Hydrophobic Particles as Antifoamers

Many solid particles with some degree of hydrophobicity cause destabilisation of foams, e.g. hydrophobic silica, PTFE particles. These particles exhibit a finite contact angle when adhering to the aqueous interface. It has been suggested that 

With particles containing rough edges, the situation is more complex, as demonstrated by Johansson and Pugh [32], using finely ground quartz particles of different size fractions. The particle surfaces were hydrophobised by methylation. These and other reported studies confirmed the importance of size, shape and hydropho-bicity of the particles on foam stability. 

The synergetic antifoaming effect of mixtures of insoluble hydrophobic particles and hydrophobic oils when dispersed in aqueous medium has been well established in the patent literature. These mixed antifoamers are very effective at very low concentrations (10-100 ppm). The hydrophobic particles could be hydrophobised silica and the oil is poly(dimethyl siloxane) (PDMS). 

One possible explanation of the synergetic effect is that the spreading coefficient of PDMS oil is modified by the addition of hydrophobic particles. The oil-partide mixtures are suggested to form composite entities where the particles can adhere to the oil-water interface. The presence of particles adhering to the oil-water interface may facilitate the emergence of oil droplets into the air-water interface to form lenses, leading to rupture of the oil-water-air film. 

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