The permeability of soap films to gases

A stabilized gas bubble resting at a gas/liquid interface decreases in size owing to outward diffusion of gas across the liquid film at the top of the bubble. Brown, Thuman, and McBain used the rate of shrinkage to calculate the permeability of the film to air but the theory employed is inexact.

An improved theory is presented here which takes into account the exact shape of the bubble and, in the case of multicomponent gases such as air, the changing composition. It is shown that after a short period the composition of the gas in the bubble adjusts itself so that the observed permeability becomes constant.

If the equilibrium thickness of the film is known, the method can be used to measure the permeability of the monolayers on both sides of the film.

The experimental results support the modified theory. The method appears to produce accurate and reproducible values of the film permeability, and values for a number of gases at various concentrations of surfactant, at various temperatures, and with added electrolytes are presented.