Foaming in aqueous solutions of zwitterionic surfactant in presence of monovalent salts: The specific ion effect

The specific ion effect on foamability and foam stability was investigated. The foams were prepared from the aqueous solutions of a zwitterionic surfactant [i.e. N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS)] in the presence of three monovalent salts (i.e. LiCl, NaCl, and CsCl). Addition of salt to the DDAPS solution increased the adsorption of surfactant at the interface and decreased the CMC. The efficiency of these salts in decreasing the surface and interfacial tension followed the order: CsCl > NaCl > LiCl. Similarly, the efficiency of these salts in decreasing the foam stability followed the above-mentioned order. The zeta potential at the hexane-water interface was measured. The efficiency of the salts in making the zeta potential more negative followed the same order. The hydrated radii of the ions played a significant role in the foaming properties of the surfactant solutions.