Chemistry Reference
In-Depth Information
instances because of their ability to ''bridge'' between particles and their dramatic
effects on the electrical double layer, as well as their lower cost.
Although surfactant adsorption and its effect on solid surface properties often are
discussed in terms of colloidal systems, the same results can be of technological
importance for macrosurfaces, especially in the control of the wetting or nonwetting
properties of materials in waterproofing, detergency, lubrication, the control of fluid
flow through porous media (crude oil production), and corrosion control. Almost
any process or product that involves the interaction of a solid phase and a liquid
phase will be affected by the process of surfactant adsorption; thus the area repre-
sents a major segment of the technological application of surfactants.
10.7. WETTING AND RELATED PHENOMENA
As indicated in the preceding sections, the adsorption of surfactants at solid-liquid
interfaces can play a significant role in determining the nature of the interactions
between solvent and solid, and among solid surfaces, especially as related to a
phenomenon such as colloidal stability. A similar role can be played by surfactants
on essentially infinite surfaces related to wetting, spreading, adhesion, and lubrica-
tion. Although the basic phenomena are the same for the wetting of extended sur-
faces and the stabilization of colloidal particles, a number of concepts are more
uniquely applied to the more extended surfaces.
While the term ''wetting'' may conjure up a fairly simple image of a liquid
covering a surface, from a surface chemical standpoint, the situation is somewhat
less clear-cut. Three classes of wetting phenomena can be defined on the basis of
the physical process involved: adhesion, spreading, and immersion (Figure 10.15).
The distinctions among the three may seem subtle, but they can be significant from
a thermodynamic and phenomenological point of view.
''Adhesion wetting'' refers to the situation in which a solid, previously in contact
with a vapor phase, is brought into contact with a liquid phase. During the process,
a specific area of solid-vapor interface, A, is replaced with an equal area of solid-
liquid interface (Figure 10.15a). The free-energy change for the process is given by
G
¼
A
ðs
SA
þ s
LA
s
SL
Þ
ð
10
:
4
Þ
where the
's refer to the solid-air (SA), liquid-air (LA), and solid-liquid (SL)
interfacial energies. The quantity in parentheses in Eq. (10.4) is known as the thermo-
dynamic work of adhesion,W
a
, and the equation is that of Dupr´. From the equa-
tion, it is clear that any decrease in the solid-liquid interfacial energy
s
s
SL
will
produce an increase in the work of adhesion (and a greater energy decrease),
while an increase in
s
LA
would reduce the energy gain from the process.
Spreading applies to the situation in which a liquid (L
1
) and the solid are already
in contact and the liquid spreads to displace a second fluid (L
2
, usually air) as illu-
strated in Figure 10.15b. During the spreading process, the interfacial area between
the solid and L
2
is decreased by an amount A, while that between the solid and L
1
s
SA
or
