Geoscience Reference
In-Depth Information
reaction pathway, which is not available in ambient water due to a high barrier,
low solubility, or low-molecular diffusion rate, can be made accessible thermally
in supercritical water. Studies of the structure dynamics and reactivity of supercrit-
ical water lead to a better understanding of how complicated compounds are adap-
tively evolved from simple ones in hydrothermal reactions. It is important to
elucidate how the structure and dynamics of supercritical water are controlled by
intermolecular interactions (density) as well as kinetic energies (T). This is a new
frontier of solution chemistry. The PVT data for water up to 1000
C and 10 kbar is
known accurately enough (within 1% error)
[49
51]
. At very high PT conditions
(1000
C and 100 kbar), water is completely dissociated into H
3
O
1
and OH
2
, behav-
ing like a molten salt, and has a higher density of the order of 1.7
1.9 g/cm
3
.
2
Figure 4.10
shows the temperature
density diagram of water, with pressure as a
parameter
[51]
.
When aqueous solutions are heated up to the solvent critical point, the partial
mol volume of “hydrophilic” species becomes largely negative and that of “hydro-
phobic” ones largely positive due to a divergent behavior of the solvent compress-
ibility. It is well known that, as distances of the intermolecular charge separation
decreased along the reaction coordinate, these “hydrophilic” species were dehy-
drated in supercritical water due to the low dielectric constant of the solvent. The
desolvation effect was pronounced in the solvent with a low dielectric constant
over a long range. One can regard this dehydration effect on the ionic and dipolar
species as the catalytic effect of supercritical water because of its ability of lower-
ing the height of the reaction barrier. Similarly, the conductance data for dilute
alkali metal halide solutions at elevated temperatures and pressures have been usu-
ally interpreted in terms of the formation of simple, neutral ion pairs. It has never-
theless been recognized that higher order polynuclear species (e.g., triplets and
quadruplets) may form in electrolyte solutions having a low dielectric constant at
1000
5
10
25
50
1
kbar
15
25
100
kbar
800
600
bar
600
220
bar
CP
400
200
0
Gas-solid
TP
Solid
Gas-solid
0
0.5
1.0
1.5
Figure 4.10 Temperature
density diagram of water, with pressure as a parameter
[51]
.
