Physical Chemistry of Foods

g-gradient, hence the A–W interfaces can withstand a tangential
stress. Hence the interfaces act like solid walls, so that the downflow
of water is very much retarded, the more so as the lamella becomes
thinner. Thus the foam will have a lifetime that is orders of
magnitude longer than in the absence of surfactant.
Diffusion across an interface. Consider a pond containing pure water.
If the air above it is dry, water will evaporate from the surface,
especially if the wind is blowing. The air flow will readily be
turbulent, so that water vapor can be transported from the pond
surface by convection. Now a surfactant is added, enough to
produce a monomolecular layer on the pond, and the evaporation
rate is markedly reduced. It is often assumed that the surfactant
layer provides resistance to evaporation because water cannot
readily diffuse through it. However, the layer is very thin (a few
nanometers) and can only cause a small resistance to diffusion (see
Section 5.3.3). The main explanation of the reduced evaporation
must be that the wind over the surface causes ag-gradient, so that
the surface can now withstand a tangential stress; hence a laminar
boundary layer of air will be formed near the surface, and the
diffusion of water vapor through the boundary layer (which may be
about a millimeter thick) will cause a considerable decrease in
transport rate.
Film stability. The formation ofg-gradients is all that allows ‘‘stable’’
liquid films to be made. A film of pure water immediately breaks. To
be sure, a thin film is never stable in the thermodynamic sense, but
its lifetime can be quite long if it contains surfactant. Figure 10.29c
illustrates the so-calledGibbs mechanismfor film stability. If for
some reason a thin spot forms in a film, this implies a local increase
in surface area, hence a local decrease in surface load, hence a local
increase in surface tension, hence motion of the film surfaces in the
direction of the thin spot, hence a Marangoni effect, i.e., flow of
liquid toward the thin spot, hence a self-stabilizing mechanism.
Actually, a more elaborate treatment of film stability is needed (see
Section 13.4.1), but the Gibbs mechanism is essential.
Wine tears. The wine in a glass may show the formation of wine tears
on the glass wall above the wine surface, which phenomenon is
enhanced when the glass is gently rocked. It occurs especially with
wine of a fairly high ethanol content. The explanation is illustrated
in Figure 10.30. In frame 1 we see the meniscus. Ethanol will
evaporate from the thin layer in the meniscus, locally decreasing the
ethanol content. This will cause an increase of surface tension (see
Figure 10.4). Hence ag-gradient is formed, hence the Marangoni