Silicone antifoam performance: Correlation with spreading and surfactant monolayer packing

The relationship between the spreading of antifoam oils and their performan ce is much discussed in the literature, but a demonstrated connection betwe en antifoam spreading and performance has been lacking. This paper reports the performance of a poly(dimethylsiloxane) (PDMS)-based antifoam on foam p roduced by 12 surfactant solutions. These include single or mixed surfactan t systems, including impure surfactant mixtures to model fabric washing det ergents. The oil film spreading pressure, pi (o/w), is presented as a simpl e and relevant measurement of the thermodynamics of antifoam oil spreading. Antifoaming efficacy was measured as the relative reduction in the initial foam height, DeltaH(rel), using cylinder shake tests at a fixed antifoam d osage. DeltaH(rel) is shown to increase with pi (o/w), demonstrating a stro ng statistical correlation between antifoam oil spreading and its performan ce. Antifoam effectiveness varies with surfactant concentration, surfactant type, and surfactant hydrophobe size and also with increased density of su rfactant packing. Surface shear viscosity, mu (s), was used to quantify sur factant packing. Antifoam effectiveness decreases with increasing surface s hear viscosity. This finding provides a potentially useful link between ant ifoam efficacy and surfactant selection based on well-established surfactan t molecular packing parameters. The role of spreading of antifoam oil at th e air/surfactant solution interface is investigated. Oil film spreading pre ssure is shown to decrease by a power law function with increasing surface shear viscosity of the surfactant film. A new fluorescence technique was us ed to measure the extent of PDMS spreading. Initial results suggest a corre lation between the spreading distance and antifoaming performance. An antif oam mechanism is proposed that features antifoam spreading as a direct Cont ributor to bubble film rupture and incorporates surfactant type and concent ration, surfactant packing density, and antifoam oil film spreading pressur e as factors contributing to antifoam efficacy.