Reactive dissolution of organic nanocrystals at controlled pH

The development of novel protocols and techniques for waste treatment represents the state of the art in the so-called “green conversion”. Chemical wastes deriving from industrial and power-station processes, which involve organic crystals, can be very hazardous for the environment. Studying their dissolution mechanism, both theoretically and experimentally, represents a mandatory step in the critical task of their disposal. Surprisingly, most of the studies are focused on millimeter scale length, from which one could estimate the crystal dissolution rate. In these studies, where no information about the dissolution mechanism on a molecular scale is provided, etch-pit formation is recognized as the ultimate mechanism of crystal dissolution. In this work, we show the morphological evolution of organic nanocrystals on the sub-micrometer scale range in a reactive dissolution process controlled by pH. This approach allows us to explore ranges of high undersaturation, whereby crystal dissolution occurs even though etch-pit formation is suppressed. Adopting different surface and bulk-sensitive techniques (atomic force microscopy, time-of-flight secondary ion mass spectroscopy and X-ray/electron diffraction, Raman spectroscopy, respectively), we investigate the dissolution process of porphyrin thin films deposited on the basal plane of highly oriented pyrolytic graphite, proving that such films constitute a model system to unveil the dissolution mechanism of organic nanocrystals.