Counter-Ion Effect on the Surface Potential of Foam Films and Foams Stabilized by 0.5 mmol/L Sodium Dodecyl Sulfate

It is well known that the type of counter-ion affects the state of the adsorption layer of ionic surfactants and, consequently, its surface potential. Yet, it is not clear how they affect the foamability, the rate of foam decay or foam production. How is the surface potential of the air/water interface related to the properties of the foam? This work aims to answer these questions. Foam films, stabilized by 0.5 mmol/L sodium dodecyl sulfate (SDS) in the presence of added LiCl, NaCl, and KCl, were studied by means of the interferometric experimental setup of Scheludko-Exerowa. The surface potential values were derived from the equilibrium film thickness by means of the DLVO theory. A linear relation between the values of the surface potential and specific adsorption energy of the counter-ions on the air/water interface was established. The slope of this linear relation depends on the salt concentration. The foamability, the rate of foam decay, and the foam production of the same aqueous solutions of SDS and added salts were studied by means of the shaking method. A correlation was found between the derived surface potential of the foam film's surfaces and the properties of the foam. The foam production, which is the ratio between the initial foam volume and the rate of foam decay, increases with the decrease in the surface potential. Previous studies in the literature confirm that the lower surface potential promotes higher surfactant adsorption, thus boosting more foam and vice versa. It was also confirmed that the dual effect of KCl on foam production involves converting the best foam stabilizer into a foam suppressor at the highest salt concentration.