Foams film stability

Measurements have been carried out on the excess tensions, equilibrium thicknesses, and compositions of aqueous foam films stabilized by either n-decyl methyl sulfoxide or n-decyl trimethyl ammonium-decyl sulfate, and containing inorganic electrolytes. 

Fig. 3.55. Equilibrium thickness h of microscopic foam films stabilized with lyso PE as a function of...

The most important factor regulating the rate of foam column destruction are the surfactant kind, electrolyte concentration and additives that determined the structural characteristics of the foam (dispersity, film type and thickness, etc.) and foam film stability. 

As already mentioned, if the van der Waals force (or other attractive force) is not predominant, first a dimple forms in the thinning liquid films. Usually the dimple exists for a short period of time initially it grows, but as a result of the swift outflow of liquid it decreases and eventually disappears. The resulting plane-parallel film thins at almost constant radius R. When the electrostatic repulsion is strong, a thicker primary film forms (see point 1 in Figure 5.13). From the viewpoint of conventional DLVO theory, this film must be metastable. Indeed, the experiments with microscopic foam films, stabilized with sodium octyl sulfate or sodium dodecyl sulfate in the presence of different amount of electrolyte, show that a black spot may suddenly form and a transition to... 

When considering foam film stability, three rather different cases should be distinguished. Since film thickness is an essential parameter, it is useful to consult Section 11.2.2 on drainage. 

Mechanisms of Single-Foam Film Stability. Soap bubbles and soap films have been the focus of scientific interest since the days of Hooke and Newton (2—9). The stability and structure of foams are determined primarily by the relative rate of coalescence of the dispersed gas bubbles (10). The process of coalescence in foams is controlled by the thinning and rupture of the foam films separating the air bubbles. Experimental observations suggest that the lifetime (stability) of foam films is determined primarily by the thinning time rather than by the rupture time. Hence, if the approaching bubbles have equal size, the process of coalescence can be split into three stages ... 

Experimentally determined rates of thinning do not always agree with the predictions of the Reynolds model. For foam films stabilized by an anionic surfactant, sodium do-... 

Figure 14. A typical plot of V/Vrc versus l/h for nitrobenzene foam films stabilized by various concentrations of dodecanol 11,44 and 178 mM.

Schelero N, Hedicke G, Linse P, Klitzing RV (2010) Effects of counterions and co-ions on foam films stabilized by anionic dodecyl sulfate. J Phys ChemB 114 15523-15529... 

Schulze-Schlarmann J, Buchavzov N, Stuixmrauch C (2006) A disjoining pressure study of foam films stabilized by tetradecyl trimethyl ammtminm bromide cl4tab. Soft Matter 2 584-594... 

The ability to control foam stability or coalescence rate of bubbles is important in many industrial applications. Foams can persist for a few minutes to several days depending on storage conditions. To effectively utilize foams in any of these situations, it is important to have some control over their stability. Therefore it is very important to deepen our understanding of the mechanisms involved in foam persistence and decay. Most works in this area have been rather empirical and many experimental data are rendered useless because important parameters such as bubble size have not been measured. In this chapter we attempt to summarize the quantitative analysis on foam film stability in aqueous systems in terms of surface tension measurements. 

The equilibrium and dynamic aspects of surface tension and adsorption of surfactants at the air-water interface are important factors in foam film stability [82]. Dynamic adsorption models with the diffusion-controlled and mixed-kinetic mechanisms are discussed in some surfactant solution litera-... 

The main subject in this section is the quantitative analysis of the effect of humidity on foam film stability [161]. Three types of foam film were used by controlling the Marangoni effect, by changing the SDS and C12E7 concentration, and by disjoining pressure with the addition of electrolytes. In the low humidity conditions with regulating dry nitrogen gas, water evaporation from... 

Foam films stabilized by anionic surfactants can be destroyed by the addition of an oppositely charged inorganic species or near-stoichiomeric concentrations of metallic ions to form insoluble salts. For example, fatty acids and their derivatives (tall oil, stearate, etc.) can be precipitated by the addition of calcium, aluminium and zinc salts (forming insoluble salts of the acids). There have been several attempts to relate the defoaming action to the solubility product of the particles. A typical series of results for metal oleates are shown in Figure 8.4. The influence of hydrophobic calcium fatty acid soaps on foaming, and quantification of the data in terms of the solubility product of the calcium and sodium fatty acids have also been reported. Cationic surfactants added to foam stabilized by anionic surfactants could also destroy the foam. Unfortunately, due to the formation of solid contaminants and the adverse consumption of chemicals, precipitation antifoamers are used infrequently in industry. 

Figure 21.13. (a) Thin-film interference image of an individual foam film stabilized by adsorbed polymer/surfactant complexes which create a gelled network within the film, and (b) a schematic which shows a representative cross-sectional view of such a film... 

Surface Tension Gradients and Foam Film Stability... 

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