Geoscience Reference
In-Depth Information
of the dramatic decrease in the dielectric constant of water with increasing temper-
ature at constant pressure and/or decreasing pressure at constant temperature,
“completely” dissociated electrolytes at low temperatures and pressures may
become highly associated in the supercritical region. Computer simulation studies
have been carried out on this topic, which are helpful for the interpretation from
the molecular point of view. Viral expansion of Debye
H¨ckel theory affords a
basis for extrapolating thermodynamic observations of “completely” dissociated
electrolytes to high pressures and high temperatures, where they may be highly
associated. However, the Debye
H¨ckel equation fails to represent adequately the
observed activity coefficients for dilute solutions of 2:2 electrolytes. Landolt-
B¨rnstein
[42]
have estimated the compressibility of water from the PVT curves.
The hotter and less dense the fluid is, the larger is its compressibility. It is an
important factor used in the calculation of density change with pressure. The coef-
ficient of thermal expansion is also an important parameter that helps in the growth
of flawless and strain free crystals. It can be calculated from PVT data. Water has
a much increased coefficient of expansion under hydrothermal conditions. PVT
measurements for water and other solvents are rather rare. Water above critical
temperature (t
c
5
374.2
C) and critical pressure (p
c
5
22.1 MPa) is called supercrit-
ical water. At the critical point, the phases of liquid and gas are not distinguishable.
There are extensive studies on the structure of water under ambient conditions, but
the structure of water at high temperatures and high pressures are not well investi-
gated, mainly due to experimental difficulties. In recent years, there has been
increased interest in the study of the structure, dynamics, and reactivity of super-
critical and subcritical water from the chemical, physical, geological, and engineer-
ing points of view. There are several reasons for this trend. Water at high
temperatures and high pressures is believed to have played a major role in creating
organisms on the earth, the compounds necessary for them, and fossil fuels pro-
duced after their death.
Seward
[43]
, Nakahara et al.
[44]
, Yamaguchi et al.
[45]
, Nakahara et al.
[46]
,
and Ohtaki et al.
[47]
have carried out extensive experimental work on the aqueous
systems at high temperatures and high pressures using conductivity, potentiometric,
spectrophotometric, solubility, PVT, and calorimetric methods. However, insight
into the configurational aspects of ion
solvent interactions under hydrothermal
conditions has come mainly from neutron diffraction studies and more recently
from synchrotron X-ray absorption
(EXAFS)
spectroscopic measurements
[43
47]
. These latter studies have confirmed that cation-oxygen (nearest-neighbor
water) distances decrease with increasing temperature and are accompanied by an
associated decrease in the number of first shell solvated water molecules as has
been shown for Ag
1
,Sr
1
, and other cations
[43]
.
Water is an environmentally safe material and cheaper than other solvents, and
it can act as a catalyst for transformation of desired materials by tuning tempera-
ture and pressure. The larger the reaction barrier height, the larger the temperature
effect on the reaction. For example, reactions whose Arrhenius activation energies
are 42, 83, and 125 kJ mol
2
1
in water at 25
C are accelerated by factors of
10
3
, 6.6
10
7
, and 5.4
10
11
, respectively, at 400
C
[48]
. Thus, a new
8.2
3
3
3
