Adsorption of a rake-type siloxane surfactant onto carbon black nanoparticles dispersed in aqueous media

The adsorption properties of a rake-type siloxane surfactant (consisting of a poly(dimethylsiloxane) backbone with grafted polyether) on carbon black (CB) nanoparticles have been investigated. The CB particles have fractal structure according to transmission electron microscopy and small-angle neutron scattering (SANS) data. Surface compositions, determined by X-ray photoelectron spectroscopy, indicate that the CB surface is partly oxidized, prove the adsorption of the siloxane surfactant onto the CB particles, and suggest that the surface coverage is incomplete. The adsorption obeys the Langmuir isotherm at low filtrate concentrations (below the critical micelle concentration, cmc). At higher filtrate concentrations, a sharp increase in the adsorbed amount is observed. The adsorbed layer thickness was determined by viscometry and dynamic light scattering. These two methods give good agreement on the adsorbed layer thickness (∼30 nm) at surfactant concentrations above the cmc. This thickness is comparable to the hydrodynamic radius of the siloxane micelles in water. SANS experiments, performed under contrast matching conditions where either the CB particles or the siloxane surfactant was rendered "invisible", confirm that the adsorbed layer has structure and dimensions similar to those of siloxane surfactant micelles in aqueous solution in the absence of CB.