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Figure 7.44 Sequence in the formation of a thrombus at a surface involving three stages: contact, adhesion and thrombus growth.

Figure 7.44 Sequence in the formation of a thrombus at a surface involving three stages: contact, adhesion and thrombus growth.

Foams and defoamers 269 7.6 Foams and defoamers

Aqueous foams are formed from a three-dimensional network of surfactant films in air. Foams can be used as formulations for the delivery of enemas and topical products. Foams which develop in production of liquids or in ampoules are troublesome; hence there is an interest in breaking foams and preventing foam formation. The breaking and prevention of liquid foams is less well understood than the stabilisation of foams. It is recognised, however, that small quantities of specific agents can reduce foam stability markedly. There are two types of such agent:

• Foam breakers, which are thought to act as small droplets forming in the foam lamellae (see Fig. 7.45)

• Foam preventatives, which are thought to adsorb at the air/water interface in preference to the surfactants which stabilise the thin films

The latter, however, do not have the capacity, once adsorbed, to stabilise the foam. It is well established that pure liquids do not foam. Transient foams are obtained with solutes such as short-chain aliphatic alcohols or acids which lower the surface tension moderately; really persistent foams arise only with solutes that lower the surface tension strongly in dilute solution - the highly surface-active materials such as detergents and proteins. The physical chemistry of the surface layers of the solutions is what determines the stability of the system.

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 structures and this is often sufficient to rupture the thin lamellae in a well-drained film.

Antifoam

Entry

Bridging

De-wetting

Surfactant

Surfactant

Rupture

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