Stratification disjoining pressure

Stratification of asymmetric aqueous films from NaDoS, CTAB and a commercial surfactant (alpha-olefin sulphonate) solutions at C > Ccmc on a decane substrate have been studied by Bergeron and Radke [236], They found three transitions by thickness in the metastable multilayer films. They observed also stratified CTAB aqueous films on glass. The n(/i) isotherms of stratified films were analysed considering also the oscillatory structural component of disjoining pressure. 

The situation is more complicated when the aqueous solution contains surfactant micelles, which is a common experimental and practical situation. In such a case the disjoining pressure isotherm H(/j) can exhibit multiple decaying oscillations, whose period is close to the diameter of the micelles (Fig. 8b) (for details see, e.g., Ref 78). The condition for equilibrium liquid film, Eq. (42), can be satisfied at several points, denoted by Hq, h, / 2, and in Fig. 8b the corresponding films contain 0,1,2, and 3 layers of micelles, respectively. The transitions between these multiple equilibrium states represent the phenomenon stratification (see Fig. 9 and Refs 78-91). The presence of dis-... 

FIGURE 4.36 Plot of disjoining pressure, II, vs. film thickness, h comparison of experimental data for a foam film from Ref. [461] (thin-film pressure balance) with the theoretical curve (the solid line) calculated by means of Equation 4.230. The film is formed from 200 mM aqueous solution of the nonionic surfactant Tween 20. The volume fraction of the micelles (cj) = 0.334) is determined from the film contact angle the micelle diameter id = 1.2 nm) is determined by dynamic light scattering. The points on the horizontal axis denote the respective values of h for the stratification steps measured by a thin-film pressure balance. 

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