Gibbs-Marangoni mechanism

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. 

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. 

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. 

Increased surface viscosity Increased film thickness Gibbs-Marangoni effect Net surface charge Residual tertiary structure Gravitational drainage Capillary pressure drainage Mechanical shock Film permeability Surface active molecules... 

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... 

Surface elasticity, sometimes referred to as the self-heahng effect, is caused by surfactants. The mechanism behind this phenomenon is called the Gibbs-Marangoni effect and is illustrated in Figure 13.14. 

For maximum mechanical stability, the interfacial film resulting from the adsorbed surfactants should be condensed, with strong lateral intermolecular forces, and should exhibit high film elasticity. The liquid film between two colliding droplets in an emulsion is similar to the liquid lamella between two adjacent air sacs in a foam (Chapter 7) and shows film elasticity for the same reasons (Gibbs and Marangoni effects). 

Marangoni and Gibbs elasticity. The mechanism of elastic action of the adsorption layer can be represented as follows. Any deformation of the surface accompanying, for example, an increase in its area decreases the quantity of adsorbed surfactants per unit area. This decreases the surface pressure of surfactant molecules and hence increases the surface tension that counteracts further elongation of the surface. If the concentration of surfactants in the adsorption layer is small, then the two-dimensional gas of surfactant molecules is governed by the equation of state... 

Film stability. The formation of y-gradients is all that allows stable liquid films to be made. A film of pure water immediately breaks. To be sure, a thin film is never stable in the thermodynamic sense, but its lifetime can be quite long if it contains surfactant. Figure 10.29c illustrates the so-called Gibbs mechanism for film stability. If for some reason a thin spot forms in a film, this implies a local increase in surface area, hence a local decrease in surface load, hence a local increase in surface tension, hence motion of the film surfaces in the direction of the thin spot, hence a Marangoni effect, i.e., flow of liquid toward the thin spot, hence a self-stabilizing mechanism. Actually, a more elaborate treatment of film stability is needed (see Section 13.4.1), but the Gibbs mechanism is essential. 

On the basis of a differential equation Ivanov (1977) described all stages of thin liquid film evolution. He distinguished the effects of Marangoni-Gibbs and of surface viscosity. Additionally, the substantial effects of surface diffusion and slow adsorption (barrier or kinetic controlled mechanisms) are taken into consideration. A selection of basic equations can be find in Chapter 4. 

Besides this bulk phase elfects surfactants will adsorb at the liquid-liquid interface. Their influence on mass transfer may then be on different mechanism. A blocking effect of adsorption layers in a diffusional transport regime is well known and results in a reduction of mass transfer [54-57] and even Marangoni instabilities [58,59] are found. However, in the kinetical mass-transfer regime, both an enhancement and retartion of mass transfer [59] is with Gibbs surfactant layers. With extracting ionic species, ionic surfactants will induce an electrostatic double layer, which can be related to the -potential. As a result, there exists, in addition to the chemical potentials, an... 

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