The structure and stability of black foam films

The black film (50 to 100 $\overset{\circ}{\mathrm A}$ thick) has a simple lamellar structure in which two monolayers of surface-active molecules are spread on a thin aqueous core. The thickness of the film surface layers has been estimated from the simultaneous measurement of the total film thickness and the thickness of the aqueous core. From these measurements it has been concluded that the surface-active molecules are orientated perpendicularly to the film plane. Radiotracer experiments have shown that the composition of the film surface is identical to the composition of the surface layer at the bulk solution/air interface. These surface concentrations are in excellent agreement with the limiting value calculated from the solution surface tensions by application of the Gibbs adsorption equation. The factors influencing the stability of the thin films have been discussed in terms of the evaporative equilibrium of the volatile film component. The equilibrium vapour pressure over a thin aqueous film should be greater than that over the bulk liquid phase. It has been suggested that the stability of a black film can be maintained by an excess of solute, relative to the bulk solution, in the aqueous film core.