ASPHALTENE RADICALS AND THEIR INTERACTION WITH MOLECULAR-OXYGEN - AN EPR PROBE OF THEIR MOLECULAR CHARACTERISTICS AND TENDENCY TO AGGREGATE

The paramagnetic species nature of different geological origin asphalt enes are discussed on the basis of Electron Paramagnetic Resonance (EP R) results. Free organic radicals are present in petroleum asphaltenes but their molecular nature is poorly known owing to the multiplicity of their molecular structures which causes the appearance of a single unresolved EPR signal with a linewidth of 4-6 Gauss. In spite of the p oorly resolved signals the microwave power dependence of EPR line inte nsities gives some insights into the nature of the aromatic rings cond ensation. More aromatic asphaltenes show higher saturation power of th e EPR lines: this fact has been explained by a spin exchange mechanism between pi-pi electronic clouds of adjacent molecules, supported also by some XRD evidences. Moreover, asphaltenes in argon (not paramagnet ic gas) show a maximum intensity at much lower microwave power than in oxygen atmosphere (paramagnetic gas); besides, the differences in the saturation behavior for the different asphaltenes are much more evide nt in oxygen atmosphere. This phenomenon is suggested to be a conseque nce of a weak spatial complex between aromatic moieties and oxygen mol ecules that deeply contribute to the relaxation pathway. Synchronous f luorescence spectroscopy confirms the difference in the size of aromat ic cores in asphaltenes molecules as supposed on the basis of the satu ration mechanism of EPR line intensities.