Dynamical Crossover and Breakdown of the Stokes−Einstein Relation in Confined Water and in Methanol-Diluted Bulk Water

Abstract

Using nuclear magnetic resonance and quasi-elastic neutron scattering spectroscopic techniques, we obtain experimental evidence of a well-defined dynamic crossover temperature T(L) in supercooled water. We consider three different geometrical environments: (i) water confined in a nanotube (quasi-one-dimensional water), (ii) water in the first hydration layer of the lysozyme protein (quasi-two-dimensional water), and (iii) water in a mixture with methanol at a methanol molar fraction of x = 0.22 (quasi-three-dimensional water). The temperature predicted using a power law approach to analyze the bulk water viscosity in the super-Arrhenius regime defines the fragile-to-strong transition and the Stokes-Einstein relation breakdown recently observed in confined water. Our experiments show that these observed processes are independent of the system dimension d and are instead caused by the onset of an extended hydrogen-bond network that governs the dynamical properties of water as it approaches dynamic arrest.

Possible relation of water structural relaxation to water anomalies

“…These findings on confined water are subject to the concern that water behavior may be affected by the silica pore surfaces or the confinement constraints may affect its dynamic behavior, resulting in a physical situation very different from that of bulk water. The results on nanopore-confined water, however, have been confirmed by a number of different techniques (10,12,16) that also show the crossover at ∼225 K and thus are consistent with the possibility that the tetrahedral HB structure recovers at temperatures pertinent to studies of the hypothesized liquid-liquid transition (17).…”

“…These findings on confined water are subject to the concern that water behavior may be affected by the silica pore surfaces or the confinement constraints may affect its dynamic behavior, resulting in a physical situation very different from that of bulk water. The results on nanopore-confined water, however, have been confirmed by a number of different techniques (10,12,16) that also show the crossover at ∼225 K and thus are consistent with the possibility that the tetrahedral HB structure recovers at temperatures pertinent to studies of the hypothesized liquid-liquid transition (17).To test the connection between water anomalies and water's HB network, we consider thermodynamic results on confined water and bulk water sound propagation data. In specific, we consider the velocity dispersion (18)(19)(20) asking to what extent is sound velocity in water v a function of the probe wave vector ðQÞ and the frequency (ω).…”

Communications: The fractional Stokes–Einstein equation: Application to water

“…15 The transition temperatures are rather higher than other estimates that have been used to support arguments for dynamic heterogeneities in supercooled water. [16][17][18][19][20] Chen et al 16 reported a fragile-to-strong crossover temperature of 225 K for light water based on NMR selfdiffusion measurements on confined samples a few nanometers in cross-section and on QENS spectra. The StokesEinstein relation was written in the form D Ϸ − , where is a translational relaxation time derived from the QENS experiments.…”

“…1͒ shows a transition closer to 240 K. A later paper 18 from the same group, apparently based on a reworking of the same data, gives a higher crossover temperature of 290 K and exponents of 1 and 0.6. Still more recent work 20 reverts to the lower temperature, with exponents dependent on the degree of confinement. The capillary work was regarded as "quasi-onedimensional," experiments on protein surface water as "quasi-two-dimensional," and water in methanol solution as "quasi-three-dimensional."…”

“…5͒ shows a transition at approximately 240 K. In each of these examples, the transition is taken as evidence for dynamic heterogeneity at low temperatures. 20 The transition temperatures and FSE slopes listed in this summary appear to depend on the technique employed, the method of data analysis, and the parameters used to approximate the self-diffusion coefficient and the viscosity where measurements were indirect. What is clear from the bulkproperty data used in the analysis of this paper is that ͑a͒ water isotopomers show FSE over a wide range of temperature from 238 to 693 K; ͑b͒ there is a transition temperature between low and high temperature FSE behaviors at about 258 K for light water and 265 K for heavy water; ͑c͒ the slopes for the two isotopomers are almost identical within experimental error in both temperature regions, being 0.94-0.95 in the high temperature region and 0.67 on the lower.…”