Physical structure of asphaltenes

While colloidal models of asphaltenes were extremely beneficial in the early stages of understanding asphaltene behavior, these analogies are often overused. The evidence often cited showing asphaltenes to be particles several nanometers in size is small-angle neutron and X-ray scattering (SANS and SAXS) results. Data from other techniques are often interpreted with the assumption that the asphaltenes form colloids or micelles. Asphaltenes are molecules which can undergo a thermodynamic liquid-liquid phase separation from solution. The asphaltene-rich phase is often below its glass transition and therefore appears solid, exhibiting a fractal-like morphology only because of the kinetic inability for the "droplets" to coalesce. Its colloidal appearance at micrometer to millimeter lengths should not be confused with or be used to imply such colloidal structure for solvated asphaltenes at nanometer length scales. From SAXS and SANS we show that the scattering often attributed to asphaltene colloidal aggregates is the result of ephemeral dynamically fluctuating compositional inhomogenieties similar to those observed in any one-phase liquid mixture of unlike molecules. By freeing ourselves from taking the colloidal analogies too seriously, we can potentially make substantial progress toward understanding the role of asphaltenes in effecting phase behavior, morphology, and viscosity.