Three-phase foam stability, effect

Of course the Marangoni effect is not the only stabilizing factor in the three phase foam. Another critical factor is droplet size. Smaller droplet size is accomplished by lower interfacial tension, wherefore it is found that C AOS yields more stable foam... 

There appear then three primary mechanisms for stabilizing (or destabilizing) a three phase foam. The first derives from the micelle structuring in the film and depends directly upon surfactant concentration and electrolyte concentration. The second is a surface tension gradient (Marangoni) mechanism which relates to the short range intermolecular interactions and the rate of surface expansion. And the third is an oil droplet size effect which depends upon the magnitude of the dynamic interfacial tension. 

The interactions between an oil phase and foam lamellae are extremely complex. Foam destabilization in the presence of oil may not be a simple matter of oil droplets spreading upon foam film surfaces but may often involve the migration of emulsified oil droplets from the foam film lamellae into the Plateau borders where critical factors, such as the magnitude of the Marangoni effect in the pseudoemulsion film, the pseudoemulsion film tension, the droplet size and number of droplets may all contribute to destabilizing or stabilizing the three phase foam structure. 

The stability of emulsion and foam films have also been found dependent upon the micellar microstructure within the film. Electrolyte concentration, and surfactant type and concentration have been shown to directly influence this microstructure stabilizing mechanism. The effect of oil solubilization has also been discussed. The preceding stabilizing/destabilizing mechanisms for three phase foam systems have been shown to predict the effectiveness of aqueous foam systems for displacing oil in enhanced oil recovery experiments in Berea Sandstone cores. 

A closer relationship between foam stability and HLB has been reported for two- or three-phase systems surfactant solution-oil or oil-surfactant phase-water [60,109-111]. The effect of various parameters changing HLB on the stability of foams and emulsions has been studied in [111]. These were the concentration of amyl alcohol and sodium chloride, the number of the ethylene oxide groups in the molecule of the oxyethylated octylphenol. As a general parameter of HLB the authors used the surfactant affinity difference concept (SAD) which is an empirical generalised formulation. It measures the deviation from the optimum formulation for three phase behaviour. For anionic surfactants... 

In general, one can define three classes of foams unstable, metastable, and solid. While all foams containing fluid phases are thermodynamically unstable, their degree of stability or persistence can vary from seconds to weeks. Unstable or low-persistence foams, as the term implies, remain for a very short time and collapse as a result of the overwhelming effects of surface tension and gravitational forces. More-or-less persistent foams can, however, be produced in the presence of extremely small amounts of amphiphilic substances or in the presence of polymers. As little as 5 ppm of saponin, a natural polymeric surfactant extracted from certain trees, in water can produce a foam of finite, though transient, stability. 

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