids the temperature dependence of viscosity or structural relaxation time τ is usually characterized by the fragility index (9):Materials such as SiO 2 and BeF 2 display Arrhenius-like τ(T) behavior with m ∼20-22 and are called strong, whereas those with fragility indices m ∼80 and higher exhibit pronounced superArrhenius variations of τ and are called fragile. Recent dielectric studies discovered an extremely weak temperature dependence of τ in low-density amorphous (LDA) water, with m ∼14 (10). This is by far the lowest fragility known for any liquid and even below its accepted lower limit, m ∼16 (9). Recent speculations ascribe this "superstrong" behavior of water to the impact that zero-point quantum fluctuations can have on structural dynamics (7). Because of the eminent role the atomic mass plays for quantum effects, H/D isotope substitution should have a significant bearing on the low-temperature dynamics of water. To address this question, we performed dielectric measurements on H 2 O and D 2 O prepared as LDA and vapor-deposited water (amorphous solid water, ASW). Details regarding the preparation of LDA water were presented earlier (10) and are briefly summarized in Materials and Methods together with ASW preparation, measurements details, and data analysis (for more details, see also Supporting Information). All of the measurements were repeated several times to confirm data reproducibility. The relaxation times τ for protonated LDA (H-LDA) water (Fig. 1A) as well as for ASW (Fig. 1B), although differing for the reasons given in Supporting Information, both confirm their extremely low fragility, m ∼14 ± 1. An Arrhenius ap...

Quantum effects in the dynamics of deeply supercooled water

“…The provided crude estimates and analysis of literature data on crystalline ices, all support the idea about significant probability of tunneling in supercooled water at temperatures close to its T g . In addition, recent studies of LDA and ASW samples also revealed large isotope effect in T g of water [90], being again consistent with the proposed here idea about tunneling in water at temperatures close to its T g .…”

Isotope effects in liquid water probed by transmission mode x-ray absorption spectroscopy at the oxygen K-edge

“…In particular, the bulk structure of the liquid [1,2], the structure of water at interfaces [3], the thermodynamical properties of water [4], the translational and rotational motions in liquid water [5] and the hydrogen-bond (H-bond) environment [6,7] and its ultrafast dynamics [8] were investigated in light of these quantum effects. Such experimental results can serve as benchmark tests for theoretical treatments of quantum effects in hydrogen bonding (Hbonding) in water [9,10] and H-bonds in general [11].…”

Identification of Structural Relaxation in the Dielectric Response of Water

“…PACS: 82.30.Rs, 77.22.Gm, 83.60.Bc, 78.35.+c Being in the focus of intensive research for the last few centuries [1], water still presents many challenging scientific puzzles. They include complex phase diagram [2], possible liquid-liquid transition [3] and significant role of quantum effects [4]. Among them is also the anomalously large dielectric constant that makes water an excellent solvent and is exploited on a daily basis in microwave heating.…”

“…They include complex phase diagram [2], possible liquid-liquid transition [3] and significant role of quantum effects [4]. Among them is also the anomalously large dielectric constant that makes water an excellent solvent and is exploited on a daily basis in microwave heating.…”