Defoaming heterogenous mechanism

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). 

On the basis of the heterogeneous mechanism of foam inhibition Kruglyakov and Koretskaya [56,91] have given an explanation of the reasons for inversion of the defoaming ability within a homologous series of alkyl alcohols with the increase in surfactant concentration and the maximum effectiveness of the intermediate members of the series at intermediate surfactant concentrations. 

In the study of the mechanism of heterogeneous defoaming along with the spreading coefficients the so-called enter coefficient (destruction coefficient) is used to estimate the instability of aqueous foam films... 

Detailed analysis of these approaches is presented by Garrett [19]. As far as these processes are valid for homogeneous defoaming, the impossibility for a complete inhibition of foam formation in many systems, even when the solutions are saturated with antifoams, indicates that these kinetic phenomena are not sufficient for heterogeneous defoaming. Therefore, none of the considered points of view gives a satisfactory explanation of the mechanism of action of antifoams in heterogeneous systems. 

The results about spreading of antifoams as well as the inhibition of black spot formation in microscopic foam films, permit to draw a conclusion about the existence of a special mechanism of heterogeneous defoaming, based on the stability analysis [14,25,48]. The details of this mechanism are given later [18,55,56]. For the defoaming action of hydrophobic solid particles in heterogeneous system another (bridge) mechanism has been proposed [57]. Later it was applied on drops, mixtures of hydrocarbon oil and hydrophobic particles [19,20,53,54]. 

In this Section instability of asymmetric films is explained by decrease in the surfactant adsorption. Another reason for this instability can be the presence of solid particles at the water-oil interface. Such a heterogeneous defoaming is created when a foam is broken down by the antifoam drops that contain solid hydrophobic particles. The mechanism of action of such types of antifoams will be discussed in Section 9.4. 

In conclusion, let us compare some features of the considered mechanisms of heterogeneous defoaming by droplets and solid particles. A necessary condition for both mechanisms appears to be the formation and rupture of wetting films of the substrate/water/air... 

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