Salt effects in foam films
Measurements have been made of the excess tensions, equilibrium thicknesses and compositions of aqueous foam films stabilised by either n-decyl methyl sulphoxide or n-decyltrimethylammonium n-decyl sulphate and containing inorganic electrolyte. Two types of equilibrium film were obtained. The “second black” film formed at low electrolyte concentrations had a thickness of about 5 nm which was almost independent of electrolyte concentration, while the thickness of the “first black” film formed at higher electrolyte concentrations was much larger but decreased as the ionic strength increased. It is postulated that the abrupt transition from one type of film to the other was a specific ion adsorption effect, probably adsorption of the anion since the transition was more sensitive to changes in the anion than the cation. The variation of excess tension with electrolyte concentration in the first black films was in excellent agreement with the predictions of the DLVO theory of colloid stability using a simple single frequency dispersion force summation model to calculate the van der Waals energy. The large variations of excess tension of the second black films could not be explained quantitatively however. The excess tensions of both first and second black films were independent of temperature over the range 298 to 308 K, and there was no difference between the concentrations of surface active agent in the first and second black films.