Figure 7.45 A schematic drawing of the antifoaming mechanism of antifoam droplets (i) entering a foam lamella, (ii) bridging-de-wetting and (iii) rupturing the foam wall.

Reproduced from D. Perry, J. Zeng and V. O'Neil, Foam Control in Aqueous Coatings, Dow Corning Corporation, 2001.

The most important action of an antifoam agent is to eliminate surface elasticity - the property that is responsible for the durability of foams. To do this it must displace foam stabiliser. It must therefore have a low interfacial tension in the pure state to allow it to spread when applied to the foam, and it must be present in sufficient quantity to maintain a high surface concentration. Many foams can be made to collapse by applying drops of liquids such as ether, or long chain alcohols such as octanol. Addition of ether, which has a low surface tension, to an aqueous foam will locally produce regions with a low surface tension. These regions are rapidly pulled out by surrounding regions of higher tension. The foam breaks because the ethereal region cannot stretch. Long-chain alcohols also break foams because the surface is swamped by rapidly diffusing molecules so that changes in surface tension are rapidly reversed (that is, elasticity disappears).

Generally more effective and more versatile than any soluble antifoams are the silicone fluids, which have surface tensions as low as 20 mN m1. Quantities of the order of 1-60 ppm prevent foaming in fermentation vats, sewage tanks and dyebaths. Polyfluorinated hydrocarbons will lower surface tensions to the order of 10 mN m1.

Antifoam droplets are seen in Fig. 7.45 entering the foam lamella, bridging between the surfaces, dewetting and then rupturing the foam wall.

Table 7.7 In vivo effects of polydimethylsiloxane (PDMS)a

Treatment by mouth Volume Dose (mg/rat) Mean percentage reductionb in

PDMS Silica SEM).

PDMS containing 65 w/v silica 0.005 4.7 0.2 45 ± 6

a Reproduced from R. D. N. Birtley, J. S. Burton etal., J. Pharm. Pharmacol., 25, 859 (1973). b 10 rats in each group; foam induced by saponin.

Table 7.8 In vitro froth test: time taken for antifoam agent to remove experimental foama

Frothing system

Antacid preparationb

Froth reduction

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