Structural analysis of soluble and insoluble fractions of asphaltenes isolated using the PNP method.: Relation between asphaltene structure and solubility

Both, the toluene-insoluble (A1) and toluene-soluble (A2) asphaltene fractions, isolated using the PNP method, have been characterized by elemental analysis, molecular weight (VPO, SEC, and LDMS), and NMR (H-1 and C-13). The most prominent results of the analysis were the differences in hydrogen aromaticity f(H), high content in both fractions of hydrogen bonded to aliphatic carbons joined to aromatics (f(alpha)), and differences in carbon aromaticity f(C). Thus, low f(H) and high f(alpha) in A1 were consistent with a single, rigid, and flat core formed by fusion of polycyclic aromatic and naphthenic units (a single and large PANU), whereas for A2, high f(H) and high f(alpha) were consistent with a more flexible structure where several smaller PANU are joined by aliphatic chains. Using a MM program, models for A1 and A2 were built and the solubility parameters calculated were found in keeping with solubility difference suggesting that the above structural differences account for the solubility difference. Similar molecular weight and heteroatom content found for these fractions suggest that these play a minor or insignificant role in solubility. A dispersion mechanism of A1 by A2, relevant to solubility of asphaltene in organic solvents, is proposed.