Geoscience Reference
In-Depth Information
than it has been. Automation of the whole calculation process involving the assess-
ment and selection of data, the determination of basis sets, the formation of equa-
tions, and the methods by which results are evaluated are now within reach. This
should be seen as the provision of technical assistance rather than as a substitute
for human expertise. Nevertheless, by reducing errors and by ensuring that assump-
tions are recorded systematically, many of the problems that have been so apparent
in the literature may be better controlled in the future
[16,17]
.
Sterner et al.
[18]
have developed a new computer algorithm, INSIGHT, which
facilitates the nonlinear thermodynamic analysis of large heterogeneous data sets
for the purpose of developing accurate equations of the state of aqueous solutions
and their interactions with other substances. The code is capable of incorporating
a diverse array of primary thermophysical data including: osmotic and vapor
pressure, freezing point depression, solubility [fixed pH and pCO
2
], electromotive
force data, speciation information obtained from spectroscopic measurements and
molecular dynamic simulations. Also it can in corporate several other parameters
such as enthalpies of mixing, dilution and solution heat capacity, volumetric, and
compressibility data. These data, together in a global, polythermal polybaric, ther-
modynamic analysis simultaneously solve chemical equilibria within the aqueous
phase via Gibbs energy minimization.
A hydrothermal solution is generally considered as a thermodynamically ideal
one, yet in the case of strong and specific interaction between the solute and the sol-
vent or among the components of the soluble substance in them, significant devia-
tions from Raoult's law occur. Consequently, real hydrothermal solutions differ
from ideal solutions and their understanding requires knowledge of the influence of
the solvent in the process of dissolution and crystallization of various compounds.
Obviously, as shown in most of the experiments, the type of solvent and its concen-
tration determine a specific hydrothermal process and its important characteristics
such as the solubility of the starting materials, quantity of the phases, their composi-
tion, output of the phases, kinetics, and growth mechanism of single crystals. The
value of the changes in the Gibbs free energy in reaching an equilibrium condition
varies with the transition from one solvent to another and it can be shown as:
Δ
G
5
Δ
H
T
Δ
S
RT ln K
;
or K
exp
ð
2
Δ
H
=
T
Þ
exp
ðΔ
S
=
R
Þ
ð
4
:
16
Þ
5
5
where K
equilibrium constant.
The above expression shows the influence of enthalpy and entropy on the equi-
librium constant, so that the enthalpy and entropy of the solubility (at constant P
and T) are different for different solvents. Also, the solubility of one and the same
solid substance changes with the solvent.
At the moment, there is no theory which can explain and estimate the solubility
in real solutions. However, many of the problems connected with solubility can be
explained on the basis of overall physicochemical principles or laws.
In some cases, it is better to use the empirical rule which agrees with the fact
that solubility becomes high in solvents with higher dielectric constant (
5
) and
types of chemical bond which are closer to those of the solute substance. Deviation
ε
