Atomic force microscopy of humic substances: Effects of pH and ionic strength

Tapping mode atomic force microscopy was employed to determine the shape, s ize, and properties of the Suwannee River humic acid and a U.K. Geological Survey peat humic acid sorbed to mica. Humic acid macromolecules are shown to have a globular shape with an adsorbed height between 0.5 and 2 nm for a concentration of 10 mg L-1, pH > 5, and I < 500 mM NaCl. Aggregation of th e humic macromolecules was not systematically observed at low pH and ionic strength. Instead aggregation may have been to be related to the hydrophobi city of the sample: no aggregates were observed for the relatively hydrophi lic Suwannee River humic acid between pH 3 and pH 10 (I = 5-50 mM), while a ggregates of peat humic acid were observed at low pH (pH < 5) and high ioni c strength (I similar to 500 mM). Calculated and measured hydrodynamic diam eters determined under the same conditions were found to be systematically slightly larger than AFM heights,in good agreement with expectations given the different nature of the hive measurements.