Direct measurement of particle-bubble interactions in aqueous electrolyte: Dependence on surfactant

The aim of this study was to determine the influence of surfactant on the particle-bubble interaction. Therefore we constructed an experimental setup with which the interaction between colloidal particles and air bubbles in aqueous medium can be directly measured. The particles were attached to atomic force microscope cantilevers that served as force sensors. The separation between particle and bubble was adjusted with a calibrated piezo translator. Force and distance resolution were 0.1 nN and 1 nm at full dynamic ranges of roughly 1 mu N and 15 mu m, respectively. With this setup the force between hydrophilic and hydrophobized silica particles (5 mu m diameter) and an air bubble was measured. For both cases the influence of dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate (SDS) was determined. These surfactants were chosen because of their different electric charge and adsorption behavior. DTAB is positively charged and adsorbes strongly to silica. SDS is negative and shows only negligible adsorption to silica. The measured forces could be interpreted in terms of the electrostatic double-layer force, the van der Waals repulsion, the hydration repulsion, a hydrophobic attraction, and the contact angle.