Gibbs-Marangoni

After the creation of a fresh surface (or an almost bare surface) the adsorption increases until equilibrium has been reached. Liquid flow lateral to the surface creates concentration and surface tension gradients which induce additional surface flow. This effect is called the Marangoni-Gibbs effect. 

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. 

These conclusions can change if micellisation in surfactant solutions is taken into account. If the surfactant concentration exceeds the critical concentration of micellisation (CMC) the surface tension does not change with surfactant concentration. It means that the Marangoni-Gibbs effect and the surface retardation of a bubble also disappear. 

Finally, the effect of the ionic character of the surfactant upon the surface forces. Let us consider first of all the diffusion - electric analog of the Marangoni-Gibbs effect. Since stretching of the film results in adsorption, both film surfaces are charged simultaneously and are pushed away from each other. This makes film thinning difficult. This repulsion of a nonequilibrium double layers takes place at distances which are many times greater than the double layer thickness. 

In order to enhance coalescence in systems stabilized by the Marangoni-Gibbs effect, the interfacial activity of the surfactant must be high enough to account for the interfacial tension gradient created [39,41,42-44]. Some researchers found that the emul-... 

Figure 12 Schematic of the plateau-Marangoni-Gibbs effect As the surface area of the air interface is impacted, a surface tension gradient is created locally resulting in the movement of phospholipid molecules toward areas of stress. (From Ref 99.)...

A is the area of the surface. In a foam, where the surfaces are interconnected, the time-dependent Marangoni effect is important. A restoring force corresponding to the Gibbs elasticity will appear, because only a finite rate of absorption of the surface-active agent, which decreases the surface tension, can take place on the expansion and contraction of a foam. Thus the Marangoni effect is a kinetic effect. 

It is also important that the emulsifier films have sufficient elasticity to enable recovery from local disturbances (see Gibbs-Marangoni effect page 274). 

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 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... 

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. 

The elasticity depends on the rate of film expansion. Under quasistatic equilibrium conditions its values are very low and in such a case it is called Gibbs elasticity. When there is no equilibrium it is called Maiangoni elasticity. The largest value of the elasticity modulus, acquired when the adsorption layer behaves as insoluble one, is called Marangoni dilatation modulus Em). 

Two types of elasticity could be distinguished equilibrium (Gibbs elasticity) and dynamic (Marangoni elasticity). According to Gibbs the modulus of elasticity of the film is... 

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