Geoscience Reference
In-Depth Information
these conditions, there is no chemical reaction occurring between the modifier and
the material surface, but it is only through a strong hydrogen bonding the modifier
can attach to the nanoparticle surface. On the contrary, at higher pH than pK
a
, dis-
sociation of the modifier takes place and results in a chemical reaction between the
dissociated part of modifier and OH
1
from the particles' surface. Thus, by dehydra-
tion reaction, the modifier attaches to the surface of the particles or crystal. Mass
balances, charge balance in the actual system, pH, and the surfactant can be fixed
for most of the systems by considering the chemical reactions
[32]
. The role of sur-
factants is described in detail in Chapter 10.
1.5 Natural Hydrothermal Systems
The beginning of hydrothermal research is firmly associated with the study of the
natural systems by earth scientists who were interested in understanding the gene-
sis of various rocks, minerals, and ore deposits through laboratory simulations of
the conditions existing in the earth's crust. Therefore, it is appropriate to discuss
briefly the research on natural hydrothermal systems and its contribution to the
development of this field to its present status. Starting from the earliest experiment
by Schafthaul in 1845 on the synthesis of quartz
[3]
, over 130 mineral species were
synthesized by the end of nineteenth century, and the experiments were carried out
on various geological phenomena ranging from the origin of ore deposits to the ori-
gin of meteorites. Today, it is being popularly used by geologists to solve several
existing problems in petrology, geochemistry, mineralogy, ore genesis, and paleon-
tology. The impetus for the experimental investigations during the nineteenth cen-
tury was provided not only by a desire to explain geological phenomena but also
by greatly improved equipment and techniques fostered by the industrial revolu-
tion. Such investigations helped in unraveling the hitherto unknown natural geolog-
ical processes of mineral formation. Also they have helped in finding uses for
the artificially synthesized single crystals like ruby, emerald, sapphire, quartz and
diamond in the gemstone industry.
In order to understand the formation of several ore deposits including that of noble
metals, it is necessary to discern the physicochemical conditions which govern the
transport and precipitation mechanisms of these metals in hydrothermal solutions.
Several thermodynamic models have been proposed to explain these mechanisms in
nature. Relatively, much is known, for example, about the hydrothermal chemistry of
gold
[33,34]
. Similarly, the behavior of common rock-forming minerals in a variety
of electrolytic solutions has been studied in detail. Here, the authors present only the
salient features of these works to provide the background for hydrothermal technique
since the main theme of this handbook is crystal growth and materials processing.
Besides, a quantitative model of the transport and deposition mechanisms is still
impeded by a dearth of reliable high temperature and high pressure, experimentally
based, thermodynamic data. However, there is some remarkable progress being made
in this direction, thanks to the advances in thermodynamic modeling, the direct
