Studies of asphaltenes by the use of pulsed-field gradient spin echo NMR

Pulsed field gradient spin echo nuclear magnetic resonance (PFG-SE NMR) has been used to study the self-diffusion of asphaltenes in deuterated toluene. Diffusion coefficients of n-heptane asphaltenes were measured in the concentration range of 0.044-5 wt%. It is shown that the self-diffusion of asphaltenes is well described by a log-normal distribution of diffusion coefficients and that the width of the obtained distribution is broad and independent of the asphaltene concentration in the interval studied. Both asphaltene and toluene diffusion indicates that asphaltenes have a disc-like shape as revealed by a strong concentration dependence on the diffusion coefficients. The diffusion coefficient of asphaltenes at infinite dilution was 2.2×10^-10 m²/s.