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 materials play crucial roles in the bioavailability and mobility of organic and inorganic pollutants in aquatic and terrestrial ecosystems. Scanning tunneling microscopy (STM) was used to image and analyze the morphological character of CA aggregates on graphite. Nano-scale images of CA also were simulated and interpreted using a molecular mechanics technique. 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. Computer-assisted STM proved to be a powerful technique to investigate structure, morphology, and chemical reactivity of soil organic matter components and their aggregates. The CA molecule aggregation and hydration do not affect the electronic properties significantly. However, electrostatic potential is highly sensitive to the conformation of the molecule and the morphology of the ensuing molecular aggregate.