Gibbs 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... 

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. 

Many surfactant solutions show dynamic surface tension behavior. That is, some time is required to establish the equilibrium surface tension. If the surface area of the solution is suddenly increased or decreased (locally), then the adsorbed surfactant layer at the interface would require some time to restore its equilibrium surface concentration by diffusion of surfactant from or to the bulk liquid. In the meantime, the original adsorbed surfactant layer is either expanded or contracted because surface tension gradients are now in effect, Gibbs—Marangoni forces arise and act in opposition to the initial disturbance. The dissipation of surface tension gradients to achieve equilibrium embodies the interface with a finite elasticity. This fact explains why some substances that lower surface tension do not stabilize foams (6) They do not have the required rate of approach to equilibrium after a surface expansion or contraction. In other words, they do not have the requisite surface elasticity. 

A different antifoaming mechanism was suggested by Kulkarni et al. (96). They found that surfactants adsorb on the surface of hydrophobic particles during antifoaming, and this adsorption results in deactivation of the particles. On the basis of this observation, they postulated that the adsorption of surfactants onto the hydrophobic particles is so fast that it results in surfactant depletion around the particle in a foam film, and this effect breaks the film. However, no direct proof was presented on this theory. Moreover, depletion of surfactant would cause the film liquid to flow toward the particle because of the increased surface tension (Gibbs— Marangoni effect), and thus cause a stabilizing effect. 

Figure 5. Illustration of the Gihhs-Marangoni effect in a thin liquid film. Reaction of a liquid film to a surface disturbance, (a) Low surfactant concentration yields only low differential tension in film. The thin film is poorly stabilized, (b) Intermediate surfactant concentration yields a strong Gibbs-Marangoni effect which restores the film to its original thickness. The thin film is stabilized, (c) High surfactant concentration (> cmc) yields a differential tension which relaxes too quickly due to diffusion of surfactant. The thinner film is easily ruptured. (From Pugh [109]. Copyright 1996 Elsevier, Amsterdam.)...

Another mechanism important in foam stability is the Gibbs-Marangoni effect, and this plays a role in preventing catastrophic thinning of the fluid films and subsequent bubble collapse. Consider two adjacent air bubbles in a foam, divided by a fluid film coated with surfactant molecules. As the bubbles grow, the dividing film will increase in area and become stretched. This means that the distribution of surfactant molecules... 

Figure 1.2 shows how these two very different types of molecules stabilize emulsion systems. Surfactants rely on rapid diffusion to dissipate any disturbances to the interface. This rapid motion wiU drag fluid along into the inter-lamellar space between droplets, keeping them separated. This activity is known as the Gibbs-Marangoni mechanism. On the other hand, proteins... 

As shown in Figure 1.2, emulsions can be stabilized by surfactants or emulsifiers employing the Gibbs-Marangoni mechanism, which has a very low inter-facial viscoelastic modulus, or by protein-like molecules, which employ a viscoelastic mechanism with a naturally high viscoelastic modulus. Both mechanisms result in stable systems individually, but in many commercial emulsions there is often a mixture of these two molecule types. 

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... 

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