Excess of Proton Mean Kinetic Energy in Supercooled Water

Abstract

We find, by means of a deep inelastic neutron scattering experiment, a significant excess of proton mean kinetic energy hE k i in supercooled water, compared with that measured in stable liquid and solid phases.The measured values of hE k i at moderate degrees of supercooling do not fit the predicted linear increase with temperature observed for the water stable phases. This anomalous behavior is confirmed by the shape of the measured momentum distribution, thus supporting a likely occurrence of ground-state quantum delocalization of a proton between the O atoms of two neighboring molecules. These results strongly suggest a transition from a single-well to a double-well potential felt by the delocalized proton, with a reduced first neighbor O-O distance, in the supercooled state, as compared to ambient condition. The anomalous properties of water have attracted great attention from the scientific community for a long time and are still a topic of intense research. These remarkable properties, most pronounced in the supercooled metastable state [1], can be ascribed to water's unique structure, consisting of a random and fluctuating three-dimensional network of hydrogen bonds [2]. Despite the combined effort of powerful molecular dynamics simulation and novel experimental methods, a complete description of water is still missing. Therefore, this apparently simple liquid still represents a challenging puzzle [3][4][5]. These issues have received great interest in recent years thanks to possibilities opened by novel experimental techniques, detailed theoretical predictions, and computer simulation methods. In particular, the development of pulsed neutron sources has allowed the remarkable advance of the deep inelastic neutron scattering (DINS) technique [6,7]. DINS is based on measurements at high energy, @!, and high momentum, @q, transfers, thus providing a probe of both the short-time (t 10 ÿ15 s) dynamics and local (r 1 A) environment of the atoms in materials [6,7]. The high energy and momentum transfers achieved allow us to describe the scattering process within the framework of the impulse approximation (IA) [7][8][9]. The scattering cross section is then expressed in terms of the single particle momentum distribution, np, whose variance is related to the mean kinetic energy hE k i. In the case of water protons, for instance, the np and hE k i provide a richness of information about the potential surface that the proton experiences, including the effects of hydrogen bonding. This complements microscopic structural studies and allows a direct comparison with quantum Monte Carlo simulations [10,11]. These possibilities make the DINS technique a unique and well established tool to investigate the hydrogen bonding of water under various conditions [12 -16].Here we report on an experimental determination of the proton np and hE k i for a sample of bulk moderately supercooled water. This experiment was performed on a large sample of bulk water, thus avoiding the perturbation introduced by a confining substrate, oft...

Temperature dependence of the zero point kinetic energy in ice and water above room temperature

“…Increased kinetic energy in the solid with respect to the liquid is also reported in Path Integral simulations of water using rigid models [54]. Such combined efforts should help to shed some light on the effects of isotopic substitution (H-D) or the behaviour of hE K ðTÞi in the region below room temperature or near the triple point, where the large discrepancies between experiments on water's protons and deuterons, and theories, are still unexplained [33,35]; we note, in passing, that indications of slight if not almost negligible differences in the deuteron kinetic energies were reported between room temperature liquid and solid heavy water close to the triple point [55], the solid showing in this case a slightly lower kinetic energy than the liquid. Values inferred from macroscopic thermodynamic free energy data on the proton kinetic energy in ice at 269 K and liquid water at 300 K predict a $ 0.5 meV increase from the solid to the liquid [56].…”

Quantum effects in water: proton kinetic energy maxima in stable and supercooled liquid

“…The additional intensities in the wings of the data at T = 271.00 K (black) and T = 269.00 K (purple), must be due to the protons being more localised in the supercooled phase. The minimum in 4πp 2 n(p) at˜14Å −1 suggests that the protons are confined in double well hydrogen bond potentials [15].…”

“…In the stable phase around the density maximum, <E K > increases with density, while an even larger < E K > is found in the metastable phase. In the latter, counter-intuitively, the dis-tance between the water molecules decreases [15] as the density decreases, the decrease in density being primarily due to the reduction in nearest neighbors. The temperature dependence of <E K > in the stable phases can be well described taking into account translational, rotational, and vibrational contributions, and making use of the optical spectroscopic data available in the literature [5,11].…”

“…The Figure reports <E K > values in the solid [6], in the metastable phase -supercooled phase from T = 269.00 K [15] to T = 272.95 K (present data) -in the region of the density maximum from present study -at T = 274.15 K, T = 277.15 K, T = 281.15 K, T = 285.15 K, at room temperature [6,7,11] up the supercritical point [5,7]. The <E K > exhibits two local maxima, one in the stable liquid phase at …”

A combined INS and DINS study of proton quantum dynamics of ice and water across the triple point and in the supercritical phase

“…[13,14], and they are still awaiting a proper theoretical description. Over all in recent years several are the DINS and PICPMD investigations devoted to study both physical quantities in ice and water in a wide temperature range 269 K < T < 673 K [3,7,10,[13][14][15][16][17][18][19][20][21][22]. In the experimental studies one makes use of 0301-0104/$ -see front matter Ó 2013 Elsevier B.V. All rights reserved.…”