Nanoaggregates of Diverse Asphaltenes by Mass Spectrometry and Molecular Dynamics

Asphaltene nanoaggregates from three diverse source materials-coal-derived asphaltenes dominated by aromatic carbon, petroleum asphaltenes with comparable abundances of aromatic and aliphatic carbon, and immature source-rock asphaltenes dominated by aliphatic carbon-are examined by means of surface-assisted laser desorption ionization mass spectrometry (SALDI-MS) coupled with laser desorption laser ionization mass spectrometry ((LMS)-M-2). All three types of asphaltenes form nanoaggregates with aggregation numbers close to 7. Molecular dynamics calculations for proposed island molecular structures show the important roles that pi-stacking and alkane steric hindrance play in nanoaggregate formation and structure. These results are discussed in terms of entropy and enthalpy changes. All results are consistent with the Yen-Mullins model, which bodes well for its expanded use in oilfield reservoir evaluations.