Geoscience Reference
In-Depth Information
viscosity of water as a function of density and temperature
[36]
. It has been dem-
onstrated in many cases that the mineralizer solutions (typically 1 M NaOH,
Na
2
CO
3
,NH
4
F, K
2
HPO
4
, etc.) are close to the properties of water. For 1 M NaOH
at room temperature,
25
[38]
, one can expect that the viscosity
of hydrothermal solutions can be as much as two orders of magnitude lower than
“ordinary” solutions. The mobility of molecules and ions in the supercritical
range is much higher than under normal conditions. Also, electrolytes, which
are completely dissociated under normal conditions, tend to associate with rising
temperatures
[39]
.
The dielectric constant is one of the important properties of a solution. A knowl-
edge of the temperature dependence of the relative dielectric constant is of great
importance for understanding the hydration/dehydration phenomena and reactivities
of a variety of solutes in supercritical water. More than 40 years ago, Hasted et al.
[40]
observed that
η
solution
=η
H
2
O
5
1
:
the dielectric constant of electrolyte solutions (
ε
) can be
regarded as a linear function of molarity up to 1
2 M, depending on the electro-
lyte. Franck
[36]
has discussed ionization under hydrothermal conditions and made
most careful and complete conductivity studies, and shown that the conductance of
hydrothermal solutions remains high in spite of a decrease in
, because that effect
is more than compensated for by an increase in the ion mobility brought about by
decreased viscosity under hydrothermal conditions.
Figure 4.9
shows the dielectric
constant of water
[41]
. Thermodynamic and transport properties of supercritical
water are remarkably different from those of ambient water. Supercritical water is
unique as a medium for chemical processes. The solubility of nonpolar species
increases, whereas that of ionic and polar compounds decreases as a result of the
drop of the solvent polarity, and the molecular mobility increases due to a decrease
in the solvent viscosity (
ε
). Drastic changes of ionic hydration are brought about by
the decrease in the dielectric constant (
η
ε
) and density (
ρ
). Largely as a consequence
Figure 4.9 Dielectric constant of water
[41]
.
30
Saturation curve
20
390
C
400
°
C
°
10
450
°
C
500
°
C
0
0
10
20
30
40
P
(MPa)
