Multicomponent Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 88

(1983) noted that when the mole fractions were very small, UNIFAC under-predicted
the solubilities. Banerjee and Howard (1988) also noted this and appropriate empirical
corrections have been suggested. In spite of the above observations, it is generally rec-
ognized that for most complex mixtures, UNIFAC can provide conservative estimates
of activity coefficients. In fact, its remarkable versatility is evident when mixtures
are considered. As more and more data on functional group parameters in UNIFAC
become available, this may eventually replace most other methods of estimation of
activity coefficients.
Since UNIFAC activity coefficients of compounds in octanol can also be similarly
determined, it is possible to obtain directly UNIFAC predictions of octanol-water
partition constants. It has been shown that adequate corrections should be made for
the mutual solubilities of octanol and water if UNIFAC predictions are to be attempted
in this manner.
It is unnecessary to perform detailed calculations nowadays, since sophisticated
computer programs are available to calculate UNIFAC activity coefficients for
complex mixtures.

3.4.6 S OLUBILITY OF I NORGANIC C OMPOUNDS IN W ATER
One area that we have not considered is the solubility of metal oxides, hydroxides,
and other inorganic salts. As discussed in Chapter 2, because of the preponderance
of organic over inorganic compounds, there is a general bias toward the study of
environmentally significant organic compounds. As a result, less of an emphasis has
been placed on the study of inorganic reactions in environmental chemistry. Perhaps
the most significant aspect of environmental inorganic chemistry is the study of the
aqueous solubility of inorganic compounds. As observed earlier, the water molecule
possesses partial charges at its O and H atoms, which give rise to a permanent dipole
moment of 1.84 10 − 8 esu. The partial polarity of water molecules allows it to

exert attractive or repulsive forces toward other charged particles and ions in the
vicinity. The dipoles between water molecules are attracted to one another by the
H-bonds. In the presence of an ion, the water dipoles near the ion orient themselves
such that a slight displacement of the oppositely charged parts of each molecule
occurs, thereby lowering the potential energy of water. In other words, the H-bonds
between water molecules are slightly weakened, facilitating the entry of the ion into
the water structure. This discussion points to the importance of understanding the
charge distribution around the central ion in water (see Section 3.4.3.5).
Thereareimportantitemstobeunderstoodifthechemistryofmetals,metaloxides,
and hydroxides in the water environment has to be studied. The development of these
ideas requires some knowledge of the kinetics of reactions and reaction equilibria.
Elaborate discussion of ionization at mineral-water interfaces, complexation, effects
of pH and other ions, and other reactions are relegated to Chapter 5.

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3.5 ADSORPTION ON SURFACES AND INTERFACES
In Chapter 2, the thermodynamics of surfaces and interfaces were explored using
the concepts of surface excess properties defined by Gibbs. The Gibbs equation is