AN EXPERIMENTAL-STUDY OF THE ANTIFOAM BEHAVIOR OF MIXTURES OF A HYDROCARBON OIL AND HYDROPHOBIC PARTICLES

An experimental study of the synergistic antifoam behaviour of hydroph obic particle-hydrocarbon oil mixtures is presented here. The system f or study has been selected to minimise specific effects in order to hi ghlight the phenomenology of the antifoam process. Thus, liquid paraff in and a commercial sodium alkyl benzenesulphonate were chosen as oil and surfactant respectively, because this oil exhibited a negative spr eading coefficient at the air-water surface of the solution of that su rfactant under all circumstances investigated. These circumstances inc luded situations where the air-water surface was rapidly expanded to r epresent the conditions prevailing during foam generation. In the main , finely divided silica, hydrophobised with trimethylsilane, was used as the particulate component. Again, this material had no tendency to spread at air-water surfaces and had no effect on the spreading behavi our of the liquid paraffin. The study was also extended to include exa mples of intrinsically hydrophobic organic particles such as calcium s tearyl acid phosphate. Contact angles, electrophoretic mobilities, ele ctron microscopy and observations of emulsion behaviour all suggested that these oil-particle mixtures form composite entities where the par ticles tend to adhere to the oil-water surface with a contact angle (m easured through the aqueous phase) theta(ow) > pi/2. The particles als o exhibited a finite contact angle theta(aw) at the air-water surface so that theta(aq) < pi/2. Weak antifoam effects associated with the pa rticles alone probably concerned dewetting and bridging in foam films to form holes in a manner analogous to that found previously with poly tetrafluoroethylene particles. Preliminary observations indicated that the presence of particles adhering to the oil-water surface facilitat es the emergence of oil droplets into the air-water surface by rupturi ng unsymmetrical oil-water-air films. The observations reported here a re consistent with an antifoam mechanism where oil droplets form mecha nically unstable bridging lenses in foam films. Formation of such conf igurations requires rupture of oil-water-air films by particles. The c ontact angle requirement for rupture of such films is less severe than that required for symmetrical air-water-air films. Therefore particle s may promote the emergence of oil droplets into air-water surfaces to form lenses without themselves exhibiting significant antifoam effect s.