Liquid foam stabilization

From the liquid foam stability theory it follows that a liquid foam is most stable when the gas bubbles are strictly spherical in shape since, according to the Laplace and Plateau laws, the interface area and the capillary pressure have minimum values in this case. As it has been shown in Sect. 4.1 for the monodispersed... 

FIGURE 10.1 Schematic structure of a liquid foam stabilized by amphiphilic molecules. 

Both high bulk and surface shear viscosity delay film thinning and stretching deformations that precede bubble bursting. The development of ordered stmctures in the surface region can also have a stabilizing effect. Liquid crystalline phases in foam films enhance stabiUty (18). In water-surfactant-fatty alcohol systems the alcohol components may serve as a foam stabilizer or a foam breaker depending on concentration (18). 

Bendure indicates 10 ways to increase foam stability (1) increase bulk liquid viscosity, (2) increase surface viscosity, (3) maintain thick... 

Dishwashing foam stability performance of an LAS-based light-duty liquid (LDL) is strongly affected by the carbon chain distribution, by water hardness, and, under some conditions, by phenyl isomer distribution. Foaming characteristics of C)2 phenyl isomer blends have been reported previously for conditions where LAS is the single anionic surfactant in the formulation (phosphate-built laundry powder) and the level of residual water hardness is low [30,31]. Under these conditions the internal phenyl isomers of C,2 LAS gave better foam performance than the 2-phenyl isomer. 

FIG. 11 Hardness effect on foam stability of LAS homologs in a 24% LAS/6% AES/ 2% amide dishwashing liquid. Conditions 46°C, 0.05% concentration, Keen soil. (From Ref. 19.)... 

Chemically, the preparation of a "stable" foam or emulsion requires the use of a surfactant to aid in dispersion of the internal phase and prevent the collapse of the foam (or emulsion) into separate bulk phases. The selection of a surfactant is made on the basis of severity of conditions to be encountered, the gas to be entrained (N2, C02, LPG, CH, or air), the continuous phase liquid (water, alcohol, or oil), and half-life of foam stability desired. 

R=Ci2-C18 Foam stabilization Emulsifier Solubilizer, antistatic, wetting agent in PPCPs Hair shampoo, liquid soaps, shaving creams and other PPCPs No data found Contradictory data related to they biodegradability [19]. Readily biodegradable [20,21]... 

The work of Calderbank and Rennie (C4) has been criticized by Sargent and Macmillan (S2) on the basis that the liquid flow conditions used by Calderbank and Rennie (C4) are not found in distillation columns. They (S2) consider that cellular foams are formed for dilute aqueous solutions only when low gas flow rates are employed. By using an n-pentane-isopentane system, Macmillan (Ml) found that for all gas flow rates, froths with densities less than 0.15 were formed and the froth densities were independent of the factor vs(pg)112 but dependent on tray geometry. The associated problem of foam stability has also attracted considerable attention (Al, D3, Zl). 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

It has been shown (Friberg, 2003 Birdi, 2002, 2008) that there exists a correlation between foam stability and the elasticity [E] of the film (i.e., the monolayer). In order for E to be large, surface excess must be large. Maximum foam stability has been reported in systems with fatty acid and alcohol concentrations well below the minimum in y. Similar conclusions have been observed with -C12H25S04Na [SDS] + -C12H25OH systems that give minimum in y versus concentration with maximum foam at the minimum point (Chattoraj and Birdi, 1984). Because of mixed mono-layer formation it has been found that SDS + C12H25OH (and some other additives) make liquid-crystalline structures at the surface. This leads to a stable foam (and liq-... 

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. 

The stability of foams is usually measured by the volume of liquid drained from a foam during a specific time at room temperature (10. 15. 17) or by a decrease in foam volume over time. Methods employed to measure foam stability include the rate of fall of a perforated weight through a column of foam (20. 22), the penetration of a penetrometer cone (17), or the ability to support a series of specific weights (277. 

Defoamlng plates. Foam at the interface may occur as gas bubbles break out of the liquid. Foam can be stabilized with addition of chemicals at the inlet. Often a more effective solution is to force foam to pass through a series of inclined... 

In addition to the film elasticity, other factors that may affect foam stability arc surface shear viscosity, bulk viscosity of the foaming liquid, and the presence of particulate matter. 

Aerosol Shampoos. These shampoos constitute a very small percentage of the market. They have been available in two versions, ie, liquid foam types and dry spray forms. The liquid foam type, despite its convenience and appealing appearance, did not attain high general use. Factors involved in its low acceptability include not only higher product cost but also serious stability issues with can corrosion. 

The main factor determining the stability of such foams is the rate and extent of drainage from the thin liquid film. In general, this type of foam is relatively unstable. The stability may be enhanced by increasing the viscosity of the liquid by increasing the dry matter content or adding certain hydrocolloids. The foam stability may also be enhanced with hydrocolloids, in particular microcrystalline cellulose. 

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). 

Foams are always thermodynamically stable. The stability of liquid foams is largely determined by the repulsion between surfactants and the viscosity of the liquid. They decay by drainage driven by the negative Laplace pressure in the Plateau borders. 

Bendure indicates 10 ways to increase foam stability (1) increase bulk liquid viscosity, (2) increase surface viscosity, (3) maintain thick walls (higher liquid-to-gas ratio), (4) reduce liquid surface tension, (5) increase surface elasticity, (6) increase surface concentration, (7) reduce surfactant-adsorption rate, (8) prevent liquid evaporation, (9) avoid mechanical stresses, and (10) eliminate foam inhibitors. Obviously, the reverse of each of these actions, when possible, is a way to control and break foam. 

A foam is a colloidal dispersion in which a gas is dispersed in a continuous liquid phase. The dispersed phase is sometimes referred to as the internal (disperse) phase, and the continuous phase as the external phase. Despite the fact that the bubbles in persistent foams are polyhedral and not spherical, it is nevertheless conventional to refer to the diameters of gas bubbles in foams as if they were spherical. In practical occurrences of foams, the bubble sizes usually exceed the classical size limit given above, as may the thin liquid film thicknesses. In fact, foam bubbles usually have diameters greater than 10 pm and may be larger than 1000 pm. Foam stability is not necessarily a function of drop size, although there may be an optimum size for an individual foam type. It is common but almost always inappropriate to characterize a foam in terms of a given bubble size since there is inevitably a size distribution. This is usually represented by a histogram of sizes, or, if there are sufficient data, a distribution function. 

---