Particular features of circular dichroism spectra of resin-asphaltene compounds in solvent and crude oil

The circular dichroism (CD) spectra of solutions of asphaltenes, resins, and crude oils in toluene, as well as of oil residue, which is free of resin-asphaltene compounds (RACs), were studied in the spectral range 400–600 nm. It was demonstrated that, unlike, oil and RACs solutions, oil residue does not show the CD signal. This result proves that, in this spectral range, the occurrence of the CD signal in oil is determined by the presence of RACs. The concentration dependences of CD signal for the solutions under study were obtained for the maximum of the CD band in the considered spectral range. For pure toluene solutions of asphaltenes and resins, these dependences can be approximated with good accuracy by a linear function, whereas, for solutions of crude oils, the concentration curves deviate from the linear dependence at large concentrations of RACs due to the formation of particles of the disperse phase in oil. Therefore, in a certain concentration range, the structure of oil undergoes significant changes and becomes disperse, whereas solutions of molecules of RACs remain molecular.