Interfacial Rheology of Asphaltenes at Oil–Water Interfaces and Interpretation of the Equation of State

Abstract

In an earlier study, oil-water interfacial tension was measured by the pendant drop technique for a range of oil-phase asphaltene concentrations and viscosities. The interfacial tension was found to be related to the relative surface coverage during droplet expansion. The relationship was independent of aging time and bulk asphaltenes concentration, suggesting that cross-linking did not occur at the interface and that only asphaltene monomers were adsorbed. The present study extends this work to measurements of interfacial rheology with the same fluids. Dilatation moduli have been measured using the pulsating droplet technique at different frequencies, different concentrations (below and above CNAC), and different aging times. Care was taken to apply the technique in conditions where viscous and inertial effects are small. The elastic modulus increases with frequency and then plateaus to an asymptotic value. The asymptotic or instantaneous elasticity has been plotted against the interfacial tension, indicating the existence of a unique relationship, between them, independent of adsorption conditions. The relationship between interfacial tension and surface coverage is analyzed with a Langmuir equation of state. The equation of state also enabled the prediction of the observed relationship between the instantaneous elasticity and interfacial tension. The fit by a simple Langmuir equation of state (EOS) suggests minimal effects of aging and of nanoaggregates or gel formation at the interface. Only one parameter is involved in the fit, which is the surface excess coverage Γ∞ = 3.2 molecules/nm(2) (31.25 Å(2)/molecule). This value appears to agree with flat-on adsorption of monomeric asphaltene structures consisting of aromatic cores composed of an average of six fused rings and supports the hypothesis that nanoaggregates do not adsorb on the interface. The observed interfacial effects of the adsorbed asphaltenes, correlated by the Langmuir EOS, are consistent with the asphaltene aggregation behavior in the bulk fluid expected from the Yen-Mullins model.

Understanding Mechanisms of Asphaltene Adsorption from Organic Solvent on Mica

“…Such binding configuration has been confirmed by recent studies. 48,49 Adsorption of Asphaltenes on Mica. The thickness of the asphaltene layers at different concentrations and time intervals were determined.…”

Fractionation of Asphaltenes in Understanding Their Role in Petroleum Emulsion Stability and Fouling

“…The presence (formation) of solid-like (elastic dominant) asphaltene films at the oil-water interface has been shown to significantly hinder the coalescence of two contacting water droplets, as such that when two droplets interact and undergo significant compression, they continue to remain stable without coalescence. 48,51 The correlation between interfacial dilatational elasticity and overall emulsion stability has been qualitatively proven by several researchers, 49,50,[52][53][54][55][56] although there is clear disagreement at high asphaltene concentrations where emulsion stability increases and E' decreases. The discrepancy between the interfacial rheology and emulsion stability is believed to be associated with a change in the interfacial layer structure, transitioning from a compact and rigid monolayer, to a collapsed interfacial layer dominated by threedimensional structures.…”

Thermodynamic modelling of asphaltene precipitation and related phenomena

“…Results from experimental asphaltene science have advanced knowledge considerably within the past decade, and it is now accepted that asphaltenes lead an enigmatic colloidal existence based on different levels of association, as encompassed in the Yen-Mullins model. Interestingly, recent evidence seems to indicate that this model is also consistent with asphaltene adsorption at water-oil interfaces, in which adsorption of the monomer units dominates; nanoaggregates and clusters appear not to adsorb (at least not as strongly, or within the same timescale), and the overall adsorption behaviour can be treated using a simple Langmuir equation of state [223]. The significance of this lies in the role of asphaltenes in stabilising water-in-crude oil emulsions, for example, where the build-up of viscous interfacial layers ("skins") occurs [224], which, additionally, may be amenable to theoretical treatments based on ideas discussed herein relating to solution behaviour.…”