Surfactants Gibbs-Marangoni effect

An absence of the Gibbs-Marangoni effect is the main reason why pure liquids do not foam. It is also interesting, in this respect, to observe that foams from moderately concentrated solutions of soaps, detergents, etc., tend to be less stable than those formed from more dilute solutions. With the more concentrated solutions, the increase in surface tension which results from local thinning is more rapidly nullified by diffusion of surfactant from the bulk solution. The opposition to fluctuations in film thickness by corresponding fluctuations in surface tension is, therefore, less effective. 

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. 

Closely related to the above mechanism is the Gibbs-Marangoni effect [13-17], which is represented schematically in Figure 10.19. The depletion of surfactant in the thin film between approaching drops results in a y-gradient without Hquid flow being involved. This results in an inward flow of liquid that tends to drive the drops apart. 

The Gibbs-Marangoni effect also explains the Bancroft rule, which states that the phase in which the surfactant is most soluble forms the continuous phase... 

The Gibbs-Marangoni effect also explains the difference between surfactants and polymers for emulsification. When compared to surfactants, polymers produce larger drops and also give a smaller value of e at low concentrations (Figure 10.19). 

Figure 16.2 Schematic representation of the Gibbs-Marangoni effect, (a) Low surfactant concentration (cmc).

Figure 5. Gibbs—Marangoni effect in the thin-film drainage process. Surfactant is swept to the Plateau borders by flow in the film and droplet phases, and thereby create surface concentration gradients that engender surface tension gradients.

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. 

The effect on surface tension by surfactant adsorption from the bulk solution (Gibbs effect) and by diffusion along an interface (Marangoni effect) is often referred to as the combined Gibbs-Marangoni effect (Figure 11.7). 

Figure 11.7. Schematic representation of the Gibbs-Marangoni effects, (a) Unstretched film, (b) Stretched film. Film stretching causes localized areas of high surface tension, y [1]. Surfactant molecules flow from the bulk phase [2] to the surface (Gibbs effect) and along the interface [3] (Marangoni effect) to heal the stretched film [IJ...

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.)...

The Gibbs-Marangoni effect results from the surface excess of surfactant and so does not occur in pure liquids. 

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