Effect of electrolytes on foaming of nonionic surfactant solutions

Abstract

The effect of electrolytes and other additives on the foaming of solutions of the nonionic surfactant octoxynol 9 (Triton X-100) was investigated and correlated with their effect on the cloud point. Foams were generated by pouring solutions as a thin stream into a vertical tube. The height of the generally fast-draining foam columns was measured as a function of time, and their stability was rated according to the damage sustained within 13 min. The surfactant concentrations used, 0.050, 0.100 and 2.00%, were above the critical micelle concentration. Increasing concentrations increased foam height but not foam stability.

The electrolytes NaCl and Na₂SO₄ salted the surfactant out. They reduced its cloud point in proportion to their concentration. They also reduced the foam height, albeit to a lesser extent than the cloud point, but hardly reduced the foam stability below the cloud point.

Electrolytes which salted the surfactant in, raising its cloud point by complexation of their cations with the ether oxygens of the surfactant, namely HCl, H₂SO₄ and Mg(NO₃)₂, produced no major changes in either foam height or foam stability. The exception was Al(NO₃)₃, which reduced the foam height by increasing the viscosity of the solutions. Urea and NaSCN, which raised the cloud point by breaking the structure of water, and to a lesser extent citric acid, reduced the foam stability but produced no major changes in foam height.

Heating the solutions below their cloud point reduced the stability of all foams, especially of those which were the least stable at the lower temperatures. When the cloud point was exceeded by 5 C through a combination of added NaCl and heating, foam height and foam stability decreased significantly. At 10 C above the cloud point, the foam height was reduced to 1/6 of that at 12 C below the cloud point, but the former foam was slow draining and rather stable.