Foam formation isotherms

Fig. 6.2 Foam formation isotherms of aqueous surfactant solutions, foam formation rate W = 0.83 cmVs - C]o-C 3 sodium alkyl ethoxy sulphate, O - sodium lauryl sulphate, - lauryl ethoxy sulphate...

The foam collapse rate Wp as a function of surfactant concentration can be called foam formation isotherm (Fig. 6.2). As it is seen from Fig. 6.2, that Wp sharply increases in a narrow concentration range below adsorption layer saturation. The Wp° value can thus be a characteristic of the aggregation stability of foams during the formation process. 

For the evaluation of the foamability of a surfactant the bulk concentration is used at which the relative rate of foam collapse is equal to 50% of its formation (cw °). The cw ° values determined from foam formation isotherms of a number of products are given in Table 6.1. As it is seen, typical representatives of anionics (sodium dodecyl sulphate), cationics (cetyl trimethyl ammonium bromide) and nonionics (ethoxylated alkylphenols) give bubble foams at very low concentrations, and the foam stability of ionic surfactants does not differ much from that of nonionics. For anionics, the highest concentrations are required for soaps of higher carboxylic acids. 

Formation and stability studies of black foam films can be summarised as follows 1) surface forces in black foam films direct measurement of disjoining pressure isotherm DLVO- and non-DLVO-forces 2) thin foam film/black foam film transition establishing the conditions for the stability of both types of black films and CBF/NBF transition 3) formation of black foam films in relation to the state of the adsorption layers at the solution/air interface 4) stability of bilayer films (NBF) theory and experimental data. 

Fig. 3.77 depicts Ao(C) isotherms of NaDoS solution containing an electrolyte at a level chosen to ensure formation of particular types of black foam films. The curves are drawn according to the regressive spline analysis [364]. 

Let us evaluate first the static surface properties of the adsorbed layer of these same systems used in the formation of foams. Hence, Figure 10.4 shows the measured surface pressure isotherms for Tween 20 and for the mixtures with whole casein and P-casein. For each of the concentrations... 

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