Surfactants thin-liquid films

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. 

Thin-liquid-film stability and the effects of surfactants... 

THIN-LIQUID-FILM STABILITY AND THE EFFECTS OF SURFACTANTS... 

I.B. Ivanov (Ed.), Thin Liquid Films, Surfactant Sci. Ser., Marcel Dekker, New York, 1988. 

Although methods were available to prepare and investigate isolated air-suspended thin liquid films many years ago [5], they have only been developed further comparatively recently. The most extensive studies have been performed on surfactant-stabilized films using molecules such as sodium dodecyl sulfate [6]. Our apparatus has been developed from the film holders used by this Bulgarian group. 

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... 

Figure 5.6 shows an example of a total interaction energy curve for a thin liquid film stabilized by the presence of ionic surfactant. It can be seen that either the attractive van der Waals forces or the repulsive electric double-layer forces can predominate at different film thicknesses. In the example shown, attractive forces dominate at large film thicknesses. As the thickness decreases the attraction increases but eventually the repulsive forces become significant so that a minimum in the curve may occur, this is called the secondary minimum and may be thought of as a thickness in which a meta-stable state exists, that of the common black film. As the... 

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. 

In order to generate foam, surfaces of thin liquid films always have to be stabilised by layers of surfactants, polymers or particles. This is why pure liquids never foam. Foaming is always accompanied by an increase in the interfacial area and, hence, its free energy. Thus, in a thermodynamic sense foams are basically unstable and are, therefore, sooner or later destroyed. The lifetime of a foam can span a remarkable range from milliseconds to very long duration. 

The special properties of thin liquid films, in particular of foam films, involve studying various colloid-chemical aspects, such as kinetics of thinning and rupture of films, transition from CBF to NBF, isotherms of disjoining pressure, thermodynamic (equilibrium) properties, determination of the electrical parameters of surfactant adsorption layer at the liquid/gas... 

In order to understand the nature of surface forces which characterise the thermodynamic state of black foam films as well as to establish the CBF/NBF transition, their direct experimental determination is of major importance. This has been first accomplished by Exerowa et al. [e.g. 171,172] with the especially developed Thin Liquid Film-Pressure Balance Technique, employing a porous plate measuring cell (see Section 2.1.8). This technique has been applied successfully by other authors for plotting 11(A) isotherms of foam films from various surfactants solutions [e.g. 235,260,261]. As mentioned in Chapter 2, Section 2.1.2, the Pressure Balance Technique employing the porous ring measuring cell has been first developed by Mysels and Jones [262] for foam films and a FI(A) isotherm was... 

The CBF/NBF transition at pH discussed above was performed at constant ionic strength and capillary pressure. Obviously, such a transition can also be realised when the capillary pressure is altered, for instance, with the Thin Liquid Film-Pressure Balance Technique (see Section 2.1.8). Thus, it is possible to conduct the experiments at lower ionic strength which proves to be important when ri(/i) isotherms of Ci0(EO)4 and NP20 [285], and non-ionic sugar-based surfactants [260] are plotted with respect to pH (see Section 3.4.1). 

The comparison of the results for foam films with those for emulsion films has proved to be very useful, especially with respect to emulsion films of the O/W type. Reason for such a comparison provides the fact that in both cases the thin liquid film is in contact with two hydrophobic phases. It is anticipated that the effects related to adsorption and orientation of surfactant molecules at the film/hydrophobic phase interface are very similar, and there are examples illustrating it. Hence, some regularities established for foam films can be applied to emulsion films and vice versa. 

The alveolar surface represents a thin liquid film formed at the interface between the alveolar gas phase and a liquid hypophase covering the epithelium. This film is stabilised by the alveolar surfactant (AS), consisting mainly of phospholipids and proteins. AS plays an important role in alveolar stabilisation in the process of breathing. It is known that AS components exist as individual molecules and as various lipid and protein/lipid micellar structures present in the so-called hypophase and, according to some researchers, form a continuous lipid monolayer at the water/air interface [e.g. 1-4]. 

Effect of Surfactant Type and Concentration. Surfactant concentration and type is of great importance for the stability of thin liquid films and for emulsion stability. Type and concentration of surfactants are responsible for the degree of lowering the interfacial tension and for the viscoelastic properties of droplet surface, as well as for the film thickness between two droplets. 

In the biomedical applications outlined by Ward et al. (7 ), more so than in any other separation application of synthetic polymeric membranes, the goal is to mimic natural membranes. Similarly, the development of liquid membranes and biofunctional membranes represent attempts by man to imitate nature. Liquid membranes were first proposed for liquid separation applications by Li (46-48). These liquid membranes were comprised of a thin liquid film stabilized by a surfactant in an emulsion-type mixture. Wtille these membranes never attained widespread commercial success, the concept did lead to immobilized or supported liquid membranes. In... 

Oscillatory structural forces appear in two cases (1) in thin films of pme solvent between two smooth solid surfaces (2) in thin liquid films containing colloidal particles (including macromolecules and surfactant micelles). In the first case, the oscillatory forces are called the solvation... 

Malhotra, A.K. and Wasan, D.T., Interfacial rheological properties of adsorbed surfactant films with applications to emulsion and foam stability, in Thin Liquid Films, Ivanov, I.B., Ed., Marcel Dekker, New York, 1988, p. 829. 

Kralchevsky PA, Danov KD, Ivanov IB. Thin liquid film physics. In Prud homme RK, Kahn SA, eds. Foams Theory, Measurements, and Applications. Vol. 57. Surfactant Science Series. New York Marcel Dekker, 1996 1-98. 

Ivanov, LB., (Ed.), "Thin Liquid Films", Marcel Dekker, Inc. New York, Basel (Surfactant Science Series), Vol. 29, (1988)... 

Kretzschmar Voigt (1989) have recently examined the contribution of interacting forces in surfactant adsorption layers to the film pressure. A detailed knowledge of the geometry of the electrical double layer with respect to the plane of the interface is an essential item in the theoretical description of charged monolayers, thin liquid films and membranes. Fig. 2.12. shows an illustration of structural and energetic aspects of the surfactant monolayer formation. 

Grimson, M.J., Richmond, P. and Vassiliev, C.S., in "Thin Liquid Films", Surfactant Science Sen, l.B. Ivanov, Ed., Vol. 29, Marcel Dekker, 1988 Guggenheim, E.A., Thermodynamics, North-Holland Publishing Company, Amsterdam, (1959)... 

Israelachvili, J., "Intermolecular and Surface Forces", Academic Press, London, San Diego, New York, Boston, Sydney, Tokyo, Toronto, 1992 Ivanov, LB. and Toshev B.V., Colloid Polymer Sci. 253(1975)593 Ivanov, I.B., Ed., Surfactant Science Ser., "Thin Liquid Films", Vol. 29, Marcel Dekker, 1988 Jaycock M.J. and Parfitt, S.D., Ellis Horwood Ltd., John Wiley Sons, New York, Chichester, Brisbane, Toronto, (1981)... 

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