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 experimen tal setup with which the interaction between colloidal particles and a ir bubbles in aqueous medium can be directly measured. The particles w ere attached to atomic force microscope cantilevers that served as for ce sensors. The separation between particle and bubble was adjusted wi th 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, r espectively. With this setup the force between hydrophilic and hydroph obized silica particles (5 mu m diameter) and an air bubble was measur ed. For both cases the influence of dodecyltrimethylammonium bromide ( DTAB) and sodium dodecyl sulfate (SDS) was determined. These surfactan ts were chosen because of their different electric charge and adsorpti on behavior. DTAB is positively charged and adsorbes strongly to silic a. SDS is negative and shows only negligible adsorption to silica. The measured forces could be interpreted in terms of the electrostatic do uble-layer force, the van der Waals repulsion, the hydration repulsion , a hydrophobic attraction, and the contact angle.