Hydrophobic interactions exist between two hydrophobic surfaces in aqueous media. Air bubbles are hydro-phobic, and studies on bubble hydrophobicity and bubble-bubble interaction can provide insights into froth flotation. This paper describes the use of the thin film pressure balance technique to determine the surface forces in foam films stabilized by flotation frothers such as methyl isobutyl carbinol and polypropylene glycol in the presence of 0.1 M NaCl. The Reynolds lubrication approximation was used to estimate the hydrophobic force from measured film thinning rates while the capillary wave theory (Valkovska et al., 2002) was used to calculate the hydrophobic force from measured critical rupture thicknesses. The hydrophobic force was expressed by a power law in the same form as the van der Waals force, which makes it possible to directly compare these two forces by means of the hydrophobic force constant (K_(232)) and the Hamaker constant. It was found that the hydrophobic forces were substantially larger than the van der Waals forces. Also, the K_(232) values were insensitive to surface mobility of the foam films.
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