Foaming metastable

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. 

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,...

A comparison of the properties of the bulk micellar systems with those of the films in metastable equilibrium, in particular NBF, is of special interest. The existence of a correlation between the temperature dependent phase transition in NBF stabilised with phospholipids and the analogous phase transition taking place in the bulk phase is to be further discussed (see Section 3.4.4). Undoubtedly, for the systems considered the establishment of a similar correlation between the foam films and the bulk solubilising phases is worth studying. 

The quantitative investigation of the films in metastable equilibrium under constant capillary pressure was to a considerable extent impeded by the delayed film drainage (being due to the higher viscosity of the bulk solutions [348]). In that sense the Pressure Balance Technique for investigation of microscopic single foam films allowed to detect all the metastable states of those films. Thus, a phenomenon not previously described has been... 

Dependence of the lifetime of foam bilayers on the concentration of dissolved surfactant. The stability of foam, emulsion and membrane bilayers can be characterised by their mean lifetime r which is the time elapsing form the moment of formation of a bilayer with a given radius until the moment of its rupture. Obviously, this is a kinetic characteristic of the bilayer stability and can only be applied to thermodynamically metastable bilayers. 

The concentrations Ce and Cc of the monomer surfactant in the solution are important parameters for a given bilayer/solution system, as they determine the ability of the foam bilayer to exist in metastable equilibrium in the range Cc < C < Ce. For C > Ce the bilayer is... 

Because of the strong t(C) dependence at and below a given surfactant concentration Cc the foam bilayer cannot be observed experimentally, as it ruptures instantaneously. Hence, Cc is the experimental limit of bilayer metastability and is determined by the resolution of the measuring equipment. For NaDoS foam bilayers Cc - 1.2-10 4 mol dm 3, a value which coincides with that of the lowest bulk surfactant concentration at which maximum packing of the adsorption monolayer at the solution/air interface is attained [332,366]. 

The adsorption isotherms of NaDoS vacancies calculated in [424], from Eq. (3.13) with the aid of the values of Q and Co given previously are shown in Fig. 3.90 for 10°C (curve 1), 22°C (curve 2) and 30°C (curve 3). The equilibrium NaDoS concentrations Ce above which NaDoS foam bilayers of infinitely long lifetime are to be obtained are marked by arrows. In practice, this cannot be achieved because CMC < Ce. The hatched area shows the investigated concentration interval in which the gas of NaDoS vacancies in the foam bilayer undergoes a 2D first-order phase transition from a dilute phase (existing metastable bilayer) into a condensed phase (ruptured bilayer). 

The determination of the binding energy of DMPC molecule in the foam bilayer was carried out using the experimental results for the temperature dependence of the critical concentration for formation of foam bilayer (Fig. 3.95) and the theory of Kashchiev-Exerowa (see Section 3.4.4.2). The concentrations Cc and Ce (Eq. (3.129)) are specific constants of each system which determine the ability of a foam bilayer to exist in a metastable state within the concentration range Cc< C < Ce. When C >Ce the foam bilayer is thermodynamically stable (there is no driving force for the whole nucleation process in the foam bilayer). It follows from the theory that the critical concentration of amphiphile molecules in the solution equals the equilibrium one (Cc = Ce) in the case of a missing metastable region when only thermodynamically stable foam bilayers are formed. As mentioned above, the DMPC foam... 

By many properties emulsion aqueous films are analogous to foam films. There are several review articles dedicated to properties of emulsion aqueous films [e.g. 320,503-506]. The properties of microscopic emulsion aqueous films (kinetics of thinning, determination of equilibrium thickness, etc.) are studied employing devices quite similar to those used for foam films [503]. Analogous to foam films, stable (metastable) emulsion films are formed only in the presence of surfactants (emulsifiers) at concentrations higher than the critical concentration of formation of black spots C or the concentration, corresponding to... 

At large surfactant concentrations emulsion films as well as foam films exhibit a layer-by-layer thinning (stratification) and metastable black films are formed [31,347,512], Such a behaviour has been reported for hydrocarbon films obtained from solutions of lecithin in either benzene or a mixture of chloroform and decane at concentration higher than 0.6-0.8% as well as in films from oxidised cholesterol in decane [31,512]. Manev et. al. [347] have reported stratification of O/W type emulsion films, toluene being added as a disperse phase, occurring within a surfactant (NaDoS) concentration range of 0.017-0.14 mol dm 3. The number of metastable states was 5-6. Compared to foam films of analogous composition, the respective emulsion films were thicker, due to the weaker intermolecular attraction and the stratification occurred at lower surfactant concentrations. 

For aqueous films on organic substrate the condition s < 0 is most often realised. Hence, these films would be unstable over a large range of thicknesses (except diluted adsorption layers) but a possibility to form thicker metastable (II >0) films stabilised by surfactants cannot be neglected (analogous to foam and emulsion films). 

A number of equations for estimation of the coalescence rate in a foam with metastable films are given in [37], For example, the following expression is proposed for the average bubble radius... 

Individual structural elements of the foam, such as films and borders, can be under hydrostatic equilibrium and can correspond to a true metastable state. Therefore, when there is no diffusion expansion of bubbles in a monodisperse foam, its state can be regarded as metastable in the whole disperse system. Krotov [5-7] has performed a detailed analysis of the real hydrodynamic stability of polyhedral foam by solving two problems determination of... 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

The structural and rheological properties of emulsions, blends, and foams are of great importance in the food, cosmetics, oil-field, and packaging industries. By definition, such fluids are thermodynamically unstable or at best metastable. Hence, conditions of preparation are of extreme importance to both the scientific study and the engineering of these fluids. 

---