Effect of Surface Charge Density on the Affinity of Oxide Nanoparticles for the Vapor−Water Interface

Using in-situ X-ray photoelectron spectroscopy at the vapor−water interface,the affinity of nanometer-sized silica colloids to adsorb at the interface is shown to depend oncolloid surface charge density. In aqueous suspensions at pH 10 corrected Debye−Hückeltheory for surface complexation calculations predict that smaller silica colloids have increasednegative surface charge density that originates from enhanced screening of deprotonatedsilanol groups (Si−O−) by counterions in the condensed ion layer. The increased negativesurface charge density results in an electrostatic repulsion from the vapor−water interface thatis seen to a lesser extent for larger particles that have a reduced charge density in the XPSmeasurements. We compare the results and interpretation of the in-situ XPS and correctedDebye−Hückel theory for surface complexation calculations with traditional surface tensionmeasurements. Our results show that controlling the surface charge density of colloid particles can regulate their adsorption tothe interface between two dielectrics.