Thin-liquid-film elasticity surfactants

Thin-liquid-film stability. The effect of surfactants on film and foam stability. Surface elasticity. Froth flotation. The Langmuir trough and monolayer deposition. Laboratory project on the flotation of powdered silica. 

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. 

Surfactants also reduce the coalescence of emulsion droplets. The latter process occurs as a result of thinning and disruption of the liquid film between the droplets on their close approach. The latter causes surface fluctuations, which may increase in amplitude and the film may collapse at the thinnest part. This process is prevented by the presence of surfactants at the O/W interface, which reduce the fluctuations as a result of the Gibbs elasticity and/or interfacial viscosity. In addition, the strong repulsion between the surfactant layers (which could be electrostatic and/or steric) prevents close approach of the droplets, and this reduces any film fluctuations. In addition, surfactants may form multilayers at the O/W interface (lamellar liquid crystalline structures), and this prevents coalescence of the droplets. 

Surfactants improve foam formation by inducing film elasticity, which is helpful when the film is stretched as a gas bubble emerges from a liquid solution to become part of the foam matrix. As the film stretches, differentia surfactant adsorption at the interface leads to surface tension gradients and healing of the film (so it thins with approximately uniform thickness). The optimum surfactant concentration for foam formation (although not foam stability) is around the CMC. 

One complication that arises with thin-liquid foam film studies is the need to have surface-active components present in order to stabilize the films. Without adequate film stability, measmement of the interactions between the two air-water interfaces caimot be accomplished. These surface-active species provide film stability via surface elasticity and repulsive force interactions between the interfaces (i.e., DLVO-type interactions). In addition, surfactants may interact with polymers added to the system, which can mediate and change the polymer configuration, surface adsorption, and thin-film interactions. Therefore, to determine the role of a polyelectrolyte one must understand independently the various interfacial and polymer-surfactant interactions. Theodoly and colleagues [18,19] have accomplished this through a judicious choice of combined polymer-surfactant mixtures. Two systems... 

Flow of trains of surfactant-laden gas bubbles through capillaries is an important ingredient of foam transport in porous media. To understand the role of surfactants in bubble flow, we present a regular perturbation expansion in large adsorption rates within the low capillary-number, singular perturbation hydrodynamic theory of Bretherton. Upon addition of soluble surfactant to the continuous liquid phase, the pressure drop across the bubble increases with the elasticity number while the deposited thin film thickness decreases slightly with the elasticity number. Both pressure drop and thin film thickness retain their 2/3 power dependence on the capillary number found by Bretherton for surfactant-free bubbles. Comparison of the proposed theory to available and new experimental... 

The second case concerns a thin film. Here the change in 77 is governed by the limited amount of surfactant in the bulk liquid in the film, since by far most of the surfactant will be in the adsorbed layers. The time for diffusional transport (normal to the surfaces) is taken to be negligible in the thin film it would nearly always be < 0.1s. The modulus then is purely elastic. Rather than the modulus, the Gibbs elasticity of the film is given, by... 

The surface tension gradient in the thinning film, which is created by the efflux of liquid from the film and the sweeping of surfactant along the film surfaces to the Plateau borders (Figure 5), can be characterized by the dimensionless elasticity number, Es, which is defined for one surface-active component (21) by... 

Here, is the so called foam parameter, and t is the viscosity m the surfactant-containing phase (Liquid 1 in Fig. 15) the influence of the transition zone film - bulk liquid is not accounted for in Eq. (76). Note that the bulk and surface diffusion fluxes (see the terms with and Z) in the latter equation), which tend to damp the surface tension gradients and to restore the uniformity of the adsorption monolayers, accelerate the film thinning (Fig. 1). Moreover, since Din Eq. (76) is divided by the film thickness h, the effect of surface diflhsion dominates that of bulk diffusion for small values of the film thickness. On the other hand, the Gibbs elasticity Eq (the Marangoni effect) decelerates the thinning. Equation (76) predicts that the rate of... 

---