The structure of asphaltenes in bitumen

The definition of asphaltenes and some of its chemical properties like polarity and molecular weight is discussed, based on available knowledge. A critical examination of the currently common claim that asphaltenes is dispersed in bitumen in the shape of micelles is made. It is concluded that it is not likely that any micelles exist in bitumen. An alternative model, a thermodynamic solubility model is suggested. As a support for the solubility model, the Hansen solubility parameter is used to describe the solubility properties of bitumen, asphaltenes and maltenes. In the solubility model bitumen is considered to be a mixture of million of different molecules which is kept in solution by its mutual solubility. A continuum of different polarities and molecular weights are necessary for complete stability. The model predicts that if certain fractions are removed from bitumen the stability may be lost resulting in formation of precipitate and phase separations. The usefulness of the model is illustrated by the ability to relate polarity to the elasticity of the bitumen at normal service temperatures (<70°C).