MECHANISMS FOR ANTIFOAMING ACTION IN AQUEOUS SYSTEMS BY HYDROPHOBIC PARTICLES AND INSOLUBLE LIQUIDS

The antifoaming of aqueous surfactant solutions by mixtures of nonpola r oils and hydrophobic solid particles was investigated and a new mech anism was suggested. According to this mechanism, the antifoam drops c ontaining the particles flow from the lamellae of a static foam into i ts plateau borders and get trapped there. The trapped oil drops enter the bubble surfaces and form lenses because the pseudoemulsion films, i.e., the aqueous films between the antifoam drops and the gas phase, are destabilized by the solid particles. On further foam drainage thes e lenses get trapped in the thinning plateau borders and form a bridge whereby the hydrophobic particles destabilize the film between the le ns and the opposite surface of the plateau border. The driving force o f the antifoaming action is the capillary pressure in the plateau bord ers of foam. Antifoaming by oil alone is much less effective because t he process is controlled by the instability of the pseudoemulsion film and, without hydrophobic particles, this film is stable. The hydropho bic particles alone are less effective than the mixture because the pa rticles alone penetrate less into the aqueous phase than the oil + sol id lenses. For high efficiency, the antifoam drops must be large enoug h because smaller drops are less likely to get trapped in the plateau borders. The effect of oil viscosity, the presence of solid particles in the oil, and the antifoam deactivation process on the antifoam drop size and activity are discussed. (C) 1994 Academic Press, Inc.