SANS study of asphaltene aggregation:: Concentration and solvent quality effects

The structure of asphaltene solutions in toluene was studied by small-angle neutron scattering (SANS) as a function of temperature and concentration. Temperature alters solvent quality, flocculation being expected at low temperature. SANS measurements were carried out at four different temperatures (from 73 down to 8 degreesC) for solute (asphaltene) volume fractions Phi ranging from similar or equal to0.3 to similar to 10%. Asphaltenes were found to form nanometric aggregates, whose average masses (M-w) and radii of gyration (R-GZ) increased as temperature decreased. These parameters hardly varied with concentration in the dilute regime Phi less than or equal to 3-4%, in which no evidence of dissociation was found. At higher Phi, apparent values of the same parameters (M-w and R-GZ) decreased as repulsive interactions or aggregate interpenetration reduced the normalized intensity, I/Phi, a phenomenon reminiscent of the semidilute regime of polymers and fractal aggregates. At the two lowest temperatures studied, 8 and 20 degreesC, a strong scattering at low q signaled flocculation, as some of the asphaltenes formed dense domains of micronic size. This phenomenon occurred throughout the studied concentration range and entailed some limited hysteresis for time scales of the order of a few hours.