Applications of Noncontact Atomic Force Microscopy in Petroleum Characterization: Opportunities and Challenges

Abstract

Despite a tremendous amount of research, molecular characterization of petroleum remains a significant scientific challenge because petroleum is an exceptionally complex molecular mixture. This paper reviews recent advances in the application of noncontact atomic force microscopy (nc-AFM) to the petroleum field. The complexity of petroleum presents a number of experimental and computational challenges to achieve the full potential of this new technique. Initial results on categorizing structures as either alternant or nonalternant polycyclic aromatic hydrocarbons (PAHs), recognizing empirical structural patterns, quantifying the local aromaticity and bond orders, and computing the electronic structures will be presented. The effect of sample preparation in AFM and the influence of petroleum structural parameters on imaging results, such as molecular weight, thermal cracking of weak linkages, nonplanar geometries or conformations, the presence of heteroatoms, and trace amounts of free radicals will be discussed. This review also highlights the application of the AFM imaging technique, including recent results from characterizing molecules in petroleum pitch M-50. The observed predominant methyl substituents on M-50 pitch led to the design of dimethylpyrene, confirming the role of methyls in promoting molecular weight growth of aromatic hydrocarbons and revealing new insights into the polymerization mechanism. The knowledge of definitive petroleum structures may enable new reaction pathways and sustainable uses for petroleum molecules in applications such as carbon materials and infrastructures.