Structural dynamics of supercooled water from quasielastic neutron scattering and molecular simulations

Abstract

One of the outstanding challenges presented by liquid water is to understand how molecules can move on a picosecond time scale despite being incorporated in a three-dimensional network of relatively strong H-bonds. This challenge is exacerbated in the supercooled state, where the dramatic slowing down of structural dynamics is reminiscent of the, equally poorly understood, generic behavior of liquids near the glass transition temperature. By probing single-molecule dynamics on a wide range of time and length scales, quasielastic neutron scattering (QENS) can potentially reveal the mechanistic details of water's structural dynamics, but because of interpretational ambiguities this potential has not been fully realized. To resolve these issues, we present here an extensive set of high-quality QENS data from water in the range 253-293 K and a corresponding set of molecular dynamics (MD) simulations to facilitate and validate the interpretation. Using a model-free approach, we analyze the QENS data in terms of two motional components. Based on the dynamical clustering observed in MD trajectories, we identify these components with two distinct types of structural dynamics: picosecond local (L) structural fluctuations within dynamical basins and slower interbasin jumps (J). The Q-dependence of the dominant QENS component, associated with J dynamics, can be quantitatively rationalized with a continuous-time random walk (CTRW) model with an apparent jump length that depends on low-order moments of the jump length and waiting time distributions. Using a simple coarse-graining algorithm to quantitatively identify dynamical basins, we map the newtonian MD trajectory on a CTRW trajectory, from which the jump length and waiting time distributions are computed. The jump length distribution is gaussian and the rms jump length increases from 1.5 to 1.9 Å as the temperature increases from 253 to 293 K. The rms basin radius increases from 0.71 to 0.75 Å over the same range. The waiting time distribution is exponential at all investigated temperatures, ruling out significant dynamical heterogeneity. However, a simulation at 238 K reveals a small but significant dynamical heterogeneity. The macroscopic diffusion coefficient deduced from the QENS data agrees quantitatively with NMR and tracer results. We compare our QENS analysis with existing approaches, arguing that the apparent dynamical heterogeneity implied by stretched exponential fitting functions results from the failure to distinguish intrabasin (L) from interbasin (J) structural dynamics. We propose that the apparent dynamical singularity at ∼220 K corresponds to freezing out of J dynamics, while the calorimetric glass transition corresponds to freezing out of L dynamics.

Low-temperature dielectric measurements of confined water in porous granites

“…A review of supercooled water studies can be found in Debenedetti (2003). Typically, these experiments involve techniques such as nuclear magnetic resonance (Mallamace et al, 2006), quasielastic neutron scattering (Qvist et al, 2011), small angle X-ray diffraction (Huang et al, 2010), and others.…”

X-ray and Neutron Scattering of Water

“…as the mass diffusion motions and vibrations 105 but provides a more accurate description of the low temperature regime with respect to the roto-translational model.…”

“…This method, recently applied on high-quality QENS data 105 , leads to the identification of two highly entangled motional components, corresponding to two distinct types of structural dynamics:…”

Diffusion in membranes: Toward a two-dimensional diffusion map

“…It has been previously reported by Krynicki et al [11] that the diffusion constant of bulk water at standard pressure and a temperature of 298.2 K is D = 2.30 × 10 −9 m 2 /s [11] in their paper utilizing the nuclear magnetic resonance spin echo technique. Teixeira et al and Qvist et al have published results from quasi-elastic incoherent neutron scattering from bulk water at a variety of temperatures where they found that two relaxation processes were associated with the quasi-elastic peak [12,13], which is how we have chosen to analyze our lateral diffusion data thus far. Qvist et al 's diffusion constants as a result of experiment and molecular simulations at 297.1 K are 2.16 × 10 −9 m 2 /s [13] and 2.20 × 10 −9 m 2 /s [13], respectively.…”

“…Teixeira et al and Qvist et al have published results from quasi-elastic incoherent neutron scattering from bulk water at a variety of temperatures where they found that two relaxation processes were associated with the quasi-elastic peak [12,13], which is how we have chosen to analyze our lateral diffusion data thus far. Qvist et al 's diffusion constants as a result of experiment and molecular simulations at 297.1 K are 2.16 × 10 −9 m 2 /s [13] and 2.20 × 10 −9 m 2 /s [13], respectively. We can at this point not comment on the difference between our diffusion constants with respect to bulk water.…”