Foams liquid crystal stabilization

The role of the liquid crystal in stabilizing a foam can be related to its effect on several mechanisms involved in foam loss, including hydrodynamic drainage, the mechanical strength of the liquid film, and the diffusion rate of... 

The role of various surfactant association structures such as micelles and lyotropic liquid crystals (372), adsorption-desorption kinetics at liquid-gas interfaces (373) and interfacial rheology (373) and capillary pressure (374) on foam lamellae stability has been studied. Microvisual studies in model porous media indicate... 

The structure of whipped topping is thus completely different from that of whipped dairy or liquid imitation creams. In the latter systems the air bubbles appear to be covered in a monolayer of fat globules, which are rarely deformed and which protrude with a substantial part of their volume into the air phase of the bubbles. If large fat crystals are present, they are considered detrimental to foam stability, in contrast to whipped toppings6 (Figure 7). 

In view of the importance of macroscopic structure, further studies of liquid crystal formation seem desirable. Certainly, the rates of liquid crystal nucleation and growth are of interest in some applications—in emulsions and foams, for example, where formation of liquid crystal by nonequilibrium processes is an important stabilizing factor—and in detergency, where liquid crystal formation is one means of dirt removal. As noted previously and as indicated by the work of Tiddy and Wheeler (45), for example, rates of formation and dissolution of liquid crystals can be very slow, with weeks or months required to achieve equilibrium. Work which would clarify when and why phase transformation is fast or slow would be of value. Another topic of possible interest is whether the presence of an interface which orients amphiphilic molecules can affect the rate of liquid crystal formation at, for example, the surfaces of drops in an emulsion. 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [411]. For example, in some cases the addition of a small amount of non-ionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer, which is possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [25,110], In general, some very stable foams can be formed from systems in which a liquid crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid crystal phase is present, stable foams are not produced. In this connection foam phase diagrams may be used to delineate compositions that will produce stable foams [25,110],... 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

Liquid crystal structures are important not only in the viscosity modification of surfactant solutions, but also in the stabilization of foams and emulsions, in detergency, in lubrication (Boschkova, 2002), and in other applications. 

The presence of a third phase can promote or impair foam stability, and in some cases, even prevent foaming. As mentioned previously, stable foams can be formed from mixtures of an isotropic liquid with a liquid-crystal phase The foam lamellae become covered with layers of liquid-crystal the foam stability is increased through surface viscosity. Foam stability can also be affected by the presence of other dissolved species, an additional liquid phase such as oil in an aqueous foam, or fine solids. In these cases, whether the effect is one of stabilizing or destabilizing depends on several factors. First, it depends on whether or not the third-phase species have a strong affinity for the liquid phase, and therefore whether they tend to accumulate at the gas—liquid interface. Second, once accumulated, any effect they may have on the interfacial properties is important. 

Because of the limited magnitude of surface tension gradients and absence of electric double-layer effects, the stabilization of foams in nonpolar liquids requires other ways of retarding the thinning of foam lamellae. These include the high liquid-phase viscosity that has been discussed earlier and increased surface viscosity because of presence of highly viscous or even rigid liquid-crystal films. 

As we have seen, the stability of foams depends on a wide variety of factors involving several aspects of surface science. The potential importance of liquid crystal (LC) formation to emulsion stability was pointed out in the previous chapter. Not surprisingly, an equally important role for such structures has been identified in foaming applications. Although the phenomenon of LC stabilization of aqueous foams has been recognized for some time, their role in nonaqueous foaming systems has been less well documented. Recently, it has been shown that the presence of a liquid crystalline phase can also serve as a sufficient condition for the production of stable foams in organic systems. 

FIGURE 12.7. The presence of surfactant liquid crystals may add stability to a normal foam (a) by forming a semirigid structure in the plateau border regions (b) and/ or thick lamellar films that provide mechanical as well as colloidal resistance to drainage. 

In addition to thin film models, the stabilizing action of liquid crystals in foams has been well established, especially in the case of nonionic surfactant systems. At low concentrations, no significant structure buildup has been detected. However, in the early 1960s convincing evidence of a stabilizing effect of liquid crystals at high concentrations was reported. It was suggested that the crystals had a twofold function, in that... 

Foams are coarse dispersions of gas in a relatively small amount of liquid. In solid foams, the continuous phase is a solid which, at one point in time, has been a liquid. Pure liquids cannot form foams and nearly all liquid foams are thermodynamically unstable. Only liquids containing a surface-active eomponent can form foams. The surface-active component may be a solute dissolved in the liquid or insoluble matter at the interface, such as a solid in the form of insoluble particles, a liquid-crystal phase, or an insoluble monomolecular film. Surfactants can stabilize but also destabilize foams and cause their collapse. 

As already mentioned above, the functional properties of whippable emulsions depend largely on the properties of the fat globules they contain. The fat globules form the skeleton of the foam. The crystallization behaviour inside the fat globules of whippable emulsions is decisive for the stabilization of the foam structure after aeration. It is a well-known fact in the food industry that whippable emulsions made with liquid fats are totally devoid of functionality. 

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