Adsorption Kinetics of Asphaltenes at the Oil–Water Interface and Nanoaggregation in the Bulk

Abstract

Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop-drop coalescence. As a result, asphaltenes unfavorably impact water-oil separation processes and consequently oil production rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water-oil interfaces. This study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barrier-controlled regime rather than to a conformational relaxation regime.

Understanding Mechanisms of Asphaltene Adsorption from Organic Solvent on Mica

“…The results in Figure 5 show that the confined layer thickness follows a linear relationship with the square root of adsorption time. This result is in good agreement with the result of asphaltene interfacial tension study conducted by Rane et al 9 For higher concentrations (0.1 wt% and 1 wt%) the confined layer thickness does not follow a linear relationship as well as the case of 0.01 wt%. Thus, for our experimental conditions, asphaltene adsorption from its lowest concentration solution, where layer compressibility was assumed to be negligible, is controlled by the diffusion of asphaltene molecules from the bulk solution to the substrate.…”

“…The interactions and assembly of adsorbed molecules or their aggregates lead to formation of a protective layer that resists droplet-droplet coalescence. [1][2][3][4][5][6][7][8][9][10][11] Stable emulsions have been a major challenge in petroleum production, affecting process throughput and downstream operations such as upgrading. It is necessary to develop knowledge on the formation of these protective layers to design suitable protocols for film disruption and prevention of stable emulsion formation.…”

Molecular Dynamics Simulation of Self-Aggregation of Asphaltenes at an Oil/Water Interface: Formation and Destruction of the Asphaltene Protective Film

“…Disputes are focused on the orientations of the plane of the aromatic rings, i.e., whether they are parallel, perpendicular, or at a certain angle with the liquid/liquid interfaces. 35,36,55,56 Therefore, simulations were performed with a number of coal or petroleum asphaltenes at the water surface.…”

Density Anomalies in Crude Oil Blends Reflect Multiple Equilibrium States of Asphaltene Colloidal Aggregates

“…Hence, the aggregation paradigm has been changed quantitatively, though not qualitatively. The most popular current model is that at a "critical nanoaggregate concentration" ("CNAC" ≈ 100-200 mg/L) of asphaltenes in native crude oils, or in "good" solvents, there is a one-step transition from a phase of individual asphaltene monomers to a phase of colloidal "nanoaggregates" of 4-6 monomers (further clustering of primary aggregates is usually regarded as a dynamic random process and is not discussed in terms of a phase transformations) [41,42]. • The dismissed paradigm [40]:…”

“…Recently, it was experimentally proven [48][49][50][51][52] that basic asphaltene molecules (unimers, monomers) typically include very small, 1-3 ring, aromatic systems, in contrast to the popular notions about the predominance of large multiring fused systems [41,42]. These basic molecules become predominant equilibrium species only after dissolution of solid asphaltenes in "good" solvents (benzene, toluene, etc.)…”