Foamed high-dispersive systems

The assignment of ol meric foams to fine dispersion systems is of considerable importance since it enables to approach the study and modification of structure by using the concepts of dispersion media. The advantages of such an approach are evident if one takes into account the variety of ideas and methods developed in the study of high dispersion systems of non-polymeric nature and the first results of the application of this approach to oligomeric foams 1. 

Foams, as well as other liophobic disperse systems are thermodynamically unstable due to their high interfacial free energy. This high free energy provokes processes that lead to foam coarsening and eventual destruction, i.e. to separation of the liquid from the gas phase. 

The heat transfer in a foam, as in any other physical system, occurs through thermal conductivity, heat radiation and convection [87]. It was established that in disperse systems the heat transfer through radiation is only significant at high temperature (> 100°C) and in the presence of large pores, while convection is effective only if the particles (bubbles in the foam) are large (> 1 mm). This means that thermal conductivity is the basic mechanism of heat transfer at not very high temperatures. 

Foam films and a foam from the aqueous and organic phases of an extraction system containing a 30% solution of tri-buthyl phosphate (TBP) in kerosene and nitric acid (1 mol dm 3) have been studied in a parallel mode [137]. The reasons for foaming and the effect of emulsion formation on foam stability were elucidated. Thus, a foam with a measurable lifetime was obtained when TBP was in concentration of 0.8 mol dm 3 which corresponded to the concentration of black spot formation. When the volume ratio of the organic to the aqueous phase was 1 5, the foam formed in the system was stabilised additionally by a highly disperse O/W emulsion. This was due to the reduced rate of drainage. These results are confirmed by the experimental data acquired with a specially constructed centrifugal extractor [136]. It makes it possible to perform an extraction process under conditions close to those in industry. 

The thinnest-black films have been found to play a particularly important role in the formation of highly stable foams. They are used as models in the study of surface phenomena at various interfaces, molecular interactions between two contacting phases at short distances, including at bilayer contact. This fact in itself is of the utmost importance in studying the formation and stability of concentrated disperse systems and in modelling the contact between the two biomembranes. For this reason the book discusses different aspects of black foam films and some intriguing perspectives for future development, for instance, as a self-organising nanomolecular system, have been pointed out. 

As shown above, the minimal of microcells is in the range of 10 jum (R < 1000 A) and S in the range of 10 cm. Thus, recent data on the structure of oligomeric foams, at least those described above, allow assignment of these foams to high or fine dispersion systems. 

The fact that foamed plastic can be classified among fine-dispersed systems is highly important since their structure may be studied not only from the conventional polymer point of view, but also by applying a different novel and promising approach — the physics and chemistry of disperse materials. 

Foam is a disperse system with a high surface area, and consequently foams tend to collapse spontaneously. Ordinarily, three-dimensional foams of surfactant solutes persist for a matter of hours in closed vessels. Gas slowly diffuses from the small bubbles to the large ones (since the pressure and hence thermodynamic activity of the gas within the bubbles is inversely proportional to bubble radius). Diffusion of gas leads to a rearrangement of the foam stmctures and this is often sufficient to rupture the thin lamellae in a well-drained film. 

When the aqueous system contains finely divided solids, then foaming of the system may be influenced greatly by the nature of the dispersed solid particles. If the particles have a surface that is hydrophobic, and if the particles are divided finely enough, then the particles may adsorb onto the surface of any air bubbles introduced into the system and stabilize them against coalescence. They adsorb at the air-solid interface from the aqueous system because their solid-aqueous solution interfacial tension, ySL, is high and their solid/(nonpolar) air interfacial tension, ySA, is low because of their nonpolar surface. Consequently, their contact angle, 0, with the aqueous phase, from equation 6.3... 

Foam is a disperse system in which the dispersed phase is a gas (most commonly air) and the dispersion medium is a liquid (for aqueous foams, it is water). Foam structure and foam properties have been a subject of a number of comprehensive reviews [6, 17, 18]. From the viewpoint of practical applications, aqueous foams can be, provisionally, divided into two big classes dynamic (bubble) foams which are stable only when gas is constantly being dispersed in the liquid 2) medium and high-expansion foams capable of maintaining the volume during several hours or even days. In general, the basic surface science rules are established in foam models foam films, monodisperse foams in which the dispersed phase is in the form of spheres (bubble foams) or polyhedral (high-expansion foams). Meanwhile, real foams are considerably different from these models. First of all, the main foam structure parameters (dispersity, expansion, foam film thickness, pressure in the Plateau-Gibbs boarders) depend... 

Foams are highly concentrated dispersions of gas (dispersed phase) in a liquid (continuous phase). Concentrated emulsions (biliquid foams) are similar systems in which the dispersed phase is a liquid. The terms foam and concentrated emulsion are often used interchangeably in the literature because the phenomena responsible for their behavior are essentially the same. 

Like most disperse systems, foams can be obtained by condensation and dispersion methods. The condensation methods for generating foam involve the creation of gas bubbles in the solution by decreasing the external pressure, by increasing temperature or as a result of chemical reaction. Thus, bubble formation may occur through homogeneous nucleation at high supersaturation or heterogeneous nucle-ation (e.g. from catalytic sites) at low supersaturation. 

Uses Defoamer for aq. coatings, solv.- and binder-free pigment pastes Features No negative effect on gloss development good foam knockdown in systems which are free from soivs./binders and require high dosage of dispersants Properties Wh. opaque liq. emulsifiable in water vise. = 1000 mPa-s Use Level 0.5-1.5% on total formulation... 

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