| Asphaltenes constitute high molecular weight constituents of crude oils that 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. The first chapter of this study elucidates aspects of these effects based on new data on interfacial tension (IFT) in such systems from pendant drop experiments. 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 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.;In the next chapter measurements performed to study interfacial rheology with the same fluids are reported and interpreted. 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 insight on the adsorption of asphaltenes at oil-water interfaces based on the Langmuir EOS is consistent with the bulk phase molecular architecture of asphaltenes suggested by the Yen-Mullins model.;In the next chapter, using the same system, we investigate adsorption kinetics at longer times and higher coverages, as well as the jamming behaviour of asphaltenes at the oil-water interface. From dynamic IFT curves, we could confirm that Langmuir equation with application of bilayer permeability and moduli data controls the later stage of adsorption (at longer times and high coverage). It is verified for from dynamic IFT curves for different concentration of asphaltenes (below and above CNAC) and different bulk viscosities of the oil phase. In the last chapter, we introduce a study of the ferromagnetism of asphaltenes, an effect that was accidentally discovered when setting up a pendant drop experiment using a magnetic stirrer. To quantify the magnetic behavior, SQUID magnetometry is performed on asphaltene powder and asphaltenes dissolved in toluene. Data shows diamagnetic behavior of extracted asphaltene powder but ferromagnetic behavior of asphaltenes dissolved in toluene. It is conjectured that the ferromagnetism of asphaltenes could possibly arise because of the graphene-like nature of their polyaromatic cores. This property of asphaltenes leads to the observed movement of asphaltene aggregates in bulk oil phase,and that of asphaltene covered water droplets in magnetic fields, reported here. One potentially important implication of this phenomenon could be to enhance separation of asphaltene stabilized water-oil emulsions. (Abstract shortened by UMI.). |
