In situ AFM study of sorbed humic acid colloids at different pH

Humic acid colloids adsorbed on the basal plane of cleaved muscovite are investigated under in situ conditions by non-contact mode atomic force microscopy (AFM) in liquid (also called fluid tapping-mode AFM). Structures are found to be of nanometer scale, consisting of flat particles (8–13 nm in diameter), aggregates of particles (20–100 nm), chain-like assemblies, networks and torus-like structures. In contrast to former investigations colloids are investigated in aquatic solution and structures are not influenced by sample preparation. Nanostructure, surface coverage and particle sizes are found to depend on solution pH. Humic colloids can be distinguished from surface roughness and background noise by image processing. Furthermore, an approach to quantify the surface coverage is discussed. Therefore, non-contact mode AFM in liquid is shown to be a powerful method to study the interaction of colloids at solid–liquid interfaces.