Determination of the Number Average Molecular Mass of Asphaltenes (<i>M</i>_n) Using Their Soluble A2 Fraction and the Vapor Pressure Osmometry (VPO) Technique

Fractions A1 and A2 have been isolated from asphaltenes using the para-nitrophenol (PNP) method,(20, 21) with fraction A2 being characterized as one with a tendency for aggregation smaller than asphaltenes and the opposite being observed for fraction A1. Hence, measuring molecular mass (MM) properties of fraction A2 would suggest MM properties of asphaltenes where aggregation is either smaller or insignificant. In this work, using several samples of asphaltenes and different solvents and temperatures, we investigated the aggregation tendency of fraction A2 using the vapor pressure osmometry (VPO) technique. When measurement of the number average molecular mass (Mn) was performed in toluene and o-dichlorobenzene (ODB), Mn ≈ 1000 g mol−1 was found. Evidence for fraction A2 aggregation was detected in chloroform and nitrobenzene. We found that ODB was the best solvent for VPO, where a constant value close to 1000 g mol−1 was measured for fraction A2 when the temperature was changed from 80 to 120 °C. In all cases studied, the Mn ratio of fractions A1/A2 was greater than the one suggesting that aggregation properties of asphaltenes are mainly due to fraction A1.