title: 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, 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.
authors.affiliation: Department of Physics and Astronomy and The Center for Supramolecular Studies, California State University at Northridge , Northridge, California 91330, United States.
authors.forename: Ida
authors.initials: I
authors.lastname: Peric
authors.forename: Dalibor
authors.initials: D
authors.lastname: Merunka
authors.forename: Barney L
authors.initials: BL
authors.lastname: Bales
authors.forename: Miroslav
authors.initials: M
authors.lastname: Peric
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D057446:Hydrodynamics; D008962:Models, Theoretical; D033081:Shear Strength; D013696:Temperature; D014216:Triacetoneamine-N-Oxyl / Q000737:chemistry; D014783:Viscosity; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D014216:Triacetoneamine-N-Oxyl