Chapter 15
The Relationship between the Self-Diffusivity of Supercooled and Amorphous Solid Water Downloaded by PENNSYLVANIA STATE UNIV on September 14, 2012 | http://pubs.acs.org Publication Date: June 3, 2002 | doi: 10.1021/bk-2002-0820.ch015
R. Scott Smith, Z. Dohnálek, Greg A. Kimmel, K. P. Stevenson, and Bruce D. Kay *
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 3335 Q Avenue, Mail Stop K8-88, Richland, WA 99352
We summarize the existing experimental data for the self-diffusivity of supercooled liquid water and review two proposals for the temperature dependence of these data. These data are compared to the recently published measurements of the self-diffusivity of amorphous solid water. We discuss the implications of these data regarding the continuity between ASW and supercooled liquid data. The advantages and limitations of using nanoscale thin films to measure the physical properties of metastable amorphous materials are discussed.
Introduction The properties of supercooled liquid water have been the subject of much experimental and theoretical research and several excellent reviews of this work are available. The experimental properties have been summarized in a review article by Angell (1). The theoretical interpretation of the experimental data are thoroughly discussed by Debenedetti (2). More recently, Mishima and Stanley
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© 2002 American Chemical Society
In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
Downloaded by PENNSYLVANIA STATE UNIV on September 14, 2012 | http://pubs.acs.org Publication Date: June 3, 2002 | doi: 10.1021/bk-2002-0820.ch015
199 discuss some of the current puzzles and outstanding issues regarding liquid, supercooled liquid, and glassy water (3). One issue is the relationship between amorphous forms of solid water and liquid water. Water forms an amorphous solid when vapor deposited on cold (
Supercooled 10i-6 230
240
250
260 270 Temperature (Κ)
Liquid
280
290
300
Figure 1. Liquid and supercooled liquid self-diffusivity data for water.
One approach is to use the Vogel-Fulcher-Tamman (VFT) equation, D(T)=D exp{-B/(T-T )} 0
(1)
0
where Τ is temperature, and T„, D , and Β are fit parameters. This equation is often used to parameterize the viscosity of supercooled liquids and is highly accurate over several orders of magnitude for many substances. In Eqn 1, T is the temperature where the diffusivity descends to zero (conversely, where the viscosity goes to infinity). The magnitude of Β determines the steepness of this descent. Although originally used as an empirical formula (75), the Β and T parameters are thought to have thermodynamic significance and are used to used to classify supercooled liquids as either strong or fragile (2,16). The other approach is to use a power law equation of the form, G
a
0
l/2
D(T) = D T ( T / T - l ) 0
Y
(2)
;;
where Τ is temperature, and T , D
