5.1 Nucleation of Ice
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[6]. Density of liquid water keeps falling with cooling below 4 °C. The results
indicate that the density of subcooled liquid water approaches the low density of
ice with cooling, and if crystallization had not occurred, the density of ice and that
of subcooled water would match around −45 to −50 °C [3]. In short, the tetrahedral
coordination established by ice results in progressively open structures with cooling
[3]. The open structures can be interrupted by the presence of hydrophilic solutes such
as ions and other strongly hydrogen-bonding molecules [3]. For example, structuring
in subcooled liquid water increases with the addition of ethanol [3].
The origin of water’s unusual physical properties is the tendency of the H 2 O
molecules to attract each other strongly through hydrogen bonds. This self-loving
nature of water molecules is the source of so-called hydrophobic effects [7]. The
resulting low density causes a loss of orientational entropy. At sufficiently low
temperatures, hydrogen bonds render (1) the energy and the volume to be negatively correlated and (2) the entropy and the volume to be negatively correlated. In
an ordinary liquid, these correlations should be the opposite [6].
Interactions of water with hydrophilic and hydrophobic solutes may provide some
insight. A hydrophilic molecule reduces the viscosity of water, whereas ethanol that
has a hydrophobic ethyl group increases the viscosity of water. The temperature of the
density maximum of water increases by doping of ethanol at low concentrations [3].
The diffusivity of the non-hydrogen-bonding molecule acetonitrile in water decreases
with heating, while the diffusivity of ethanol shows increases more with pressure
than the self-diffusion of water [3]. In short, the hydrogen bonding of liquid water is
strengthened by the presence of hydrophobic groups and weakened by the presence
of hydrophilic groups [3]. The enhancement of the hydrogen bonding of liquid water
by the presence of hydrophobic groups or molecules is called hydrophobic hydration.
Two distinct amorphous (glassy) water phases exist [6] in addition to the polymorphism of ice (ice has 17 known phases as of 2020 [8]). It is possible to quench
liquid water faster than it crystallizes, and homogeneous nucleation can be effectively bypassed. In other words, homogeneous nucleation temperature is not a unique
temperature but is a function of the cooling rate and the observation time [6]. The
viscosity of such glassy water shoots up around −80 °C and 200 MPa [3]. Thus, if
one cools water to lower than −80 °C under 200 MPa so fast that does not allow sufficient response time, the water can be “petrified” in the middle of a motion, without
attaining a thermodynamically stable crystalline structure. Such viscous slowdown is
a significant factor that could invalidate an application of classical nucleation theory
to the determination of nucleation rates [3]. If a system is initially placed neither
in a thermodynamically equilibrium state nor in a metastable local minimum, the
system is more likely to eventually transition to the global minimum than to a local
minimum [6]. Glassy water formed by rapid quenching of the liquid water has a glass
transition temperature of around 136 K or 165 K. When such glassy water is heated,
it crystallizes spontaneously to ice I c at around 150 K [6].
113
[6]. Density of liquid water keeps falling with cooling below 4 °C. The results
indicate that the density of subcooled liquid water approaches the low density of
ice with cooling, and if crystallization had not occurred, the density of ice and that
of subcooled water would match around −45 to −50 °C [3]. In short, the tetrahedral
coordination established by ice results in progressively open structures with cooling
[3]. The open structures can be interrupted by the presence of hydrophilic solutes such
as ions and other strongly hydrogen-bonding molecules [3]. For example, structuring
in subcooled liquid water increases with the addition of ethanol [3].
The origin of water’s unusual physical properties is the tendency of the H 2 O
molecules to attract each other strongly through hydrogen bonds. This self-loving
nature of water molecules is the source of so-called hydrophobic effects [7]. The
resulting low density causes a loss of orientational entropy. At sufficiently low
temperatures, hydrogen bonds render (1) the energy and the volume to be negatively correlated and (2) the entropy and the volume to be negatively correlated. In
an ordinary liquid, these correlations should be the opposite [6].
Interactions of water with hydrophilic and hydrophobic solutes may provide some
insight. A hydrophilic molecule reduces the viscosity of water, whereas ethanol that
has a hydrophobic ethyl group increases the viscosity of water. The temperature of the
density maximum of water increases by doping of ethanol at low concentrations [3].
The diffusivity of the non-hydrogen-bonding molecule acetonitrile in water decreases
with heating, while the diffusivity of ethanol shows increases more with pressure
than the self-diffusion of water [3]. In short, the hydrogen bonding of liquid water is
strengthened by the presence of hydrophobic groups and weakened by the presence
of hydrophilic groups [3]. The enhancement of the hydrogen bonding of liquid water
by the presence of hydrophobic groups or molecules is called hydrophobic hydration.
Two distinct amorphous (glassy) water phases exist [6] in addition to the polymorphism of ice (ice has 17 known phases as of 2020 [8]). It is possible to quench
liquid water faster than it crystallizes, and homogeneous nucleation can be effectively bypassed. In other words, homogeneous nucleation temperature is not a unique
temperature but is a function of the cooling rate and the observation time [6]. The
viscosity of such glassy water shoots up around −80 °C and 200 MPa [3]. Thus, if
one cools water to lower than −80 °C under 200 MPa so fast that does not allow sufficient response time, the water can be “petrified” in the middle of a motion, without
attaining a thermodynamically stable crystalline structure. Such viscous slowdown is
a significant factor that could invalidate an application of classical nucleation theory
to the determination of nucleation rates [3]. If a system is initially placed neither
in a thermodynamically equilibrium state nor in a metastable local minimum, the
system is more likely to eventually transition to the global minimum than to a local
minimum [6]. Glassy water formed by rapid quenching of the liquid water has a glass
transition temperature of around 136 K or 165 K. When such glassy water is heated,
it crystallizes spontaneously to ice I c at around 150 K [6].