Metastable foams

Alcohol sulfates are excellent foaming surfactants. According to the Kitchener and Cooper classification [148], alcohol sulfates form metastable foams. However, quantitative values cannot easily be compared because foam largely depends not only on the instrument used to produce and evaluate foam but also on the concentration of surfactant, impurities, temperature, and many other factors. In addition, a complete characterization of the foam capacity should take into account the initial amount of foam, its stability, and its texture. 

Mechanical agitation of the cream - a process called whipping - creates a metastable foam (i.e. it contains much air). Further whipping causes this foam to collapse some water separates out, and the major product is yellow butter. Incidentally, butter is a different form of colloid from milk, since its dispersed medium is water droplets and its dispersal phase is oil (milk is an oil-in-water colloid). Forming butter from milk is a simple example of emulsion inversion. 

In order to investigate foam stability, it is important to consider the role of the plateau border under dynamic and static conditions. Foam films with intermediate life times - that is, between unstable and metastable foams - should also be considered. 

Figure 5.15 shows an example of a disjoining pressure isotherm in which the steric force contributions have been superimposed on the classical DLVO force contributions. It can be seen that this creates two regions for metastable foam films. One region is the thick, common black film region, with film thicknesses of approximately 50 nm or so. The other region is the thin, Newton black film region, with film thicknesses of 4nm. While the common black films are mostly stabilized by electrostatic forces, the Newton black films are at least partly stabilized by the steric forces. 

Some mildly surface active compounds will stabilize weakly stable, transient foams. Examples include short-chain alcohols, short-chain fatty acids, aiuline, phenol and pine oil. These weak frothers tend to produce foam films having stabilities on the order of seconds. More strongly surface active compounds can stabilize quite strong, metastable foams. Examples include long-chain alcohols and fatty acids and proteins. These strong frothers tend to produce foam films having stabilities on the order of minutes to hours. 

The above metastable foams are produced from surfactant solutions near or above the critical micelle concentration (c.m.c.). The stability is governed by the balance of surface forces (see below). Film thickness is comparable to the range of intermolecular forces. In the absence of external disturbances, these foams may stay stable indefinitely They are produced using proteins, long-chain fatty acids or solid particles. 

Metastable foams are capable of withstanding ordinary disturbances (thermal or Brownian fluctuations, etc.) but collapse in an irregular manner from abnormal disturbances (evaporation, temperature gradients, etc.). 

High-stability (metastable) foams are prepared from detergents, proteins, long-chain fatty acids (near or above the critical micelle concentration (CMC)), particles, etc. These foams usually have a lifetime up... 

Unstable (transient) and high-stability (metastable) foams... 

Figure 2.2. Examples of (a) transient and (b) metastable foams (from V. Bergeron in ref. (2)), with kind permission from Kluwer Academic Publishers...

For convenience, foams have been classified into two extreme types unstable or transient foams with lifetimes of seconds and metastable (or pamanrait) foams with lifetimes measured in days. Metastable foams can withstand ordinary disturbances (Brownian fluctuations), but coUq)se from abnormal disturbances (e.g. evq)oration or temperature gradients). 

Uses Foaming agent for unstable and metastable foam coatings Properties YIsh. cl. liq. 

Metastable foams have a persistence lasting from a few minutes to months. They are stabilized at the liquid-gas interface by the presence of amphiphilic and/or polymeric materials that retard the loss or drainage of liquid from the area between bubbles, or form a somewhat rigid, mechanically strong bilayer that maintains the foam stmcture. Because the stabilizing structure of the metastable foams is fluid, it can be disrupted by a number of factors such as vibration, dust particles, evaporation, pressure, and other environmental changes. Even the most stable of... 

However, certain kinds of foams are known to persist for very long periods of time, and many attempts have been made to explain their metastability. 

To produce a foam, stable or metastable, it is necessary for surface-active molecules... 

Let us discuss the structure of a metastable polyhedral foam in a bit more detail. In pioneering experimental studies Joseph Plateau5 established some simple rules in the second half of the 19th century. Three of these... 

The thin liquid films bounded by gas on one side and by oil on the other, denoted air/water/oil are referred to as pseudoemulsion films [301], They are important because the pseudoemulsion film can be metastable in a dynamic system even when the thermodynamic entering coefficient is greater than zero. Several groups [301,331,342] have interpreted foam destabilization by oils in terms of pseudoemulsion film stabilities [114]. This is done based on disjoining pressures in the films, which may be measured experimentally [330] or calculated from electrostatic and dispersion forces [331], The pseudoemulsion model has been applied to both bulk foams and to foams flowing in porous media. 

Surface forces measurements with microscopic foam films permitted to study in details the long-range/short-range interaction transition, including the reversal transition occurring in some cases. A fluctuation zone of existence of metastable films is found, governed by the two types of forces. 

The 9(Cei) dependence has been studied with microscopic foam films from other surfactants [324], For instance, films from cationic surfactants exhibit the same course of the dependence. It has to be noted, however, that the metastable region of such films lays within a... 

Therefore, the study of foam film behaviour under a-irradiation is an excellent detector for the CBF/NBF transition. Curve 2 in Fig. 3.65,b indicates Ta for CBF. The dashed line refers to ra for NBF up to the transition. These curves distingish the region in which metastable CBF exist. The values of Cei.Cr found under a-particle irradiation are presented in Table 3.9. They are in good correlation with the ones obtained by other techniques [253,323]. 

Fig. 3.76 presents an analogous P(h) isotherm of foam films obtained from system n. Here stratified foam films were also observed. At constant p0 (measuring cell A), seven metastable states of the films (in the various experiments) with thicknesses ranging from 82.1 to 45.2 nm were distinguished. The latter thickness was the lowest that could be realised by a spontaneous stepwise thinning. Spontaneous and forced transitions followed upon pressure increase, similar to those shown in Fig. 3.75. The final thickness reached was about 5.6 nm, i.e. a bilayer film. Therefore, on imposing a definite pressure on the films of both systems,...

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