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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 °C) for solute (asphaltene) volume fractions Φ ranging from ≃0.3 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 Φ ≤ 3−4%, in which no evidence of dissociation was found. At higher Φ, apparent values of the same parameters ( M w and R GZ ) decreased as repulsive interactions or aggregate interpenetration reduced the normalized intensity, I /Φ, a phenomenon reminiscent of the semidilute regime of polymers and fractal aggregates. At the two lowest temperatures studied, 8 and 20 °C, 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.
Roux et al. (Tue,) studied this question.