SANS Study of Asphaltene Aggregation:  Concentration and Solvent Quality Effects

Abstract

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 (Mw) and radii of gyration (RGZ) increased as temperature decreased. These parameters hardly varied with concentration in the dilute regime Φ e 3-4%, in which no evidence of dissociation was found. At higher Φ, apparent values of the same parameters (Mw and RGZ) 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.

Aggregate structure analysis of Colombian heavy crude oil-derived asphaltenes using small angle X-ray scattering

“…In these spectra two regions can easily be distinguished: a first one corresponding to small q-values (q < 3 x 10 -2 Å -1 ) in which the scattered intensity is a fastly decreasing function of q (or power-law scattering), and a second one at large q-values (q > 3 x 10 -2 Å -1 ) where the scattered intensity shows a plateau (or Guinier region) followed by a slight decrease of the intensity at higher q. Similar scattering profiles have been obtained previously in small angle scattering studies for some concentrated Asphaltene solutions and crudes (Espinat et al, 1998;Roux, Broseta, & Deme, 2001;Sheu, 2006;Gawrys, Blankenship, & Kilpatrick, 2006;Zielinski, Saha, Freed, Hürlimann, & Liu, 2010).…”

Diffusion of asphaltene molecules through the pore structure of hydroconversion catalysts

“…Polydispersity of asphaltene molecules can be estimated by size exclusion chromatography (SEC) (Kodera et al, 2000;Merdrignac et al, 2002;Reerink and Lijzenga, 1975) and ultracentrifugation (Fenistein, 1998;Fenistein and Barre, 2001;Ray et al, 1957;Winniford, 1963). Several fractions of asphaltenes were isolated by preparative ultracentrifugation and a scaling law dependance between molar weight and radius of gyration was observed, very similar to polymers behavior (Fenistein, 1998;Fenistein and Barre, 2001;Roux et al, 2001). Concerning diffusion, recently a large literature concerning the determination of bulk-phase diffusion coefficient with experiments in the fluid phase (solute +solvent pair) has been published (Ballard Andrews et al, 2006;Schneider et al, 2007;Durand et al, 2008).…”

Model molecules mimicking asphaltenes

“…Since experimental techniques used to measure the molecular weight generally required the measurements at higher concentration, the obtained results were influenced by the presence of asphaltene aggregates in solution. For instance Small-Angle X-ray and Neutron Scattering (SAXS and SANS) experiments were generally performed at concentrations close to 10 g.L -1 for asphaltenes dissolved in model solvents 17,18,[48][49][50][51] . It was not until 1999, using techniques such as fluorescence depolarization technique [52][53][54][55][56] , fluorescence correlation spectroscopy [57][58][59] and mass spectrometry 60,61 that reliable results, accounting for the molecular aggregation at higher concentrations, became available.…”