Speaker
Prof.
Iwao Ohmine
(Institute for Molecular Science, Myodaiji, Okazaki)
Description
Water is the most ubiquitous substance on earth and known to
have anomalous properties, arisen from the characteristic
structure and dynamics of hydrogen bond network in water.
There exist intermittent collective molecular motions
associated with the rearrangement of the hydrogen bond
network and concomitant fluctuation and relaxation in liquid
water. Two aspects of the water dynamics will be discussed;
(1) Water dynamics in the low temperatures, 2) (2) Molecular
dynamical mechanism of freezing3) and melting of water,
Upon cooling, water freezes into ice. This process is a
most familiar phase-transition, occurring in many places in
nature, but is extremly hard to be simulated by a computer
simulation. Since the global potential energy surface of HBN
rearrangement is rugged and complex, water is much harder to
freeze than simple liquid We will discuss how an initial
nucleus is created and grows on the rugged potential energy
surface of water and the role of fluctuation in the freezing
process.
Resilient hydrogen bonds render ice melting complex. A key
step to break the resilient hydrogen bonds of ice is not the
formation but rather the spatial separation of defect pairs.
We find that once it is separated, the defect pair—either an
interstitial (I) and a vacancy (V) defect pair (a Frenkel
pair), or an L and a D defect pair (a Bjerrum pair)9—is
entropically stabilized, or ‘entangled’. In this state,
defects with threefold hydrogen-bond coordination persist
and grow, and thereby prepare the system for subsequent
rapid melting.
We carry out extensive molecular dynamics simulations from
room temperatures down to as low as 130K, without attaining
the freezing to ice. Relaxation time is found to vary over
twelve orders of magnitude in traversing this range, with
occurrence of multiple anomalies. Structural, dynamical and
thermodynamic properties all show a crossover, around 220K,
to a different, low density liquid state with different
dynamical properties. On further cooling, this low density
liquid again undergoes a dynamical transition around 197K
region where (i) the density reaches its minimum (ii) the
dynamical heterogeneity starts to decrease after reaching
maximum. The temperature dependence of relaxation times
reveals three distinct branches, with discontinuities around
220 K and 197K. Molecular pictures of the dynamics in theses
three branches will be discussed.
