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 temp erature. SANS measurements were carried out at four different temperatures (from 73 down to 8 degreesC) for solute (asphaltene) volume fractions Phi r anging from similar or equal to0.3 to similar to 10%. Asphaltenes were foun d to form nanometric aggregates, whose average masses (M-w) and radii of gy ration (R-GZ) increased as temperature decreased. These parameters hardly v aried 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 interac tions 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 s trong 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 t ime scales of the order of a few hours.