Foam stabilising properties of surfactants determined by

Foam stabilising properties of surfactants determined by Foam Pressure 

It is well know that the direct comparison between the methods of estimation of foam stability (in most of the cases it is determined by the foam lifetime) is not possible. Each of the existing methods involves different parameters, for example, time for destruction of a foam column of a definite height (or part of it), rate of decrease in the specific foam surface, etc. The main reason for the impossibility to make such a comparison is that foam stability is determined at different pressures in the foam liquid phase. This means that the rate of drainage as well as the time of reaching an equilibrium state of the films in the foam is different. Another reason could be attributed to the possibility both foam formation (i.e. foam volume 

These two examples with the homologous series of alkylsulphonates and alkylsulphates indicate the undoubted advantages of Foam Pressure Drop Technique for determining the foam stabilising properties of surfactants. This technique allows to distinguish small differences in the foam stabilising ability of surfactants. 

The effect of the foam film type has been discussed in Section 7.5.1. However, a question arises as to whether the Ross-Miles method can be employed in testing foams built up by the three types of foam films. The answer is given by the data presented in Fig. 7.12. 

It depicts the H(i) dependence for NaDoS foams. Curve 1 refers to common thin films, curve 2 to CBF and curve 3 to NBF. It is clearly seen that the curves differ from one another. This indicates that the Ross-Miles test can be used to distinguish foams constituted of the different types of foam films. This is in agreement with the xp(Ap) dependence for NaDoS foams (seeFig. 7.6). As mentioned in the beginning of this Section a quantitative comparison of the results by the two techniques cannot be done. 

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