Scanning force microscopy of spin-coated humic acid

We report on investigations of spin-coated peat humic acid solutions by scanning force microscopy allowing, for the first time, direct imaging of individual single humic acid molecules with a minimum diameter of 1.5-3.5 nm. The measured height of the molecules deposited onto a mica substrate increases with decreasing pH value of the solution. This behaviour can be explained by means of the random coil model if one takes the existence of charge carriers in organic macromolecules into consideration. With increasing humic acid concentration the molecules arrange into a few monolayer-thick agglomerates formed during the process of film deposition. In unfiltered solutions and in 1000 nm filtrates we find additional particles well distinct from the above-mentioned molecules in size and properties. They possess an equivalent spherical diameter of 70-160 nm and behave inert when the ambient conditions are changed.

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