Determination of Surface Characteristics and Fractal Dimensions of Natural and Synthetic Eumelanins from Nitrogen Adsorption Isotherms

The concept of eumelanins as aggregates of particles (originating from biosynthetic mechanisms in melanosomes or from purely chemicophysical processes in the case of the synthetic pigment) has modified our view concerning their chemical reactivity, that now must be interpreted in terms of heterogeneous reactions kinetics. It is therefore crucial to reach a detailed knowledge of the geometrical and physical characteristics of the melanin particle surface. Light-scattering and small-angle X-ray scattering (SAXS) experiments have suggested a fractal structure for these aggregates. Ten years ago an analysis of N-2 adsorption isotherms, performed following the method of Brunauer, Emmett, and Teller (BET), permitted a calculation of surface parameters (such as specific surface area and pore volume) for some kinds of natural and synthetic eumelanins. In the present work the same data are interpreted in the framework of more recent theories developed specifically for materials of technological interest, such as active carbon, silica gel, TiO2, and so forth. Analysis of the experimental data with Dubinin-Radushkevitch, Avnir-Jaroniec, and Frankel-Halsey-Hill theories has confirmed the fractal structure of all the eumelanins examined and has allowed a deeper insight into the physical structure of the particles.