PULSED FIELD GRADIENT SPIN-ECHO NMR STUDIES OF ASPHALTENES

摘要

Studies of the diffusion behaviour of asphaltenes in different solvents have been performed by the use of pulsed field gradient spin-echo (PFG-SE) NMR. PFG-SE NMR allows us to study asphaltenes on a molecular level and valuable information about size and shape of the molecules may be obtained. A concentration serie of asphaltenes in pure toluene was prepared and it was observed that the PFG-SE NMR signal decay of the asphaltenes can be well described by a log-normal distribution of diffusion coefficients. By combining the diffusion behaviour of asphaltenes and solvent molecules, information about the shape of the asphaltenes was extracted. The information indicated that the asphaltene molecules had a disc-like geometry with an axial ratio of 20, which was in good agreement with literature data. With the aim to induce flocculation of the asphaltenes, heptane-d16 or pentane-d12 was added to asphaltenes that had been dissolved in an aromatic solvent (toluene-d8 or ethylbenzene-d10). The flocculation of asphaltenes was minor when 0 to 30 wt-% of flocculant was added. Thus it is proposed that a certain threshold point has to be reached before flocculation starts, which has also been suggested by others.