| Abstract
| - The relationship between the spreading of antifoam oils and their performance is much discussed inthe literature, but a demonstrated connection between antifoam spreading and performance has beenlacking. This paper reports the performance of a poly(dimethylsiloxane) (PDMS)-based antifoam on foamproduced by 12 surfactant solutions. These include single or mixed surfactant systems, including impuresurfactant mixtures to model fabric washing detergents. The oil film spreading pressure, πo/w, is presentedas a simple and relevant measurement of the thermodynamics of antifoam oil spreading. Antifoamingefficacy was measured as the relative reduction in the initial foam height, ΔHrel, using cylinder shake testsat a fixed antifoam dosage. ΔHrel is shown to increase with πo/w, demonstrating a strong statistical correlationbetween antifoam oil spreading and its performance. Antifoam effectiveness varies with surfactantconcentration, surfactant type, and surfactant hydrophobe size and also with increased density of surfactantpacking. Surface shear viscosity, μs, was used to quantify surfactant packing. Antifoam effectivenessdecreases with increasing surface shear viscosity. This finding provides a potentially useful link betweenantifoam efficacy and surfactant selection based on well-established surfactant molecular packingparameters. The role of spreading of antifoam oil at the air/surfactant solution interface is investigated.Oil film spreading pressure is shown to decrease by a power law function with increasing surface shearviscosity of the surfactant film. A new fluorescence technique was used to measure the extent of PDMSspreading. Initial results suggest a correlation between the spreading distance and antifoaming performance.An antifoam mechanism is proposed that features antifoam spreading as a direct contributor to bubblefilm rupture and incorporates surfactant type and concentration, surfactant packing density, and antifoamoil film spreading pressure as factors contributing to antifoam efficacy.
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