Speaker
Prof.
Mikhail Anisimov
(University of Maryland)
Description
Properties of supercooled water exhibit spectacular anomalies
which have been a subject of debates for decades [1]. Twenty
years ago Poole et al. suggested that the anomalous
properties of supercooled water may be caused by a critical
point that terminates a line of metastable (and hidden below
the line of homogeneous ice nucleation) liquid-liquid
separation of lower-density and higher-density water [2].
This phenomenon can be viewed as “water’s polyamorphism”.
Most recent accurate simulations of the ST2 model of water
favor the existence of this metastable transition [3]. A
phenomenological model, in which liquid water at low
temperatures is considered as a “solution” of two hydrogen-
bond network structures with different entropies and
densities explains why supercooled water may unmix and
nicely describes the thermodynamic anomalies in supercooled
water [4] and in the popular water-like models, mW and ST2
[5,6]. The existence of two alternative structures in water
may [6] or may not [5] result in the liquid-liquid separation,
depending on the nonideality of mixing of these structures.
The two-state thermodynamics has been recently generalized
to aqueous solutions for describing the liquid-liquid
transitions stemming from the hidden liquid-liquid transition
in pure water [7].
In supercooled water, anomalies in dynamics closely follow
the thermodynamics anomalies. However, contrary to the
vapor-liquid transitions, the anomalies in supercooled water
have been attributed to a structural relaxation [8]. While the
dispersion of sound near the vapor-liquid critical point is
solely an effect of the relaxation of critical density
fluctuations, the dispersion of sound in supercooled water is
most likely a viscoelastic relaxation phenomenon [9].
Coupling between the viscoelastic structural relaxation and a
diffusive decay of density fluctuations could be an important
factor in understanding the supercooled-water dynamics.
Moreover, supercooled water fundamentally differs from
water in the vapor-liquid critical region due to a non-
conserved nature of the order parameter associated with the
orientation of hydrogen bonds in water [10].
Phenomenologically, this order parameter can be viewed as
the extent of “reaction” between two alternative structures of
water [7]. Rather than obeying the diffusive space-dependent
decay, the relaxation of the non-conserved order parameter
is independent of the wave number. This would have far-
reaching implications for various dynamic properties of
supercooled water.
1. Angell, C. A., Supercooled water, in Water: A
comprehensive treatise. Vol. 7, Ed. Franks, F. Plenum Press,
New York, 1982, 215-338.
2. Poole, P.H.; Sciortino, F.; Essman, U.; Stanley, H. E., Phase
behavior of metastable water. Nature 1992, 360, 324-328.
3. Palmer, J. C.; Martelli, F.; Liu. Y.; Car, R.; Panagiotopoulos,
A. Z.; Debenedetti, P. G., Metastable liquid– liquid transition
in a molecular model of water. Nature 2014, 510, 385-388.
4. Holten, V.; Anisimov, M. A., Entropy driven liquid–liquid
separation in supercooled water. Sci. Rep. 2012, 2, 713/1-
713/6; supplement: www.nature.com/scientificreports.
5. Holten, V.; Limmer, D. T.; Molinero, V.; Anisimov, M. A.,
Nature of the anomalies in the supercooled liquid state of the
mW model of water. J. Chem. Phys. 2013, 138, 174501/1-
174501/10.
6. Holten, V.; Palmer, J. C.; Poole, P. H.; Debenedetti, P. G.;
Anisimov, M. A., Two-state thermodynamics of the ST2 model
for supercooled water”. J. Chem. Phys. 2014, 140, 104502/1-
104502/13.
7. Biddle Biddle, J. W., Holten, V.; Anisimov, M. A., Behavior
of supercooled aqueous solutions stemming from hidden
liquid-liquid transition in water. J. Chem. Phys. 2014, 141,
074504/1-074504/10.
8. Mallamace, F.; Corsaro, C.; Stanley, H. E., Possible relation
of water structural relaxation to water anomalies. Proc. Natl.
Acad. Sci. U.S.A. 2013, 110, 4893-4904.
9. Cunsolo, A.; Nardone, M., Velocity dispersion and viscous
relaxation in supercooled water. J. Chem. Phys. 1996, 105,
3911/1-3911/17.
10. Tanaka, H., Importance of many-body orientational
correlations in the physical description of liquids. Faraday
Discussions 2013, 167, 9-76.
