Circular dichroism of crude oils: Influence of micelle formation

Circular dichroism (CD) and absorption spectra of toluene solutions of crude oils, collected from two different reservoirs, have been measured in the spectral range, where the optical activity of crude oil solutions is due to the aggregation of asphaltene molecules and for asphaltene concentrations up to values over the critical micelle concentration (cmc). Concentration dependences of CD and the absorption signal in the maximum of the registered CD band (410 nm) in the range of low asphaltene concentrations are described by linear functions. Micelle formation of asphaltenes in crude oil solutions leads to a sharp increase of the CD signal and a violation of the linear concentration dependence of the absorption signal. A further increase of the asphaltene concentration is accompanied by saturation of the CD signal, which is possibly due to the formation of a stable phase by micelles in solution.