AFM on humic acid adsorption on mica

Atomic force microscopy (AFM) was used to measure the thickness of (purified) Aldrich humic acid (HA) molecular layers adsorbed on mica. It was found that the drying methods used in preparing a sample affect the configuration of HA molecules adsorbed on micaʹs surface and the overall shape of the adsorbed HA layers. Adsorbed HA exhibited a more flexible configuration in freeze-dried samples than in air-dried samples. For the freeze-dried samples, results showed that 90% of HA islands have a height of 4.2–5.7 nm, while the similar range of the air-dried samples is 3.1–3.7 nm. Based on results of bearing and particle-size analyses, it is proposed that HA molecules adsorb at discrete sites to form monolayer islands at the mica/water interface. Assuming a cylindrical polyelectrolyte configuration for the HA molecules, their average diameter in solution is expected to fall within the freeze-dried height range.