Aggregation−Dissociation Studies of Asphaltene Solutions in Resins Performed Using the Combined Freeze Fracture−Transmission Electron Microscopy Technique

Abstract

Solutions of asphaltenes in resins (3%) were studied combining the freeze fracture and transmission electron microscopy techniques (FF-TEM). These studies were performed by analyzing samples prepared at different temperatures ( T) from 25 up to 250 °C. In this temperature range, mean particle diameters ( D p) were found to drop from about 5 to about 3.5 nm. The smallest diameter measured at each temperature ( D S) reached a constant value equal to 2.5 nm in the range between 100 and 250 °C. Frequencies f( D) of all but D S values were found to decrease at some temperatures reveling that diameters D > D S correspond to aggregates (Ag). Moreover, during this heating, we found that whereas for big (B) and smallest diameters frequencies f( D) either decrease (big) or increase ( D S) continuously, maxima were found for intermediate (I) diameters. This suggests that under the conditions of this experiment (temperature and time) conversion from intermediate to smallest diameter meets some energy barrier high enough for the accumulation of intermediate size diameters. These results were interpreted in terms of the consecutive process A g ( B ) → T k 1 A g ( I ) → T k 2 A 1 M where A1M stand for A1-type molecules and k 1 > k 2; consequently, because of the low solubility of A1M, conversion from intermediate aggregates to molecules is a difficult step. The above results and many others in the literature are in agreement with the colloidal, solubility, and aggregates hypothesis related to A1 and A2 fractions of asphaltenes.