Effects of counterions and co-ions on foam films stabilized by anionic dodecyl sulfate

The influence of counterions and co-ions on the stability and thickness of foam films stabilized by anionic dodecyl sulfate (DS(-)) has been studied in a thin film pressure balance. Particularly, the effect on the properties of foam films of (i) the counterions Li(+), Na(+), and Cs(+) of DS(-) and (ii) monovalent inorganic salts added to sodium dodecyl sulfate solutions is considered. Generally, addition of salt improved the stability of the foam films. As a second order, an increasing ionic size led to an increased adsorption, which in the case of cations gave thinner and less stable films and in the case of anions led to thicker and more stable films. Hence, an effect of anions was observed though the film surfaces were already negatively charged by the anionic DS(-), leading to the conclusion that adsorption of anions to the film surface is governed by ion specific rather than electrostatic interactions. At a fixed surfactant and varying salt concentration, a maximum in film thickness could be identified at a salt concentration well below the surfactant concentration. We anticipate that (i) at low salt concentration salt mainly affects the charging of a film interface, whereas (ii) at high salt concentration salt mainly affects the screening of the electrostatic repulsion between the two interfaces of the film.