Exploring nanoaggregate structures of model asphaltenes using two dimensional infrared spectroscopy

Abstract

Asphaltenes have been an enigma in the scientific community; studies on the molecular masses have differed by orders of magnitude and structures have been debated between island or archipelago structures. Thus, the asphaltene community defines asphaltenes by their solubility. Asphaltenes are n-heptane-insoluble and toluene-soluble. The known nanoaggregation of asphaltenes at different timescales and concentrations causes issues to determine the molecular weight and structure of asphaltene molecules. This thesis is the first step to using two dimensional infrared (2D IR) spectroscopy to study the nanoaggregate structure of model asphaltenes. 2D IR spectroscopy is a vibrational spectroscopy that is advantageous over linear IR absorption due to the ability to spread the spectral information over two axes. The 2D IR spectra give rise to structurally sensitive cross-peaks, affording the ability to probe the structure of the nanoaggregates. The model asphaltenes used in this work are violanthrone-79 and lumogen orange, a perylene derivative. These model asphaltenes consist mostly of polycyclic aromatic hydrocarbons (PAHs), similar to asphaltenes. Violanthrone-79 and lumogen orange also have carbonyl functional groups, which provide vibrational probes. The carbonyl stretching and ring breathing vibrations are used to probe the stacked structure of the nanoaggregates. A quinone series of one, two and three ring systems was used to first study the coupling between the carbonyl stretching and ring breathing vibrational modes. The quinone series provided the foundation for the larger ring systems that emulate asphaltenes. The data from studying the stacked structure of nanoaggregate model asphaltenes can be used to reveal properties of nanoaggregate asphaltenes. This work will allow for continued study of the kinetics of nanoaggregation using 2D IR waiting time experiments for dynamic information. Thus, this work demonstrates the use of 2D IR spectroscopy, which offers femtosecond time resolution, as a viable technique for studying nanoaggregation.