Mechanisms of action of mixed solid-liquid antifoams: Exhaustion and reactivation

A major problem in the practical application of antifoams (substances used to avoid undesirable foam) is the gradual loss of their activity in the cou rse of foam destruction. Several experimental methods are combined in the p resent study to reveal the origin of this phenomenon, usually termed as the antifoam "exhaustion" or "deactivation". A typical mixed antifoam, compris ing silicons oil and hydrophobized silica aggregates of fractal shape and m icrometer size, has been studied in solutions of the anionic surfactant sod ium dioctylsulfosuccinate (AOT). The results unambiguously show that the ex haustion in this system is caused by two interrelated processes: (1) segreg ation of oil and silica into two distinct populations of antifoam globules (silica-free and silica-enriched), both of them being rather inactive; (2) disappearance of the spread oil layer from the solution surface. The oil dr oplets deprived of silica, which appear in process 1, are unable to enter t he air-water interface and to destroy the foam lamellae. On the other side, the antifoam globules enriched in silica trap some oil, which is not readi ly available for spreading on the solution surface. As a result, the spread layer of silicone oil gradually disappears from the solution surface (proc ess 2) due to oil emulsification in the moment of foam film rupture. Ultima tely, both types of globules, silica-enriched and silica-free, become unabl e to destroy the foam films, and the antifoam transforms into an inactive ( exhausted) state. The introduction of a new portion of oil (without any sil ica) on the surface of an exhausted solution results in a perfect restorati on of the antifoam activity-reactivation of the antifoam. The experiments s how that the reactivation process is due to restoration of the spread oil l ayer and to rearrangement of the solid particles from the exhausted antifoa m with freshly added oil into new antifoam globules having optimal silica c oncentration. The results provide deeper insight into the mechanisms of ant ifoam action and suggest ways for improving the antifoam efficiency and dur ability.