Molecular Imaging of Petroleum Asphaltenes by Scanning Tunneling Microscopy: Verification of Structure from 13C and Proton Nuclear Magnetic Resonance Data

Authors

G. W. Zajac, Amoco Corporation
N. K. Sethi, Amoco Corporation
J. T. Joseph, Amoco Oil Company

Abstract

Scanning tunneling microscopy (STM) was used to verify the molecular structure of Maya asphaltene which had been derived from combined ¹³C and proton nuclear magnetic resonance (NMR) experiments. Petroleum asphaltenes are known to contain large polynuclear aromatic centers with aliphatic sidechains. Average molecular models of Maya asphaltenes were derived using studies which included combined proton and ¹³C NMR data to determine total aromatic carbon content and the ratio of peripheral to internal aromatic ring carbons. These parameters permitted estimating the average number of aromatic rings per condensed cluster.

These Maya asphaltenes were imaged by scanning tunneling microscopy (STM) in a dilute solution of tetrahydrofuran on highly oriented pyrolytic graphite. The sizes and structures of the asphaltenes as observed by STM are in reasonable agreement with these average molecular models. We observed asymmetric structures whose largest dimension averaged 10.4 Å ± 1.9 Å from 24 separate images. The condensed ring portions of three representative NMR derived molecular models yielded an average dimension of 11.1 Å ± 1.4 Å.

Recommended Citation

Zajac, G. W.; Sethi, N. K.; and Joseph, J. T. (1994) "Molecular Imaging of Petroleum Asphaltenes by Scanning Tunneling Microscopy: Verification of Structure from 13C and Proton Nuclear Magnetic Resonance Data," Scanning Microscopy: Vol. 8: No. 3, Article 4.
Available at: https://digitalcommons.usu.edu/microscopy/vol8/iss3/4