Effects of oppositely charged surfactants on the stability of foam films

The present work deals with the influence of different anionic surfactants on the properties of foam films from mixtures of oppositely charged surfactants. For this purpose foam films are formed from 10−4 M cationic alkyl trimethylammonium bromides (CnTAB) and variable amounts of the anionic surfactants sodium decanoate, sodium sulfonate and sodium styrene sulfonate, respectively, are investigated. The concentration of the anionic surfactants was varied between 10−6 M and 0.1 M while the amount of CnTAB was kept constant. With increasing concentration of the anionic surfactant the film stability decreases to a minimum. The concentration at which the minimum in stability occurs depends on the type of anionic surfactant. The film thickness is continuously reduced by the increasing amount of anionic surfactant. The comparison between the different systems leads to the conclusion that the headgroup–headgroup interaction as well as the hydrophilic/hydrophobic balance of the surfactant molecules dominate the functionality and (de)stability of foam films from these mixtures. The work clearly demonstrates that the type and amount of anionic surfactant presents a tool for tailoring the thickness and stability of foam films. The same component can act as a salt or a cosurfactant depending on its molecular functionality and its concentration.