Chemistry Reference
In-Depth Information
and they serve as the basis for describing the mechanism of film elasticity
under various conditions.
It is to be expected, as is observed, that the effects of normal surfactants and poly-
mers or polymeric surfactants can be very time-dependent because of the significant
differences in the rates of adsorption of the different species at the interface and, in
some cases, the time required for specific polymer adsorption phenomena to take
place. For example, in a polymer with a range of molecular weight species present,
the smaller chains will routinely adsorb first, but they also desorb more readily and
are, over time, replaced by the higher-molecular-weight chains. Such an exchange
over time can produce a ''ripening'' effect in the adsorbed film leading to a more
viscous or rigid structure. In systems that contain both monomeric amphiphiles and
polymers, the original rapid adsorption of surfactant may be followed by a slow
displacement of those species by the adsorption of the polymer.
The fundamental impact of surfactant concentration and diffusion rate in lamel-
lar films can be illustrated as shown in Figure 8.3; as the lamellar film between adja-
cent bubbles is stretched as a result of gravity, agitation, drainage, and other motion,
new surface is formed at some locations in the film having a lower instantaneous
surfactant concentration, and a local surface tension increase occurs (a). A surface
tension gradient (b) along the film is produced, causing liquid to flow from regions
of low s toward the new stretched surface, thereby opposing film thinning. Addi-
tional stabilizing action is thought to result from the fact that the diffusion of new
surfactant molecules to the surface must also involve the transport of associated sol-
vent into the surface area, again countering the thinning effect of liquid drainage.
The mechanism can be characterized as producing a ''healing'' effect at the site of
thinning.
Even though the Gibbs and Marangoni effects are complementary, they are gen-
erally important
in different surfactant concentration regimes. The Marangoni
Figure 8.3.
Schematic representation of the ''healing'' action of the Gibbs and Marangoni
effects.
