Direct measurement of the variation of double-layer repulsion with distance
Soap films can be subjected to compressive stresses exceeding 1 atm while their thickness is measured optically in an apparatus which is described in detail. It permits forming the film within a ring of porous porcelain whose pores communicate to the outside, whereas the film is in an enclosure in which the air pressure can be varied. The applied pressure is balanced primarily by the double-layer repulsion between the monolayers of the film. Hence as the pressure is increased, the film thickness decreases showing how double-layer repulsion varies with the distance between the monolayers. The agreement with theory is satisfactory as far as it pertains to the region of low potentials which determines the slopes of the distance dependence. The absolute values involve the less certain high potential region of the theory as well as assumptions about the structure of the films but can be brought into reasonable agreement. The effect of van der Waals forces can also be seen at higher ionic strengths.