Evidence for Island Structures as the Dominant Architecture of Asphaltenes

TLDR

The study investigates which model compound families fragment differently from asphaltenes to identify the dominant structural type. Desorption was achieved with a 10.6 µm CO₂ laser and single‑photon ionization with a 157 nm fluorine excimer laser, and fragmentation was monitored versus pulse energy. Mass spectra reveal that asphaltenes and single‑core aromatic models fragment similarly while multi‑core models fragment differently, confirming an island‑like dominant structure and matching previous average molecular weight measurements.

Abstract

Laser desorption laser ionization mass spectra of 23 model compounds and 2 petroleum asphaltene samples are presented. These experiments involved desorption by irradiation with the 10.6 μm output of a CO2 laser followed by single-photon ionization with the 157 nm output of a fluorine excimer laser. The average molecular weight of the asphaltene samples agrees closely with that found previously using multiphoton ionization with the 266 nm output of a Nd:YAG laser. The fragmentation behavior as a function of ionization laser pulse energy is studied to evaluate which families of model compounds fragment differently from asphaltenes and, hence, can be excluded from being dominant in asphaltenes. All model compounds having one aromatic core with and without various pendant alkyl groups show little to no fragmentation, mimicking the behavior observed for the two asphaltene samples, whereas all model compounds having more than one aromatic core show energy-dependent fragmentation. These observations support the contention that the dominant structural character of asphaltenes is island-like.