COMBINING SCANNING-TUNNELING-MICROSCOPY AND COMPUTER-SIMULATION OF HUMIC SUBSTANCES - CITRIC-ACID, A MODEL

Citric acid (CA) is a model for soil humic substances and an important participant in many biochemical processes. Humic and humic-like mater ials play crucial roles in the bioavailability and mobility of organic and inorganic pollutants in aquatic and terrestrial ecosystems. Scann ing tunneling microscopy (STM) was used to image and analyze the morph ological character of CA aggregates on graphite. Nano-scale images of CA also were simulated and interpreted using a molecular mechanics tec hnique. The effects of intermolecular association and hydration on the electronic properties and chemical reactivity of CA were analyzed in relation to the behavior of similar compounds in natural processes. Co mputer-assisted STM proved to be a powerful technique to investigate s tructure, morphology, and chemical reactivity of soil organic matter c omponents and their aggregates. The CA molecule aggregation and hydrat ion do not affect the electronic properties significantly. However, el ectrostatic potential is highly sensitive to the conformation of the m olecule and the morphology of the ensuing molecular aggregate.