Geoscience Reference
In-Depth Information
from this group has a place in the case where specific interaction between solid
substance and the solvent occurs.
The synthesis and recrystallization of a specific compound and the growth of
single crystals on the seed are all carried out using different solvents on the basis
of physicochemical considerations. The following conditions are adopted in select-
ing the most suitable mineralizers:
i. Congruence of the dissolution of the test compounds.
ii. A fairly sharp change in the solubility of the compounds with changing temperature or
pressure.
iii. A specific quantitative value of
the absolute solubility of
the compound being
crystallized.
iv. The formation of readily soluble mobile complexes in the solution.
v. A specific redox potential of the medium ensuring the existence of ions of the required
valence.
Additionally, the solvent should have the desired viscosity, insignificant toxicity,
and very weak corrosion activity with respect to the apparatus.
These factors fulfill the requirement of the hydrothermal mineralizer in addition
to determining the values of solubility of the compound under investigation.
The interaction between the solute and the solvent is called solvation. Solvent
molecules become linked to the dissolved molecules or ions, and the resulting spe-
cies containing solvent molecules are solvates. Depending upon the reactants, the
linkage between the solvent and the solute may exhibit different specific (coordina-
tion, hydrogen bond formation, etc.) and nonspecific (electrostatic) factors, or sol-
vation processes of different strength, and hence, solvates include very varied
formations. Therefore, it is not possible to find a single physical parameter charac-
terizing the solvent which, in itself, could rationalize the solvation process. Simple
ions may differ considerably in their electronic structures, charges, and radii, and
even larger differences are exhibited by solvent molecules representing different
types of chemicals. Hence, solvated ions may also be very different as regards the
number of solvent molecules bound to the ion (the salvation number), the forces
giving rise to the binding, and the resulting physical and chemical properties like
spatial requirements (there are various ways of determining the solvation number—
viscosity measurements, pH, conductivity measurement, density measurements, and
so on). The specific solvation process is the result of various types of complex
formation between the solvent and solute components. In the case of homoge-
neous aqueous solutions of specific solvation, an activated complex theory can
be considered
[19]
, according to which the rate constant,
ξ
, of the reaction type
X
6¼
!
reaction products (X
6¼
is the activated complex) is determined by:
A
1
B
ξ
5
ð
kTK
6¼
=
h
Þðν
A
ν
B
=ν
6¼
Þ
ð
4
:
17
Þ
The concentration of the activated complex X may be determined by the follow-
ing equation:
X
6¼
5
K
6¼
ð
C
B
Þν
A
ν
B
=ν
6¼
½
C
A
Þð
ð
4
:
18
Þ
