Chemistry Reference
In-Depth Information
+
b
a
∞
0
2
1
−
Separation distance
Figure 9.8.
Representative potential-energy diagrams: (a) electrostatic stabilization with
primary (1) and sometimes secondary (2) minima; (b) steric stabilization.
found in the comprehensive works edited by Becher (2001) and references cited
therein.
When two liquid droplets possessing adsorbed monomolecular stabilizing films
come into close proximity on a colloidal scale (Figure 9.8), the thin-film region will
develop in the area of closest approach, forming two essentially flat, parallel mono-
layers separated by a distance d (region 1), a transition region at the ends of the
parallel layers where the surface curvature is large relative to that of unaffected
droplets (region 2), and regions well away from the lamellar region where curvature
is that of the undeformed interface (region 3).
When discussing the forces acting across the lamellar film, it is necessary to con-
sider two ranges for the value of the separation distance d: (1) that in which d is
greater than twice the thickness of the monomolecular films d (d
2d ) and (2)
that in which d is less than 2d. In the first case, the significant forces are those con-
sidered to be long-range in the colloidal sense: van der Waals and electrical dou-
ble
>
layer interactions. Such interactions can be characterized by conventional
potential-energy diagrams (Figure 9.8), which relate the potential energy of inter-
action of the two drops to their distance of separation. The shape of the diagram
will be a function of the nature of the attractive and repulsive forces acting between
the approaching drops. In Figure 9.8a, the combination of attractive van der Waals
and repulsive electrostatic forces will normally show the presence of a maximum
and primary and secondary minima as a function of the distance of separation. In
general, the secondary minimum, when present, will represent a state of reversible
flocculation. The primary minimum, on the other hand, represents a state of irrever-
sible coalescence.
If the lamellar film between approaching emulsion droplets thins beyond d
>
2
d
(Figure 9.3), with no drop coalescence taking place, interactions generally referred
to as ''steric repulsions'' come into play (Figure 9.8a). Such repulsions normally
result in a steep maximum in the potential-energy curve. Interfacial film rupture,
