Asphaltenes Precipitated by a Two-Step Precipitation Procedure. 2. Physical and Chemical Characteristics

In a previous study (10.1021/ef060311g), a two-step precipitation procedure for asphaltenes from three crude oils (WA, NS-A, and NS-B) was reported. Crude oils were diluted 3:1 with n-pentane, and precipitated asphaltenes were filtrated off (first fraction). A second fraction consisting of asphaltenes still present in the crude oil was precipitated by further dilution of 18:1 n-pentane/crude oil. In the previous work, interfacial tension, aggregation size, and onset of precipitation were investigated and shown. In the current work, elemental analysis indicated that the first fractions contain relatively more heteroatoms than the second fractions and whole asphaltenes. Fourier transform infrared (FTIR) spectroscopy, proton and carbon nuclear magnetic resonance (NMR) spectroscopy, and NMR-distortionless enhancement by polarization transfer (DEPT) indicated that the less soluble fractions WA and NS-B were more aromatic and had a more polar aromatic core and a larger aromatic core consisting of more rings. Furthermore, there were indications that the more soluble fractions contained more branched aliphatic side chains with a larger degree of hydroxylic and carboxylic groups. Laser desorption ionization-mass spectroscopy (LDI-MS) molecular-weight determination indicated that the less soluble asphaltene samples had a higher average molecular weight compared to the more soluble fractions and the whole asphaltenes. The results could help explain the differences in interfacial tension and solvent properties that were reported previously.