Triplet Electronic Spin States of Crude Oils and Asphaltenes

摘要

The resolution of asphaltene nanoscience is becoming increasingly important for a variety of purposes. One key molecular attribute of the asphaltenes is the size distribution of their polycyclic aromatic hydrocarbons (PAHs). Comparison of measured spin singlet-singlet absorption and emission transitions with exhaustive molecular orbital (MO) calculations on 523 PAHs indicates that asphaltene PAHs have a population centroid of ~7 fused rings. To further test this understanding of asphaltene PAHs, it is desirable to consider the dynamics of triplet states. Nevertheless, triplet-state spectroscopy is complex, especially on polydisperse materials such as asphaltenes. For validation, we compare simple expectations for asphaltenes against both experimental and theoretical results. Measurements were conducted on erode oil and asphaltene samples of dramatically different heavy end content to identify specific transitions being investigated. Experimental results include spectra at several wavelengths, lifetimes in the presence and absence of molecular oxygen, and temperature effects. Specifically, we use classic techniques [Horrocks and Wilkinson, Proc R. Soc London A 1968,306,257-273] to measure triplet-triplet spectra for crude oils and asphaltenes. These are compared with corresponding MO calculations. Again, using classic methods [Guzeman et al,). Chan. Soc Faraday Trans. 1973, 69,708-720], quenching effects of asphaltene triplet states by molecular oxygen are measured and compared with simple diffusion expectations. The temperature dependence provides further stringent testing. Spectral comparisons versus crude oil composition rule out significant spectral contributions from free radicals. Simple expectations regarding triplet-state spectroscopy of asphaltenes and crude oils apply and corroborate previous conclusions from singlet-state spectroscopy of crude oils and asphaltenes. The data herein are consistent with asphaltene PAHs being relatively large (e.g., 7 fused rings); this, in turn, is consistent with the predominance of a single PAH per asphaltene molecule (the "island" molecular architecture). Smaller PAHs dominate the triplet transitions for the crude oil samples and optical wavelengths used herein.