Conspectus
The ability to control the icing temperature
of supercooled water
(SCW) is of supreme importance in subfields of pure and applied sciences.
The ice freezing of SCW can be influenced heterogeneously by electric
effects, a process known as electrofreezing. This effect was first
discovered during the 19th century; however, its mechanism is still
under debate. In this Account we demonstrate, by capitalizing on the
properties of polar crystals, that heterogeneous electrofreezing of
SCW is a chemical process influenced by an electric field and specific
ions. Polar crystals possess a net dipole moment. In addition, they
are pyroelectric, displaying short-lived surface charges at their
hemihedral faces at the two poles of the crystals as a result of temperature
changes. Accordingly, during cooling or heating, an electric field
is created, which is negated by the attraction of compensating charges
from the environment. This process had an impact in the following
experiments. The icing temperatures of SCW within crevices of polar
crystals are higher in comparison to icing temperatures within crevices
of nonpolar analogs. The role played by the electric effect was extricated
from other effects by the performance of icing experiments on the
surfaces of pyroelectric quasi-amorphous SrTiO
3
. During
those studies it was found that on positively charged surfaces the
icing temperature of SCW is elevated, whereas on negatively charged
surfaces it is reduced. Following investigations discovered that the
icing temperature of SCW is impacted by an ionic current created within
a hydrated layer on top of hydrophilic faces residing parallel to
the polar axes of the crystals. In the absence of such current on
analogous hydrophobic surfaces, the pyroelectric effect does not influence
the icing temperature of SCW. Those results implied that electrofreezing
of SCW is a process influenced by specific compensating ions attracted
by the pyroelectric field from the aqueous solution. When freezing
experiments are performed in an open atmosphere, bicarbonate and hydronium
ions, created by the dissolution of atmospheric CO
2
in
water, influence the icing temperature. The bicarbonate ions, when
attracted by positively charged pyroelectric surfaces, elevate the
icing temperature, whereas their counterparts, hydronium ions, when
attracted by the negatively charged surfaces reduce the icing temperature.
Molecular dynamic simulations suggested that bicarbonate ions, concentrated
within the near positively charged interfacial layer, self-assemble
with water molecules to create stabilized slightly distorted “ice-like”
hexagonal assemblies which mimic the hexagons of the crystals of ice.
This occurs by replacing, within those ice-like hexagons, two hydrogen
bonds of water by C–O bonds of the HCO
3
–
ion. On the basis of these simulations, it was predicted and experimentally
confirmed that other trigonal planar ions such as NO
3
...