Physicochemical properties of surfactants and the stabilising ability

In order to develop efficient techniques for the preparation and application of foams in industry, agriculture, firefighting, etc., it is necessary to know the physicochemical parameters of surfactants and their relationship with the foam stabilising ability of the surfactant solutions. Usually the criterion of the surfactant foaming ability is the adsorption of these compounds at the solution/air interface and the related to it properties, such as decrease in surface tension, adsorption work, maximum adsorption T. [13,39,43]. CMC is often used as a characteristic of a foaming agent (if micellisation is possible in the surfactant solution). Parameters related to foam stability, such as foam lifetime and foam column height, are also employed [12,13,39], 

The physicochemical behaviour of a foaming agent is treated below in a different unconventional manner, i.e. on the basis of parameters reflecting its stabilising ability. 

A number of physicochemical constants are derived and used to characterise the foaming agents on the basis of the properties of microscopic foam films being the most appropriate foam model. The first place belongs to the surfactant concentration Cm at which black spots are formed in the film [43,67]. As already shown, this concentration is related to the sharp increase in foam stability. Each foaming agent is characterised by its Cu value which 

The parameter Cm can provide important data to be used in estimating the antifoam efficiency, in determining the minimum (threshold) surfactant concentration in the purification of solutions by foam flotation [53], in analysing the causes of foam formation in extraction systems and in establishing the optimum conditions of effective extraction [68]. 

For NBFs (bilayer films) there is a correlation between the concentration Cm and the state of the adsorption layer Cm is close to the concentration of saturation of the adsorption layer. On the other hand, Cm is directly related to the stability of the bilayer films. Therefore, it can be concluded that Cm is identical with the critical surfactant concentration of film rupture, corresponding to a certain arbitrary time, for instance, at film lifetime xcr = 1 s. Hence, Cm of bilayer films is a function of the film lifetime. According to Eq. (3.129) 

---