Physical Chemistry of Hydrothermal Growth of Crystals - Hydrothermal Technology - page 151

It should be noted that when the pressure is very high (i.e., the density of fluid is as
high as of liquid), even if both water and organic compound are in supercritical
state, phase separation occurs, just like liquid oil phase separa-

tion. Several chemical changes arise from changes in ionic dissociation of the solu-
tion. Therefore, it is better to understand the characteristics of the pure water under
hydrothermal conditions, especially the hydrogen ions show an influence on the
solubility of various compounds under hydrothermal conditions.

water and liquid

4.4 Thermodynamic Principles of Solubility

Thermodynamic principles enable one to determine how to design a reaction to
yield phase-pure phases. Without knowledge of how to do this, it is impossible to
distinguish a process that is being controlled by thermodynamics versus kinetics. A
large number of publications have appeared on this aspect based on both the theoret-
ical and experimental data. Several models have been proposed in order to under-
stand the hydrothermal synthesis of a variety of compounds like ABO 4

(A alkaline-earth elements, B Ti, Zr, Hf), HAp, sulfides of gold, silver, iron, and

5 5

copper [30 35] . Thermodynamic studies yield rich information on the behavior of

solutions with varying pressure temperature conditions. Some of the commonly

studied aspects are solubility, stability, yield, dissolution precipitation reactions,

and so on under hydrothermal conditions. The thermodynamic principles of solubil-
ity are discussed here in brief. It is well known that:

G sol 5 Δ H sol 2 S sol Δ T Δ ð 4 : 20 Þ

S sol are the changes in the corresponding free energy,
enthalpy, and entropy of dissolution.
If

G sol ; Δ H sol ;

where Δ and Δ

G sol , G sol 5 Δ 0 ; then dissolution takes place. When Δ 0 ; the system exists in

G sol . the equilibrium state; when Δ 0 ; crystallization is thermodynamically

possible.
With a change in the enthalpy, it is observed that either:

i. In the entire temperature interval, Δ H sol , 0 :
ii. In the low-temperature region, Δ H sol . 0 ; whereas in the high-temperature region,
Δ H sol , 0 :
iii. In the entire temperature region, Δ H sol . 0 :
If entropy is considered in Eq. (4.20) then with an increase in the temperature,
just H sol ; S sol may also change. Here also we can expect like Δ the Δ three

possibilities:

1. In the entire temperature region, T Δ S sol , 0 :
2. In the low-temperature region, T Δ S sol . 0 ; whereas in the high-temperature region,

T Δ S sol , 0 :
3. In the entire temperature interval, T Δ S sol . 0 :