Foams lifetime

The common understanding of foam stability usually refers to the ability of a foam to maintain its main parameters constant with time, i.e. bubble size, liquid content (expansion ratio) and total foam volume. Foam lifetime is most often used as the simplest measure of foam stability. 

The main features of this technique are the absence of contact between the foam and the ambient space (i.e. no foam/gas interface) and constant capillary pressure along the whole foam height. This technique allows to study the kinetics of internal foam destruction at various capillary pressures, i.e. decrease in the specific foam surface area without destruction of the foam column. Thus, the influence of surface foam films on foam lifetime and the character of foam destruction can be estimated. 

The experiments performed in [49] reveal that the foam lifetime depends strongly on the humidity of the blowing air. This is illustrated in Fig. 6.8. However, a quantitative verification of Eq. (6.31) is not possible for the lack of data about film thickness, foam dispersity and rate of evaporation. 

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

In order to collect information about the influence of the pressure drop on the lifetime of foams with different types of foam films, foam columns of small heights (2-3 cm) were studied. It was found that the time needed to reach hydrostatic equilibrium pressure (the outflow of the excess liquid ceases) was considerably smaller (5-6 times) than the foam lifetime. This is realised at small pressure drops (up to 5-10 kPa). For that reason it is believed that in these experiments the foam column destruction runs mainly under equilibrium conditions (referring to hydrostatic pressure and drainage). 

The Tp(Ap) dependence for a 2 cm foam column obtained from a NP20 (0.3 g dm 3) solution containing NaCl (0.4 mol dm 3) has been studied in [18]. The foam stability sharply decreases even at small pressure drops (Ap < 1 kPa). In gravitational field the foam lifetime is xp 4 h, at Ap = 1 kPa, xp = 30 min. Further sharp reduction of the foam lifetime is observed at a pressure drop corresponding to the critical pressure Apcr - 26-27 kPa. 

The role of temperature on the foam stability has been studied in gravitational field [63,72], It was proved that temperature influences strongly the foam lifetime, especially for foams from nonionic surfactants, such as oxyethylene derivatives of alkylphenols, alcohols and acids. 

The values of foam lifetime situated to the left of the flexion points of the respective surfactants practically do not depend on temperature. It is important to note that in all systems studied this temperature is much below the cloud point of the surfactant solutions and,... 

The dependence lnTp(l/7) for a foam from the ionic surfactant NaDS (7-10"3 mol dm"3 + 0.1 mol dm"3 NaCl), presented in Fig. 6.17, has a different course. As it is seen here, both temperature and pressure drop have little influence on foam lifetime. The effective activation... 

A more important fact is the change in the mechanism of foam column destruction with the increase in the applied pressure drop. For example, at small pressure drops a slow diffusion bubble expansion along with the corresponding slow rate of structural rearrangement (either zero or very slow rates of coalescence) occurs in a NaDoS foam with CBF or NBF. This is expressed in the layer-by-layer reduction of foam column height ending with the disappearance of the last bubble layer. In such a foam the critical pressure of the foam column destruction is not reached at any dispersity, and only the foam column height and the rate of internal foam collapse determine the foam lifetime. 

To compare the stability of foams from various surfactants, or at different surfactant concentrations, it is advisable to measure the foam lifetime xp at constant pressure in the Plateau borders. Table 7.1 presents the data characterising the stability of foams formed by several surfactants. 

Foam lifetime under a-particle irradiation z n and under no irradiation zm... 

In one of the first attempts to explain foam stability in terms of thermodynamics it was assumed that the foam lifetime depends on the decrease in surface energy and increases when a definite value of A a is reached [e.g. 34],... 

Thus, it might be assumed that stabilisation of foam films will depend also on the action of other positive components of disjoining pressure. For example, equilibrium films are obtained from concentrated butyric acid solutions and, therefore, in this concentration range the foam lifetime also increases. On the basis of these concepts it should be expected that a foam consisting of films with equilibrium thicknesses at a constant capillary pressure pa = n, should be infinitely stable. In fact, a real foam decays both in bulk and as a disperse system, due to gas diffusion transfer and certain disturbances (shift of films and borders on structural rearrangement as a result of the collective effects , etc.)... 

The study of a large number of various surfactants in aqueous and non-aqueous media has shown that a sharp transition towards films of high stability at increasing surfactant concentrations is always related to the appearance of black spots in the microscopic films [17,42,43]. It was established that the surfactant concentration corresponding to black spot formation lies in the range of sharp increase in the dependence foam lifetime x on surfactant concentration C. 

Fig. 7.5. Foam lifetime r vs. surfactant concentration (a) oxyethylene dodecyl alcohol, n is the number...

The method for investigating foams at high pressure drop in Plateau borders permits the estimation of foam stability under strictly defined conditions (see Section 7.2). This method enables measurement of foam lifetime tp at a certain constant pressure as well as at reaching the critical state of the film. 

Fig. 7.6. Foam lifetime T(in hours) vs. pressure drop Ap in the foam liquid phase curve 1 - 3 1 O 3 mol...

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... 

In order to develop efficient techniques for the preparation and application of foams in industry, agriculture, firefighting, etc., it is necessary to know the physicochemical parameters of surfactants and their relationship with the foam stabilising ability of the surfactant solutions. Usually the criterion of the surfactant foaming ability is the adsorption of these compounds at the solution/air interface and the related to it properties, such as decrease in surface tension, adsorption work, maximum adsorption T. [13,39,43]. CMC is often used as a characteristic of a foaming agent (if micellisation is possible in the surfactant solution). Parameters related to foam stability, such as foam lifetime and foam column height, are also employed [12,13,39],... 

These physicochemical constants deal with the properties of the foam films. The foaming agents can be described more fully by the parameters related to the foam itself, e.g. foam lifetime xp at constant pressure and the time of foam column destruction by a-particle irradiation Xm [17,75], Table 7.4 presents xp and Xr of various surfactants. 

It is well know that the direct comparison between the methods of estimation of foam stability (in most of the cases it is determined by the foam lifetime) is not possible. Each of the existing methods involves different parameters, for example, time for destruction of a foam column of a definite height (or part of it), rate of decrease in the specific foam surface, etc. The main reason for the impossibility to make such a comparison is that foam stability is determined at different pressures in the foam liquid phase. This means that the rate of drainage as well as the time of reaching an equilibrium state of the films in the foam is different. Another reason could be attributed to the possibility both foam formation (i.e. foam volume... 

Fig. 7.10. Foam lifetime Xp vs. number n of carbon atoms in the alkylsulphonates homologues ...

Fig. 7.14. Foam lifetime Tp vs. applied Ap for NP20 foams curve I - black films (2-I0 4 mol dm"3 NP20 +...

Fig. 7.16 depicts the dependence foam lifetime r of a NP20 foam on the surfactant concentration (at constant ionic strength - 10 3 mol dm 3 and various pH). It is clearly seen that in the concentration range studied at pH = 3, ris considerably shorter than that at pH = 6.1. 

Fig. 7.16. Foam lifetime T vs. concentration C, of NP20 at constant ionic strength 10 3 mol dm 3 curve 1...

Fig. 7.17. Foam lifetime t vs. concentration C, of CiotEO) at constant ionic strength 3-Iff4 mol dm 3 ...

---