The fluctuations in shape of free liquid films which may be observed by light scattering yield information about the forces determining film stability. The main forces are the van der Waals force and the electric force caused by the overlap of diffuse electric double layers formed by the ions in solution. A detailed analysis employing the thermodynamic theory of fluctuations is made of the electric contribution to the fluctuations in shape. The thermodynamics of the doublehyphen;layer system is kept as simple as possible but account is taken of the nature of ion adsorption at the film surface.
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