Defoaming ability

Kulkarui at el. [17] have proposed another manner of expressing defoaming ability as a ratio of the rates of foaming in the absence (v°F) and in the presence (vF ) of an antifoaming agent at the same gas flow rate... 

A coefficient of defoaming ability KH has been proposed as a qualitative characteristics. It represents the ratio between the height of the foam column, that is produced... 

To estimate the defoaming ability Garrett [19,20] has used the ratio between the gas volume in the foam in the presence of an antifoaming agent to the gas volume in the foam in the absence of an antifoaming agent... 

Often the decrease in foam stability caused by an antifoaming agent is measured by its consumption, i.e. the specific defoaming ability is determined. For instance, as a measure of defoaming ability the following quantity is accepted in [21]... 

Another way to estimate the efficiency of an antifoaming agent is by the mass, necessary to reduce the foam volume to a half [22]. In yeast production defoaming ability is determined by comparing the consumption of the antifoaming agent with respect to a standard substance (oleic acid) [23]. 

The efficiency of antifoams (especially of low molecular substances) is usually strongly reduced by the increase in surfactant concentration. Fig. 9.1 illustrates the effect of concentration of two different surfactants on the defoaming ability of 10% emulsions of polydimethyl siloxane [17]. 

Fig. 9.1. Dependence of the defoaming ability Ky of polydimethyl siloxane on surfactant concentration ...

Tsuge et al. [11] present a comparison of three methods for estimation of the defoaming ability i) by the decrease in height of a foam column, produced by pouring the solution (Ross-Miles method) after antifoam introduction the height is measured immediately after foam formation and after 5 min... 

A comparison of the defoaming ability of six different compounds using the above three tests under standard conditions showed a good correlation between coefficients Kh, K h and Kf. 

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. 

Finely dispersed systems of antifoams prove to be economically more convenient in industry [26-28]. The only possibility to use low molecular antifoams (fatty alcohols, acids, etc.) of short lasting defoaming ability, is by spraying their solutions. 

The third characteristic, efficiency of defoaming ability, is determined by measuring the foam volume, broken down in unit time. This is done by adding a definite quantity of an antifoam (for example, 0.1 cm3 10% solution) as drops or aerosol. 

In some cases it is possible to have an increase in defoaming ability with time. 

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). 

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. 

Defoaming ability of antifoams with different spreading coefficients in OP-10 and sulphonol foaming systems... 

Thus, the defoaming ability of the antifoam being an individual phase in the foaming system is determined by the degree of stability of both the asymmetric films of the air/foaming solution/antifoam type and the foam films with lenses that do not cause film rupture. 

It has been established [55] that hydrocarbon (decane, benzene) promoted foam breakdown, occurring with formation of unstable films (barrierless rupture), is only possible at very low surfactant concentrations (less than 0.003-0.004% saponin and OP-10). At higher surfactant concentrations the defoaming ability of the antifoam results from the lowering of the energy barrier of film rupture. The latter is determined by the properties of the adsorption layers and other film parameters. 

One of the most striking aspects of antifoam behaviour is the synergy shown by the mixtures of hydrophobic particles and apolar oils dispersed in a foaming solution. A list of such mixtures is given in [19]. The effect of hydrophobic particles on the defoaming ability of oils is illustrated in Fig. 9.10. 

Fig. 9.11. Change in the defoaming ability in the homologous series of saturated alcohols foaming...

Fig. 9.13. Influence of the surfactant concentration on the defoaming ability of saturated alcohols ...

The range of surfactant concentrations in which the saturated alcohols should be the most effective antifoams is either absent or is very narrow and is difficult to determine because of the low stability of the foam and foam films. Thus, the dependence of the defoaming ability of alcohols on surfactant concentration in the NaDoS and NaDoBS foaming systems is not so clearly expressed (in the concentration range studied the maximum of the defoaming ability shifts by three subsequent homologues from nonyl to hexyl alcohol). 

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. 

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