The colloidal status of humic acid

Natural peat humic acid was studied by photon correlation spectroscopy in aqueous solution and scanning force microscopy after deposition (spin-coating) on a mica substrate. Two of the various particle types found on the mica surface are representative of the aqueous solution (and not only of the deposition process on the mica). One of these representative particle types is the humic acid macromolecule itself. Its conformation on the mica shows pronounced reactions to alterations of the pH value of the spin-coated solution indicating the validity of the random coil model of charged flexible polyelectrolyte molecules for humic acid. The second particle type representative of both the mica surface and the solution are small chunks of the lower submicron size range (70 to 160 nm). They are relatively inert and do not react to changes of the solution conditions. The micelle hypothesis is not supported is by our experiment. (orig.)