Chemistry Reference
In-Depth Information
Water changes dramatically when its temperature
rises, owing to the breakdown in its hydrogen-
bonded structure with temperature. The high degree
of association in the liquid causes its relative per-
mittivity (more commonly called its dielectric con-
stant) to be very high, around 80 under ambient
conditions, but as the temperature rises this value
falls, as shown in Fig. 23.1. By 205°C its dielectric
constant is equal to that for methanol (i.e. 33) at
ambient temperature. Thus, between 100°C and
200°C superheated water is behaving like a
water-methanol mixture.
The solubility of an organic compound in super-
heated water is often many orders of magnitude
higher than its solubility in water at ambient tem-
perature for two reasons. The first is the change in
dielectric constant, described above. The second is
that a compound with low solubility at ambient tem-
perature will have a high positive enthalpy of solu-
tion and thus a large increase in solubility with
temperature. Compounds that show high solubility
in superheated water are often polarisable, such as
aromatic compounds, or have some polar character.
Thus oxygenates in plant extracts, which are usually
the valuable compounds, show good solubility.
It is not appropriate in this chapter to do a full
review of solubility measurements, but an outline
will be given. An early measurement [4] showed
that naphthalene forms a 10 mass% solution in
water at 270°C. The same group of Professor Schnei-
der in Karlsruhe carried out a range of detailed
studies It was shown later [5] that the heavy hydro-
carbons benz[
e
]pyrene and nonadecylbenzene reach
the same concentration at 350°C. In recent years
many more studies have been made. In one of these
[6], an attempt has been made to rationalise solu-
bility behaviour in a simple way on a range of com-
pounds, including polyaromatic compounds and
pesticides, between 25°C and 200°C. These data have
been analysed to obtain a method of roughly esti-
mating high-temperature solubilities. The analysis
first takes into account the effect of temperature
thermodynamically and then adds in the effect of the
change in dielectric constant empirically. The analy-
sis shows clearly that solubility increases more than
would be expected by the temperature increase. For
example, the solubility of anthracene, in terms of
mole fraction, increases from 8.1 ¥ 10
-9
at 25°C to
2.1 ¥ 10
-4
at 200°C, whereas the effect of tempera-
ture alone would predict a solubility at 200°C of only
8.0 ¥ 10
-6
. Studies of the solubility of the compo-
nents of essential oils have been made [7] that also
show large increases in solubility with temperature.
The change of the properties of water with
increase in temperature allows a range of processes
to be possible, such as extraction from solids and
liquids and soil and general environmental deconta-
mination. The manipulation of water properties with
temperature to achieve various ends has been made
the subject of a patent [8].
Fig. 23.1
The dielectric constant
(relative permittivity) of water as a
function of temperature at its vapour
pressure [3]. The value for methanol at
ambient temperature is shown for
comparison.
