Effects of surface rheology and surface potential on foam stability

Here, we examine the collapse of standing aqueous foams stabilized by the anionic surfactant SDS, using DOH and sodium chloride (NaCl) respectively to produce different surface viscoelasticities and surface potentials in order to elucidate the roles of surface rheology and intermolecular forces in foam stability. Inspired from chemical kinetics, we proposed a model to quantify the foam collapse process and to compare the foam stability with two characteristic parameters: the foam half-life, tau, and the prefactor of foam collapse rate, n/tau. The presence of DOH in the SDS solutions can significantly increase the surface viscoelasticity and foam stability, despite the decreased surface potential caused by the displacement of SDS molecules by DOH molecules at the air-water interface. The correlation between surface viscoelasticity and foam stability is explained by the decrease in foam drainage and the increased liquid film stability. For SDS-NaC1 mixtures with the same value of the mean ionic product (1 mM), an increased concentration of NaC1 in the SDS solution reduces the surface potential and the Debye length because of the screening effect resulting from the binding of sodium counter ions to the sulfate head groups. The foam stability decreases because of the weakened repulsive interactions between the two interfaces of the liquid films, despite the presence of the same surface concentration of SDS molecules, as indicated by the same equilibrium surface tension. This paper highlights two different mechanisms that dominate foam stability. (C) 2015 Elsevier B.V. All rights reserved.