Scanning force microscopy of spin-coated humic acid

We report on investigations of spin-coated peat humic acid solutions by sca nning force microscopy allowing, for the first time, direct imaging of indi vidual single humic acid molecules with a minimum diameter of 1.5-3.5 nm, T he measured height of the molecules deposited onto a mica substrate increas es with decreasing pH value of the solution. This behaviour can be explaine d by means of the random coil model if one takes the existence of charge ca rriers in organic macromolecules into consideration. With increasing humic acid concentration the molecules arrange into a few monolayer-thick agglome rates formed during the process of film deposition, In unfiltered solutions and in 1000 nm filtrates we find additional particles well distinct from t he above-mentioned molecules in size and properties. They possess an equiva lent spherical diameter of 70-160 nm and behave inert when the ambient cond itions are changed. Copyright (C) 1999 John Wiley & Sons, Ltd.