Hydrodynamic and nonhydrodynamic contributions to the bimolecular collision rates of solute molecules in supercooled bulk water.

Bimolecular collision rate constants of a model solute are measured in water at T = 259-303 K, a range encompassing both normal and supercooled water. A stable, spherical nitroxide spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl, is studied using electron paramagnetic resonance spectroscopy (EPR), taking advantage of the fact that the rotational correlation time, τ(R), the mean time between successive spin exchanges within a cage, τ(RE), and the long-time-averaged spin exchange rate constants, K(ex), of the same solute molecule may be measured independently. Thus, long- and short-time translational diffusion behavior may be inferred from K(ex) and τ(RE), respectively. In order to measure K(ex), the effects of dipole-dipole interactions (DD) on the EPR spectra must be separated, yielding as a bonus the DD broadening rate constants that are related to the dephasing rate constant due to DD, W(dd). We find that both K(ex) and W(dd) behave hydrodynamically; that is to say they vary monotonically with T/η or η/T, respectively, where η is the shear viscosity, as predicted by the Stokes-Einstein equation. The same is true of the self-diffusion of water. In contrast, τ(RE) does not follow hydrodynamic behavior, varying rather as a linear function of the density reaching a maximum at 276 ± 2 K near where water displays a maximum density.

Rotation of Four Small Nitroxide Probes in Supercooled Bulk Water.

Using a precise method of least-squares nonlinear electron paramagnetic resonance (EPR) line fitting, we have obtained experimental evidence of a decoupling of the rotational motion of four nitroxide spin probes from the viscosity of bulk water at 277 K. This decoupling is about 50 K higher than another such phenomenon observed in interstitial supercooled water of polycrystalline ice by Banerjee et al. (Proc Natl Acad Sci USA 106 (2009) 11448-11453). Above 277 K the activation energies of the rotation of the probes and water viscosity are very close, while in the supercooled region the activation energies of the probes' rotation are greater than that of the viscosity of water. The rotational correlation times of the probes can be fit well to a power law functionality with a singular temperature. The temperature dependence of the hydrodynamic radii of the probes indicates two distinct dynamical regions, which cross at 277 K.