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

    Year: 2013

    Journal: Langmuir, 29, pp 4750-4759, 20130926

    Authors: Jayant P. Rane, Vincent Pauchard, Alexander Couzis, Sanjoy Banerjee

    Organizations: Energy Institute, City College of New York, New York, New York 10031, United States, Department of Chemical Engineering, City College of New York, New York, New York 10031, United States, Flow Technology Group, Department of Process Technology, SINTEF Materials and Chemistry, Trondheim, Norway FACE, the Multiphase Flow Assurance Innovation Center, Trondheim, Norway

    In an earlier study,1 oil−water interfacial tension was measuredby the pendant drop technique for a range of oil-phase asphalteneconcentrations and viscosities. The interfacial tension was found to be relatedto the relative surface coverage during droplet expansion. The relationship wasindependent of aging time and bulk asphaltenes concentration, suggesting thatcross-linking did not occur at the interface and that only asphaltene monomerswere adsorbed. The present study extends this work to measurements ofinterfacial rheology with the same fluids. Dilatation moduli have beenmeasured 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 inertialeffects are small. The elastic modulus increases with frequency and thenplateaus to an asymptotic value. The asymptotic or instantaneous elasticity hasbeen plotted against the interfacial tension, indicating the existence of a unique relationship, between them, independent ofadsorption conditions. The relationship between interfacial tension and surface coverage is analyzed with a Langmuir equation ofstate. The equation of state also enabled the prediction of the observed relationship between the instantaneous elasticity andinterfacial tension. The fit by a simple Langmuir equation of state (EOS) suggests minimal effects of aging and of nanoaggregatesor gel formation at the interface. Only one parameter is involved in the fit, which is the surface excess coverage Γ∞ = 3.2molecules/nm2 (31.25 Å2/molecule). This value appears to agree with flat-on adsorption2 of monomeric asphaltene structuresconsisting of aromatic cores composed of an average of six fused rings and supports the hypothesis that nanoaggregates do notadsorb on the interface. The observed interfacial effects of the adsorbed asphaltenes, correlated by the Langmuir EOS, areconsistent with the asphaltene aggregation behavior in the bulk fluid expected from the Yen−Mullins model.3,4