Adsorption Homogeneous mechanism of foam breakdown

The efficiency of antifoams depends not only on their physicochemical nature but also on their concentration, the way of introduction into the foam, the properties of the foaming solutions as well as many other factors. That is why the clarification of the mechanism of defoaming ability appears to be a complicate matter. Additional difficulties arise also from the fact that the reasons for foam stability still remain unclear even for simple systems (in the absence of antifoams). 

There lacks a unified concept about the defoaming mechanism. The major physicochemical properties of an antifoam determining its efficiency comprise its low solubility, high surface activity, ability to spread on the surface of the foaming solution and form a polymolecular film, etc. The influence of these properties on antifoam efficiency implies the existence of a number of mechanisms of defoaming. 

A number of authors investigated the process of foam breakdown by antifoams introduced in the foaming system or the foam such as O/W emulsion and solutions in water-soluble organic solvents. Under such defoaming conditions the mechanism of antifoaming action becomes more complex and a destabilisation of the films is possible to occur by an entirely different (heterogeneous) mechanism (see below). 

These results prove the possibility of another foam inhibition mechanism different from the adsorption, especially at high surfactant concentration. 

A special case of defoaming is observed when a soap solution (sodium oleate) is added to a foam, stabilised by saponin. It is know that addition of soap to a saponin solution leads to a decrease in foam volume and stability [e.g. 2]. Some saponin + soap mixtures do not produce a foam at all. It is not possible to explain foam inhibition in these systems with the adsorption displacement, since foam inhibition occurs when soap is added to saponin solution as well as when saponin is added to the soap solution. 

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