Thin films surfactant-stabilized

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.)...

Exerowa and co-workers [201] suggest that surfactant association initiates black film formation the growth of a black film is discussed theoretically by de Gennes [202]. A characteristic of thin films important for foam stability, their permeability to gas, has been studied in some depth by Platikanov and co-workers [203, 204]. A review of the stability and permeability of amphiphile films is available [205]. 

It is argued that the kinetics of the limited coalescence process is determined by the uncovered surface fraction 1 - t and by the rate of thinning (drainage) of the films formed between the deformable droplets [46,47], The homogeneous and monodisperse growth generated by limited coalescence is intrinsically different from the polydisperse evolution observed for surfactant-stabilized emulsions. As noted by Whitesides and Ross [48], the mere fact that coalescence halts as a result of surface saturation does not provide an obvious explanation of the very... 

Therefore, the stability and lifetime of such thin films will be dependent on these different characteristics. This is evident from the fact that, as an air bubble is blown under the surface of a soap or detergent solution, it will rise up to the surface. It may remain at the surface if the speed is slow, or it may escape into the air as a soap bubble. Experiments show that a soap bubble consists of a very thin liquid him with an iridescent surface. But, as the huid drains away and the thickness decreases, the bubble approaches the equivalent of barely two surfactant molecules plus a few molecules of water. It is worth noting that the limiting thickness is of the order of two or more surfactant molecules. This means that one can see with the naked eye the molecular-size structures of thin liquid hlms (TLFs) (if curved). 

Bibette has used this method to study the effect of osmotic pressure on the stability of thin films in concentrated o/w emulsions [96], by means of an osmotic stress technique. The emulsion is contained in a dialysis bag, which is immersed in an aqueous solution of surfactant and dextran, a water-soluble polymer. The bag is permeable to water and surfactant, but impermeable to oil and polymer. The presence of the polymer causes water to be drawn out of the emulsion, increasing the phase volume ratio and the deformation of the dispersed droplets (Fig. 10). 

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

Even closer to cell membranes than monolayers and bilayers are organized surfactant structures called black lipid membranes (BLMs). Their formation is very much like that of an ordinary soap bubble, except that different phases are involved. In a bubble, a thin film of water — stabilized by surfactants — separates two air masses. In BLMs an organic solution of lipid forms a thin film between two portions of aqueous solution. As the film drains and thins, it first shows interference colors but eventually appears black when it reaches bilayer thickness. The actual thickness of the BLM can be monitored optically as a function of experimental conditions. Since these films are relatively unstable, they are generally small in area and may be formed by simply brushing the lipid solution across a pinhole in a partition separating two portions of aqueous solution. 

Figure 1. Schematic diagram showing the possible mechanisms of thin film stabilization, (a) The Marangoni mechanism in surfactant films (b) The viscoelastic mechanism in protein-stabilized films (c) Instability in mixed component films. The thin films are shown in cross section and the aqueous interlamellar phase is shaded.

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. 

FRAP measurements of surface diffusion in surfactant or lipid-stabilized thin films... 

An improvement in foam stability was observed as R was increased to >0.15 (Figure 17). This was accompanied by the onset of surface diffusion of a-la in the adsorbed protein layer. This is significantly different compared to our observations with /8-lg, where the onset and increase in surface diffusion was accompanied with a decrease in foam stability. Fluorescence and surface tension measurements confirmed that a-la was still present in the adsorbed layer of the film up to R = 2.5. Thus, the enhancement of foam stability to levels in excess of that observed with a-la alone supports the presence of a synergistic effect between the protein and surfactant in this mixed system (i.e., the combined effect of the two components exceeds the sum of their individual effects). It is important to note that Tween 20 alone does not form a stable foam at concentrations <40 jtM [22], It is possible that a-la, which is a small protein (Mr = 14,800), is capable of stabilizing thin films by a Marangoni type mechanism [2] once a-la/a-la interactions have been broken down by competitive adsorption of Tween 20. 

When surfactants concentrate in an adsorbed monolayer at a surface the interfacial film may provide a stabilizing influence in thin films and foams since they can both lower interfadal tension and increase the interfacial viscosity. The latter provides a mechanical resistance to film thinning and rupture. 

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... 

Electric double layers can be present at the gas/liquid interfaces between bubbles in foams. In this case, since the interfaces on each side of the thin film are equivalent, any interfacial charge will be equally carried on each side of the film. If a foam film is stabilized by ionic surfactants then their presence at the interfaces will induce a repulsive force opposing the thinning process. The magnitude of the force will depend on the charge density and the film thickness. 

In pure liquids, gas bubbles will rise up and separate, more or less according to Stokes law. When two or more bubbles come together coalescence occurs very rapidly, without detectable flattening of the interface between them, i.e., there is no thin-film persistence. It is the adsorption of surfactant, at the gas-liquid interface, that promotes thin-film stability between the bubbles and lends a certain persistence to the foam structure. Here, when two bubbles of gas approach, the liquid film thins down to a persistent lamella instead of rupturing at the point of closest approach. In carefully controlled environments, it has been possible to make surfactant-stabilized, static, bubbles, and films with lifetimes on the order of months to years [45],... 

A thin film of water in air that is stabilized by surfactant. The term is used even though the film is not a film of soap and even where the surfactant is not a soap. See also Fluid Film. 

It has been assumed [30] that if with the decrease in film thickness, the elasticity modulus increases, this leads to extension of the thicker film parts, equalisation of thickness and stabilisation of the film. When the modulus of elasticity decreases with the decrease in film thickness, which is realised at low surfactant concentrations, the thin film parts enlarge during extension. This leads to greater non-uniformity in film thickness and to rupture of the thin parts. The calculations in [30] have shown that, indeed, depending on the surfactant concentration, it is possible to obtain different types of curves for the Ef(h) function and there is a decrease in elasticity with the decrease in film thickness. The poor direct experimental evidence of the relation between the elasticity of a newly formed film and its stability proves qualitatively the assumption that the films become unstable in the range of surfactant concentration where the modulus of elasticity decreases with the diminishing film thickness. 

The effect of foam film type on foam stability can be studied from the tp(Apo) dependences in a wide range of pressure drops, as mentioned above, as well as from the Yl(h) dependences (disjoining pressure isotherms) for single foam films having radii close to those of films in the foam [45,46]. Fig. 7.6 depicts the Tp(Ap0) dependence of foams obtained from NaDoS aqueous solutions with different electrolyte (NaCl) concentrations, i.e. the foams are built up of different types of foam films. The surfactant concentration used in all experiments ensured maximum saturation of absorption layer. All three curves have different courses, corresponding to different film types thin films (curve 1), CBF (curve 2) and NBF (curve 3). On increasing Ap0, the foam lifetime strongly decreases compared with the time for decay in... 

The stability of the concentrated emulsions has a kinetic origin. Repulsive double layer forces together with hydration forces are responsible for stability when the surfactant which is adsorbed upon the surface of the thin films is ionic steric repulsion as well as hydration forces are involved in stability when the adsorbed surfactant is non-ionic. 

One of the mechanisms for stabilizing an interface is shown in Figure C3-8. It concerns the film around a bubble from soapy water. Any stretching of the film causes a local depletion of surfactant so that the surface tension rises. This opposes further thinning and so stabilizes the film. 

When the lamella between two droplets thins and breaks, the droplets on either side coalesce into a single, larger droplet (41,72). Continuation of this backward" process eventually leads to the disappearance of the dispersion, if it is not balanced by the forward" mechanisms of snap-off and division. Lamellae are thermodynamically metastable, and there are many mechanisms by which static and moving thin films can rupture. These mechanisms also depend on the molecular packing in the film and, thus, on the surfactant structure and locations of the dispersed and dispersing phases in the phase diagram. The stability and rupture of thin films is described in greater detail in Chapter 7. 

In froth flow the gas and the liquid are intimately mixed as a froth, with the liquid forming very thin films surrounding the gas bubbles. This flow is created by passing the gas through a glass frit. The froth flow is usually unstable but can be stabilized by surfactants. 

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