Chemistry Reference
In-Depth Information
Figure 10.7.
Adsorption via polar, hydrogen bonding, or acid-base interactions.
An interesting example of the potential importance of such interactions can be
seen in the example of a pure, clean quartz surface. If the surface has been heated to
a sufficiently high temperature (
200
C), a drop of distilled water placed on the
surface will form a sessile drop with a non-zero-contact angle, usually in the
range of 30
. The heat treatment of the quartz produces a dehydrated surface com-
posed of Si-O-Si bonds that, while polar, are not strongly interacting in terms of
hydrogen bonding. If the same surface is allowed to hydrate so that the surface layer
is now basically composed of -SiOH groups, a drop of water will spread comple-
tely on the surface. The effect is illustrated schematically in Figure 10.8.
>
10.3.2.3. Electrostatic Interactions
At the end of the spectrum of adsorption mechanisms opposite to the universal
dispersion forces lie those interactions resulting from the presence of discrete
electrical charges on the surfactant molecules and the solid adsorbent. Those inter-
actions may be described as being either ion pairing or ion exchange (Figure 10.9).
The distinction between the two mechanisms lies in the fact that in ion exchange,
Figure 10.8.
Effects of changes in the nature of a solid surface on interactions involving
hydrogen bonding or acid-base chemistry, as illustrated for the contact angle (
y
) of water on
a silica surface.
