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

ABSTRACT: 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, ?(R), the mean time between successive spin exchanges within a cage, ?(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 ?(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/? or ?/T, respectively, where ? is the shear viscosity, as predicted by the Stokes-Einstein equation. The same is true of the self-diffusion of water. In contrast, ?(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.

SUBMITTER: Peric I 

PROVIDER: S-EPMC4076007 | biostudies-other | 2014 Jun

REPOSITORIES: biostudies-other