Viscosimetric and Neutron Scattering Study of Asphaltene Aggregates in Mixed Toluene/Heptane Solvents

Abstract

A series of viscosimetric and small-angle neutron scattering experiments on asphaltenes diluted in mixed toluene/heptane solvents has been conducted, with the purpose of characterizing the size, molecular weight, and internal structure of asphaltene aggregates as a function of solvent conditions. With increasing flocculant (i.e., heptane) content in the solvent, the intrinsic viscosities of asphaltene aggregates first decreased, went through a minimum for heptane fractions ≈ 10−20%, and then increased at the approach of flocculation. These variations paralleled those of the volume of aggregate occupied per unit mass of asphaltene, a behavior reminiscent of the Flory−Fox relationship for polymers in a solvent. This volume, proportional to the cubed radius of gyration of the aggregates divided by their molecular weight, was determined from the neutron scattering data. For increasing heptane fractions in the solvent, the molecular weight of the aggregates increased with their radius of gyration according to a power law, the exponent being in the range of 2. This exponent also characterized the self-similar internal structure of the asphaltene aggregates. With due care to the possible systematic effects of the strong polydispersity of these aggregates, these results are discussed in light of recent models of colloidal aggregation.

Characterisation of Asphaltenes and Modelling of Flocculation – State of the Art

“…According to these authors, the size depends on a reversible thermodynamic equilibrium between primary particles and aggregates. For mixtures of maltenes and asphaltenes, Fenistein et al (1998) find the same compact flocculated and open fractal structural types as with asphaltene + toluene synthetic mixtures.…”

“…At this point, it is worth referring to the extensively studied system of charged particles in aqueous medium: they are stable in an aqueous medium of low salinity-in this case, the electrostatic interactions prevail-and flocculate as soon as the water salinity exceeds a certain level, since the interactions are then masked by the salt. This analogy has been frequently mentioned to describe the structure and growth modes of asphaltenic aggregates (El Mohamed et al, 1988;Yudin et al, 1998;Fenistein et al, 1998). Nevertheless, for asphaltenic systems, a tool similar to the DLVO theory (Derjaguin-Landau-Verwey-Overbeek, see Israelachvili, 1991, Chapter 12) is required for the aqueous colloids in order to quantitatively predict the shape of the interaction potential depending on composition or pressure.…”

Adsorption of asphaltenes from toluene on mineral surface

“…The solvated asphaltene crystallites are the primary particles of the colloidal dispersed phase. These particles can associate and aggregate, forming various size particles depending on temperature, pressure and concentration [8,15,16,19,21,[26][27][28][29][30][31][32][33][34][35]42,43]. In fact, asphaltenes are natural surfactants stabilising water/crude oil emulsion [21,22,[44][45][46][47][48][49][50][51][52].…”

Adsorption of asphaltenes from toluene on typical soils of Lublin region

“…The solvated asphaltene crystallites are the primary particles of a colloidal dispersed phase. These particles can associate and aggregate forming large particles dependent on temperature, pressure and concentration [7,8,11,13,[17][18][19][20][21][22][23][24][25][26][27]30,[40][41][42][43]. It is well known that asphaltenes are natural surfactants stabilising water/crude oil emulsion [13,14,[44][45][46][47][48][49][50][51][52][53].…”