Diffusivity of coal and petroleum asphaltene monomers by fluorescence correlation spectroscopy

Using fluorescence correlation spectroscopy (FCS) we measure the translational diffusion coefficient of Iino coal asphaltenes and UG8 petroleum asphaltenes dispersed in toluene at extremely low concentrations (0.03–3.0 mg/L). Using a simple scaling argument we compare them with several molecules of known size and molecular weight. We find that Iino coal asphaltene molecules – which have a comparatively small alkane fraction – have a diffusion constant of 1.5 × 10−5 cm2/s and corresponding hydrodynamic radius close to 0.25 nm: making them significantly smaller than petroleum asphaltenes.