The thickness of equilibrium foam films of nonionic surfactants has been measured depending on the concentration of salt in the film forming solution. In the range from medium to high salt concentration, the thickness is nearly double that of the adsorption layer thickness. However, in certain intervals of salt concentration two series of peaks were observed. It is shown that the peaks in the film thickness on salt concentration dependence represent a general phenomenon for this type of surfactant. The increase of thickness always starts at a relatively well-defined critical concentration. The peak position is shifted to higher salt concentration with an increasing number of ethylene oxide units in the hydrophilic chain. The mechanism of the effect is discussed. It is shown that only a combination of a structural change of the hydrophilic chains and a change in the range of the double layer interaction is able to account for the features of the effect.

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