Atomic force microscopy of humic acid

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Abstract
Structural analysis of humic acid extracted from a Cecil soil was conducted by atomic force microscopy (AFM) of monolayer thin films of humic acid deposited on mica strips using the Langmuir‐Blodgett deposition technique. The humic acid specimens display a fibre‐like morphology showing prominent hexagonal structures of benzene rings at a molecular level of resolution. The side chains attached to these ring structures were identified by infrared spectroscopy to be carboxyl and carbonyl groups as indicated by absorption bands at 1625 and 1720 cm‐1, respectively. The presence of C‐H alkyl chains were detected by bands at 2917 and 2850 cm‐1. This AFM observation provides the first direct evidence supporting the theoretical description of humic acid molecules consisting of benzene rings with various sizes of alkyl chains.