Foams inhibition

Although the presence of certain additives can enhance the foaming ability and persistence of a surfactant system, chemically similar materials may also significantly reduce foam formation or persistence. The same material can, in fact, function as both a promoter or inhibitor under different circumstances. Materials that reduce the amount of foam formed are termed foam inhibitors, which act to prevent the formation of foam, or foam breakers or defoamers, which increase the rate of foam collapse. A foam inhibitor may function by interfering with the adsorption of surfactant at the air-solution interface or by reducing the effectiveness of adsorbed surfactant as a stabilizer. 

Foam breakers may include inorganic ions such as calcium, which counteract the effects of electrostatic stabilization or reduce the solubility of many ionic surfactants, organic or silicone materials that act by spreading on the interface and displacing the stabilizing surfactant species, or materials that directly interfere with micelle formation. 

A foam breaker that acts by spreading may do so as a monolayer or as a lens (Fig. 12.8). In either case, it is assumed that the spreading foam breaker sweeps away the stabilizing layer, leading to rapid bubble collapse from the outside of the foam. The rate of spreading of the defoamer will, of course. 

FIGURE 12.8. A foam breaker may act by one or both of two mechanisms (d) the foam breaker may displace stabilizing surfactant molecule by molecule leading to breakdown or (b) the breaker may displace the stabilizing structure by spreading as a lens at the interface. 

In some cases it is found that the action of defoaming agents may depend on the concentration of the surfactant present. If the surfactant concentration is below the cmc, the defoamer will usually be most effective if it spreads as a lens on the surface rather than as a monolayer film. Above the cmc, however, where the defoamer may be solubilized, the micelles may act as a reservoir for extended defoaming action by adsorption as a surface monolayer. If the solubilization limit is exceeded, initial defoaming effect may be due to the lens spreading mechanism with residual action deriving from solubilized material. 

---

Correlation between composition and properties of phosphate ester surfactants was exemplified by octyl phosphate with an optimum of foam inhibition and surfactant properties [301]. In separation and concentration of rare earth metals by liquid surfactant membranes 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester was used as carrier [302]. 

Polydimethylsiloxane is active in nonaqueaous systems, but it shows little foam-inhibiting effect in aqueous systems. However, when it is compounded with a hydrophobic-modified silica, a highly active defoamer emerges. 

Kreyenbuhl and Sartorius [55] found that this exchange proceeds slowly if the moisture content is less than 1 %. If it is 1 % or more so, the exchange is complete after 3-6 months and is accompanied by decreased explosive power. The addition of a moisture absorbent (e.g. urea resin foam) inhibits this exchange (French explosive Noburex, p. 552). 

Modern motor oil provides an example of some of the ways in which a number of colloidal and interfacial considerations come into play adhesion and lubrication, detergency, dispersion and suspension stabilization, foam inhibition, and viscosity and its temperature dependence. In addition to providing lubrication, a motor oil is expected to prevent corrosion and aid engine cooling and cleaning. Table 8.1 shows how a number of additives are blended in to help the oil achieve these functions [491]. 

Care must be taken that finish systems not contain de-foamers or foam destabilizers. Polyvalent metal salts are often used as catalysts in resin systems and several of these, such as zinc nitrate,strongly inhibit foam generation. This is not dissimilar to the foam inhibiting action of hard water on fatty acid soaps. 

The most detailed characteristics of the defoaming ability of an antifoaming agent should include its efficiency as depending on the ways of its application (foam inhibition or defoaming), and on the duration of its activity, since it looses activity at contacting the solution. The effect of the solvent, if the antifoam is used in solution, should also be considered. 

Three characteristics are used in the estimation of antifoam efficiency i) ability to inhibit foam formation (the time needed to produce the foam and the decrease in its stability) ii) duration of foam inhibition action iii) defoaming efficiency at introducing antifoaming agent in the foam. 

Foam inhibition is evaluated by the following procedure a definite quantity of the antifoam is added (usually - 0.1 cm3) to the foaming solution (100 cm3). Solid and paste-like antifoams are introduced as solutions. The time needed to fill up the foam vessel as well as the average foam lifetime with Tm and xam, respectively, with and without antifoams, are measured. It is convenient to express the foam inhibition ability either as the relative stability Tre[ (Eq. (9.2)) or as the defoaming coefficient [16]... 

Table 9.1 presents a comparative estimation of the efficiency of some industrial antifoams, including their foam inhibition and defoamng activity [18]. 

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. 

The possibility to use Cm as a parameter characterising foam inhibition has been demonstrated for the first time in [60]. It was shown that the increase in the concentration of silicon oil Caf (antifoam) led to increase in Cm- That is why it was proposed to used the ratio Caf/Cm as a quantitative measure of the defoaming ability. However, it should be noted that the silicon oil concentrations at which inhibition of black spot formation was observed, were very low (10 5-10 9 %). For that reason it is difficult to conclude definitely whether the system was a real solution or represented a diluted emulsion of the antifoam in the surfactant solution. 

Fig. 9.3 depicts the results from the measurements. At low surfactant concentration the course of the Caf Cm dependence follows a plateau, then it increases sharply. In the range of high surfactant concentration complete inhibition of foam formation is not possible when the usual doses of the antifoam are employed (not more than 7-10%). Similar dependence is also typical for the long-chain alcohols decyl and undecyl, which cannot ensure a complete foam inhibition even at concentration of 10% and more. 

Maximum effectiveness in foam inhibition is exhibited by the alcohols in the middle of the homologous series (C7 - C9). In the whole range of their concentrations the foam inhibition occurs under heterogeneous conditions and hence, is a consequence of the joint action of the homogenous and heterogeneous adsorption mechanisms of foam breakdown. 

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. 

Monoalkyl phosphate and phosphate esters are special types of phosphoms-contain-ing anionic surfactants that are of great industrial importance. They are used for flameproofing, as antistatic for textiles, for foam inhibition, as an extreme pressure (EP) lubricant additive, as a surfactant component for alkaline, and as acid cleaners and for special cosmetic preparations (5). The commercially available phosphate ester products are complex mixtures of monoester and diester, free phosphoric acid, and free nonionic. 

---