Adsorption kinetics of asphaltenes at the oil-water interface and nanoaggregation in the bulk

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.