Thin Liquid Films - Surface and Colloid Chemistry: Principles and Applications - page 165

exceeding 1000 D ilm 2 , the distance between the lamella (Gibbs, assuming the fluid
in film has the same viscosity and density).

d (mm) Flow (mm/s)

0.01 0.1

0.001 0.001

Also to be considered is the suction flow due to curvature (can be much greater than
the gravity effect) and the evaporation loss.
The latter will depend on the surroundings, and will be almost negligible in a
closed container.

8.3.1 f o a m S S T a b I l I T y
As is known, if one blows air bubbles in pure water, no foam is formed. On the other
hand, if a detergent or protein (amphiphile) is present in the system, adsorbed sur-
factant molecules at the interface produce foam or soap bubble. Foam can be char-
acterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase
volume. The foam, or the lamina of liquid, will tend to contract due to its surface
tension, and a low surface tension would thus be expected to be a necessary require-
ment for good foam-forming property. Furthermore, in order to be able to stabilize
the lamina, it should be able to maintain slight differences of tension in its different
regions. Therefore, it is also clear that a pure liquid, which has constant surface ten-
sion, cannot meet this requirement. The stability of such foams or bubbles has been
related to monomolecular film structures and stability. For instance, foam stability
has been shown to be related to surface elasticity or surface viscosity, η s , besides
other interfacial forces.
Foam destabilization is also a factor in the packing and orientation of mixed films,
which can be determined from monolayer studies. It is worth mentioning that foam
formation from monolayers of amphiphiles constitutes the most fundamental process
in everyday life. The other assemblies, such as vesicles and BLM, are somewhat more
complicated systems, which are also found to be in equilibrium with monolayers.
Although the surface potential, ψ, the electrical potential due to the charge on
the monolayers, will clearly affect the actual pressure required to thin the lamella to
any given dimension, we shall assume, for the purpose of a simple illustration, that
1/ k , the mean Debye-Huckel thickness of the ionic double layer, will influence the
ultimate thickness when the liquid film is under relatively low pressure. Let us also
assume that each ionic atmosphere extends only to a distance of 3/ k into the liquid
when the film is under a relatively low excess pressure from the gas in the bubbles;
this value corresponds to a repulsion potential of only a few millivolts. Thus, at about
1 atm pressure:

h ilm = 6/ k + 2 (monolayer thickness) (8.2)

For charged monolayers adsorbed from 10 −3 n -sodium oleate, the final total thick-
ness, h ilm , of the aqueous layer should be of the order 600 Å (i.e., 6/ k or 18/oc Å).