4.3 Comparison of the Ionic Model with Other Microscopic Models of Water
145
species H 3 O
+ and OH
− . Ions interact with each other and with neutral molecules.
The spacetime heterogeneity of water is determined by the concentration of ions
and the time of their relative conversion with neutral molecules by proton exchange
at sub-femtosecond time intervals. The excess protons (and proton holes) can be
considered as a proton-hole gas in a frame of reference water molecules.
Table 4.3 compares the ionic and Bernal–Fowler water models point by point.
The Bernal–Fowler model is based on the assumption of the independent movement
of its intrinsic ions, which hence have low concentrations and long lifetimes. The
ionic model takes into account the interaction of ions, and hence they have a higher
concentration and short lifetimes. The Bernal–Fowler model assumes a different
interpretation of DC conductivity, microwave absorption, and the dielectric constant,
while the ionic model considers them on the same basis. It is important to note that
the ionic model also assumes the same parameters for water and ice, which is not
the case for the Bernal–Fowler model. At the static limit, both the ionic and the
Bernal–Fowler models describe the experimental data equally well, although they
have different microscopic backgrounds.
There are several experimental facts, which go beyond the Bernal–Fowler representation of water, and can be explained in the frame of the ionic model only.
These experiments are described below (more examples can be found in Chap. 5)
and mainly concern the properties of water at ultrashort (picosecond) time periods,
or the properties of water at the nanoscale, when the spatial-time heterogeneity of
water comes to the fore, and cannot be neglected or averaged.
1. Optical Kerr effect (evidence for the spatial heterogeneity of water)
Taschin et al. showed [38] that the time-resolved optical Kerr effect allows one to
observe the response of fast vibrational dynamics in water followed by a slower
monotonic relaxation. The experimental spectra show characteristic features in the
region of 50 and 200 cm
−1 , which indicate the coexistence of two local configurations,
which are interpreted as high-density water (HDW) and low-density water (LDW).
The time interval, which corresponds to these frequencies, is considered in Sect. 2.6
and associated with the vibration dynamics of short-lived intrinsic ions of water and
the relaxation of their hydration shell following the excess-proton transfer between
ionic and molecular species. The spatial few-nanometer heterogeneity of water found
by Taschin et al. is close to the average distance between the short-lived ionic species
(see Fig. 4.5). The same period of heterogeneity was found by Huang et al. [39], who
demonstrated the presence of density fluctuations in ambient water on a physical
length-scale of about 1 nm by small-angle X-ray scattering.
2. Femtosecond mid-IR pump–probe spectroscopy (evidence for the time heterogeneity of water)
Woutersen et al. performed [40] a femtosecond mid-IR pump–probe study of the
dynamics of the OH-stretching mode of HDO dissolved in D 2 O. The orientational
relaxation of the HDO molecules was observed to occur on either a very slow or a
very fast timescale. Two discrete time constants of 0.7 and 13 ps were unambiguously
identified. This result correlated with the previously mentioned study and assumes
that two distinct molecular species exist in liquid water. The two discreet characteris-
145
species H 3 O
+ and OH
− . Ions interact with each other and with neutral molecules.
The spacetime heterogeneity of water is determined by the concentration of ions
and the time of their relative conversion with neutral molecules by proton exchange
at sub-femtosecond time intervals. The excess protons (and proton holes) can be
considered as a proton-hole gas in a frame of reference water molecules.
Table 4.3 compares the ionic and Bernal–Fowler water models point by point.
The Bernal–Fowler model is based on the assumption of the independent movement
of its intrinsic ions, which hence have low concentrations and long lifetimes. The
ionic model takes into account the interaction of ions, and hence they have a higher
concentration and short lifetimes. The Bernal–Fowler model assumes a different
interpretation of DC conductivity, microwave absorption, and the dielectric constant,
while the ionic model considers them on the same basis. It is important to note that
the ionic model also assumes the same parameters for water and ice, which is not
the case for the Bernal–Fowler model. At the static limit, both the ionic and the
Bernal–Fowler models describe the experimental data equally well, although they
have different microscopic backgrounds.
There are several experimental facts, which go beyond the Bernal–Fowler representation of water, and can be explained in the frame of the ionic model only.
These experiments are described below (more examples can be found in Chap. 5)
and mainly concern the properties of water at ultrashort (picosecond) time periods,
or the properties of water at the nanoscale, when the spatial-time heterogeneity of
water comes to the fore, and cannot be neglected or averaged.
1. Optical Kerr effect (evidence for the spatial heterogeneity of water)
Taschin et al. showed [38] that the time-resolved optical Kerr effect allows one to
observe the response of fast vibrational dynamics in water followed by a slower
monotonic relaxation. The experimental spectra show characteristic features in the
region of 50 and 200 cm
−1 , which indicate the coexistence of two local configurations,
which are interpreted as high-density water (HDW) and low-density water (LDW).
The time interval, which corresponds to these frequencies, is considered in Sect. 2.6
and associated with the vibration dynamics of short-lived intrinsic ions of water and
the relaxation of their hydration shell following the excess-proton transfer between
ionic and molecular species. The spatial few-nanometer heterogeneity of water found
by Taschin et al. is close to the average distance between the short-lived ionic species
(see Fig. 4.5). The same period of heterogeneity was found by Huang et al. [39], who
demonstrated the presence of density fluctuations in ambient water on a physical
length-scale of about 1 nm by small-angle X-ray scattering.
2. Femtosecond mid-IR pump–probe spectroscopy (evidence for the time heterogeneity of water)
Woutersen et al. performed [40] a femtosecond mid-IR pump–probe study of the
dynamics of the OH-stretching mode of HDO dissolved in D 2 O. The orientational
relaxation of the HDO molecules was observed to occur on either a very slow or a
very fast timescale. Two discrete time constants of 0.7 and 13 ps were unambiguously
identified. This result correlated with the previously mentioned study and assumes
that two distinct molecular species exist in liquid water. The two discreet characteris-