Marangoni stability

In practice, the excess oleic or aqueous phase usually emulsifies into the conjugate microemulsion phase upon agitation. The reason is that the surfactant-rich continuous microemulsion phase resists coalescence of the surfactant-poor excess phase, presumably because surfactant depletion in the thinning microemulsion phase is counteracted by surfactant diffusion to restore uniform chemical potential (Gibbs Marangoni stability). In addition, macroemulsions formed from three-phase microemulsion systems tend to... 

Linear stability analysis has been successfully applied to derive the critical Marangoni number for several situations. 

Figure 1. Schematic diagram showing the possible mechanisms of thin film stabilization, (a) The Marangoni mechanism in surfactant films (b) The viscoelastic mechanism in protein-stabilized films (c) Instability in mixed component films. The thin films are shown in cross section and the aqueous interlamellar phase is shaded.

An improvement in foam stability was observed as R was increased to >0.15 (Figure 17). This was accompanied by the onset of surface diffusion of a-la in the adsorbed protein layer. This is significantly different compared to our observations with /8-lg, where the onset and increase in surface diffusion was accompanied with a decrease in foam stability. Fluorescence and surface tension measurements confirmed that a-la was still present in the adsorbed layer of the film up to R = 2.5. Thus, the enhancement of foam stability to levels in excess of that observed with a-la alone supports the presence of a synergistic effect between the protein and surfactant in this mixed system (i.e., the combined effect of the two components exceeds the sum of their individual effects). It is important to note that Tween 20 alone does not form a stable foam at concentrations <40 jtM [22], It is possible that a-la, which is a small protein (Mr = 14,800), is capable of stabilizing thin films by a Marangoni type mechanism [2] once a-la/a-la interactions have been broken down by competitive adsorption of Tween 20. 

In concentrated emulsions and foams the thin liquid films that separate the droplets or bubbles from each other are very important in determining the overall stability of the dispersion. In order to be able to withstand deformations without rupturing, a thin liquid film must be somewhat elastic. The surface chemical explanation for thin film elasticity comes from Marangoni and Gibbs (see Ref. [199]). When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded... 

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

The principles of colloid stability, including DLVO theory, disjoining pressure, the Marangoni effect, surface viscosity, and steric stabilization, can be usefully applied to many food systems [291,293], Walstra [291] provides some examples of DLVO calculations, steric stabilization and bridging flocculation for food colloid systems. 

In the case where foam instability is desirable, it is essential to choose surfactants that weaken the Gibbs-Marangoni effect. A more surface-active material such as a poly(alkyl) siloxane is added to destabilize the foam. The siloxane surfactant adsorbs preferentially at the air/liquid interface, thus displacing the original surfactant that stabilizes the foam. In many cases, the siloxane surfactant is produced as an emulsion which also contains hydrophobic silica particles. This combination produces a synergetic effect for foam breaking. 

Cellular foam occurs at low vapor velocities in small columns, where the wall provides foam stabilization. It occurs with some systems or tray designs but not with others and is promoted by surface tension effects such as the Marangoni effect (99). Cellular foam is uncommon in industrial columns. The foam that causes problems in industrial installations is mobile foam, where the bubbles are in turbulent motion. Mobile foam is associated with the froth and emulsion regimes. Cellular foam is encountered in bench-scale and pilot-scale columns. If cellular foam occurs in the test unit, caution is required when scaling up the results. 

Surface tension effects have frequently been used to explain observed composition effects (164,187,189) or discrepancies between theory and experiment (146). Zuiderweg (146) and Dribika and Biddulph (169) argue that the Marangoni effect (Sec. 6.4.4) stabilizes the froth and therefore enhances efficiency. The enhancement is related to... 

The action of the lipase, its stability and rate of reaction are influenced by many factors, including temperature, pH, type of solvent, water activity and whether it is in an immobilized or free form (Valivety et al., 1994 Soumanou et al., 1999 Ma et al., 2002, Rousseau and Marangoni, 2002). Liquid butteroil by itself can act as a solvent as well as a substrate and interesterification is enhanced in the presence of an organic solvent such as hexane (Lee and Swaisgood, 1997). 

As has been shown in the previous section, the stability of foam, emulsion and pseudoemulsion films manifest stratification phenomena, curvature phenomena and M arangoni phenomena. We will first discuss the microstructure of the thinning films due to micellar interactions, which we have observed through stratification phenomena. We will then discuss the observed behavior of the pseudoemulsion films with curvature and finally the role of Marangoni effects in the stabilization of the foam structure in the presence of oil. 

Marangoni Effects in Foam Stability. To estimate the effect of interfacial tension gradients upon foam stability we used the maximum droplet pressure technique (19). The oil phases chosen were n-octane and n-dodecane and the surfactants used were 16 ... 

These high stability foams correlate directly with the results based upon interfacial tension gradient measurements, confirming the significance of Marangoni phenomena (31) in three phase foam stability. 

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. 

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