Molecular Probe Dynamics Reveals Suppression of Ice-Like Regions in Strongly Confined Supercooled Water

Abstract

The structure of the hydrogen bond network is a key element for understanding water's thermodynamic and kinetic anomalies. While ambient water is strongly believed to be a uniform, continuous hydrogen-bonded liquid, there is growing consensus that supercooled water is better described in terms of distinct domains with either a low-density ice-like structure or a high-density disordered one. We evidenced two distinct rotational mobilities of probe molecules in interstitial supercooled water of polycrystalline ice [Banerjee D, et al. (2009) ESR evidence for 2 coexisting liquid phases in deeply supercooled bulk water. Proc Natl Acad Sci USA 106: 11448–11453]. Here we show that, by increasing the confinement of interstitial water, the mobility of probe molecules, surprisingly, increases. We argue that loose confinement allows the presence of ice-like regions in supercooled water, whereas a tighter confinement yields the suppression of this ordered fraction and leads to higher fluidity. Compelling evidence of the presence of ice-like regions is provided by the probe orientational entropy barrier which is set, through hydrogen bonding, by the configuration of the surrounding water molecules and yields a direct measure of the configurational entropy of the same. We find that, under loose confinement of supercooled water, the entropy barrier surmounted by the slower probe fraction exceeds that of equilibrium water by the melting entropy of ice, whereas no increase of the barrier is observed under stronger confinement. The lower limit of metastability of supercooled water is discussed.

A comparative study on bulk and nanoconfined water by time-resolved optical Kerr effect spectroscopy

“…The debate on this issue is open. [11][12][13][14][15][16][17][18][19][20][21][22] All the researchers agree that some structural and dynamic properties of water at the silica interface are modied from those of the bulk liquid, but there is no agreement about if these modications involve also the water fraction situated in the inner part of the pore, not in direct contact with the surface.…”

Rotation of Four Small Nitroxide Probes in Supercooled Bulk Water

“…pDT, pDTO and pDTOH are nearly spherical molecules, whose van der Waals volumes are 170.4, 176.5 and 179.2 Å 3,17 respectively, while DTBN is an ellipsoidal molecule with its major axis about 1.7 times longer than its minor axis 18 and a van der Waals volume of 171 Å 3;17 note that van der Waals volumes are all within 5% of each other. The perdeuterated probes have been chosen because they have smaller Gaussian content contribution 19 than their hydrogenated equivalents, for example TEMPOL in ref 9,11 . The smaller Gaussian content produces narrower EPR lines and thereby a higher signal-to-noise ratio.…”

Hydrodynamic and Nonhydrodynamic Contributions to the Bimolecular Collision Rates of Solute Molecules in Supercooled Bulk Water

“…Figure 3 shows that d B dip /d c behaves hydrodynamically as well. These results might be expected since the r vdW = 3.5 Å is greater than that of the water molecule, 1.4 Å, 13 the probe perceives the collective behavior of the surrounding water molecules as a continuum. Also, d B ex /d c and d B dip /d c do not show any noticeable difference between the supercooled and normal regions.…”

“…The viscosity and diffusion are related to the free volume, thus offering an alternative view of diffusion often accompanied by ideas of “jumps.” To put values of 1/τ RE into the context of free volume, we define a simplified geometrical fraction of free volume as follows: where V is the volume of the sample and V molecule is the volume of molecules within the sample. Taking the molecules as spheres of radius r vdW = 1.4 Å, 13 we have Figure 5 shows the values of 1/τ RE as a function of the density (bottom axis) and simplified geometrical fraction of free volume (top axis). The re-encounter rate appears to be a linear function of both density and free volume; the minimum re-encounter rate occurs at ρ = 1.0 g/cm 3 and φ = 0.62, or 277 K. This may mean that this short-time diffusion behavior is influenced by the availability of free space into which the probe might be trapped.…”

“…Banerjee et al 12 have studied the rotation of the small polar spin probe 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) in interstitial deeply supercooled water of polycrystalline ice, from 90 to 300 K. They observed two fractions of TEMPOL with different mobility and fragility in interstitial supercooled water of polycrystalline ice. By varying the degree of confinement of the supercooled water fraction in ice/water mixtures, Banerjee et al 13 have recently shown that the rotational mobility of probe molecules, surprisingly, increases in water with stronger confinement. They argued that ice-like regions are present in loose confinement, while these regions are suppressed in supercooled water with stronger confinement, which increases the fluidity of water molecules and thereby increases the rotation of the probe.…”