Model Studies of the Effect of Silica Hydrophobicity on the Efficiency of Mixed Oil−Silica Antifoams

Mixtures of poly(dimethylsiloxane) oil and hydrophobized silica are used for foam control and are termed antifoam compounds or mixed oil−silica antifoams. In a previous study, we found experimentally a well-pronounced maximum in the antifoam efficiency at a certain optimal silica hydrophobicity. The reasons for this peculiar dependence are studied in the present paper by performing two series of experiments. First, compounds of silica and oil are prepared under different conditions (with and without heating and/or stirring) and the antifoam efficiency of these compounds is compared by foam tests. Second, several characteristics of the studied compounds, such as viscosity, entry barrier, and thickness of the layer of spreading oil, are measured, and their contribution to the compound performance is analyzed. The results show that the optimal hydrophobicity of the silica particles is related to a minimum in the entry barrier (measured by the film trapping technique), which determines how easily the compound globules emerge on the surface of the foam films. Another factor (particularly important for compounds containing insufficiently hydrophobized particles) is the compound yield stress, which governs the deformability of the antifoam globules. If the compound has large yield stress, its dispersiveness in the foaming solution and the oil spreading are inhibited, and the antifoam is inactive. The obtained results clarify the physicochemical properties which determine the antifoam activity of mixed oil−solid compounds.