Chemistry Reference
In-Depth Information
Repulsion
Attraction
FIGure 7.2
Solid with charges (positive-positive [repulsion] or positive-negative
[attraction]).
where the force would be attractive between oppositely charges, while repulsive in
the case of similar charges. Because the D
e
of water is very high (80 units) as com-
pared to the D
e
of air (ca. 2), we will expect very high dissociation in water, while
there is hardly any dissociation in air or organic liquids. Let us consider the F
12
for
Na
+
and Cl
−
ions (with charge of 1.6 10
−19
C = 4.8 10
−10
esu) in water (D
e
= 74.2 at
37°C), and at a separation (R
12
) of 1 nm:
F
12
= (1.6 10
−19
)(1.6 10
−19
)/[(4 Π 8.854 10
−12
) (10
−9
)(74.2)]
(7.2)
where ε
o
is 8.854 10
−12
kg
−1
m
−3
s
4
A
2
(J
−1
C
2
m
−1
). This gives a value of F
12
of −3.1
10
−21
J/molecule or −1.87 kJ/mol.
Another very important physical parameter one must consider is the size distribution
of the colloids. A system consisting of particles of the same size is called a
monodis-
perse
. A system with different sizes is called
polydisperse
. It is also obvious that sys-
tems with monodisperse will exhibit different properties from those of polydispersed
systems. In many industrial application (such as coating on tapes used for recording
music and coatings on CDs or DVDs), latter kind of quality of coatings is needed.
The methods used to prepare monodisperse colloids aim to achieve a large num-
ber of critical nuclei in a short interval of time. This induces all equally sized nuclei
to grow simultaneously, thus producing a monodisperse colloidal product.
7.1. 2 c
of l l of I d S
S
T a b I l I T y
(dlVo T
h e o r y
)
We need to understand under which conditions a colloidal system will remain dis-
persed (and under which it will become unstable). Knowing how colloidal particles
interact with one another makes possible an appreciation of the experimental results
for phase transitions in such systems as found in various industrial processes. It
is also necessary to know under which conditions a given dispersion will become
unstable (coagulation). For example, one needs to apply coagulation in wastewater
treatment so that most of the solid particles in suspension can be removed. Any two
particles coming close to each other, will produce different forces.
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