Mechanism of antifoaming action

Abstract

Short-range interfacial forces are known to play an important role in the performance of antifoams. To be effective in a given foaming medium, an antifoam agent should have a positive spreading co-efficient and form an unstable interfacial film incapable of foaming; i.e., it should be highly surface active and of limited solubility. Although these conditions are sufficient to describe the essentials of defoaming activity, they are inadequate to explain the variable effectiveness of different antifoams. In the present investigation, using silicone antifoams, we have shown that antifoam effectiveness can be strongly influenced by dispersibility and long-range interfacial electrical forces. The origin of such electrical forces is ionic surfactant adsorption which can impart identical surface charges to a foam bubble and the antifoam droplet, resulting in a repulsive interaction between them. Using the Gouy electrical double-layer model, this repulsive force has been estimated and correlated with antifoam effectiveness. It is also shown that in foaming systems containing anionic or cationic surfactants, electrical repulsive forces change significantly near the critical micelle concentrations of the foaming surfactants.