Abstract
The equilibrium thickness of free vertical liquid films stabilized by sodium or lithium dodecyl sulphate has been measured as a function of the concentration of added urea or sucrose. It was found that the presence of these organic substances lead to a thickening of the film and to the disappearance of counterion specificity. From the equilibrium thickness the dispersion force was calculated as a function of thickness d . For relatively thin films a proportionality with d −3 and a Hamaker constant of 1.7×10 −20 J was found. There are indications of the onset of retardation for d >250Å. The Hamaker constant seems to be on the low side but all other conclusions are in agreement with theoretical predictions.
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