Foam Control by Silicone Polyethers—Mechanisms of “Cloud Point Antifoaming”

Abstract

The mechanism of antifoaming in a nonionic Triton X-100 surfactant solution with silicone polyethers (SILWET L-7210, L-7230, and L-7500), the so-called “cloud point antifoams,” was investigated. The cloud point (CP) studies showed that the CP of a Triton X-100/silicone polyether mixed system is between the CP of the foaming and the antifoam surfactants, due to mixed micelle formation. It was found that the foam stability drops at a certain temperature—which we called the enhanced foam collapse temperature (EFCT)—which was a few degrees Celsius below the CP of the solutions. Single films were also formed from the same systems, and their stability and drainage were observed. Film thinning accelerated with temperature near the cloud point, but this cannot explain the lower foam stability. It was observed that while at low temperatures black film formed, above the EFCT the film ruptured just before black film formation could have begun; the rupture was probably caused by unstable black spots. It is suggested that at the EFCT (near the CP) the net interaction between the film surfaces becomes attraction due to the same reason that causes the CP: the hydrophilic parts (polyethylene oxide) of the nonionic surfactants lose their hydrate water and thus, the steric repulsion between the surfaces diminishes. The EFCT and the CP are not necessarily the same because the composition of the foam and micelle surfaces, respectively, can be different. The antifoaming action was also tested above the CP of the solution and it was observed that another antifoaming mechanism also acts there: the phase-separated surfactant drops get trapped in the thinning foam films and the trapped drops rupture the films by bridging.