Abstract
Stirred tanks experiments were conducted to investigate water droplets size in asphaltenes solutions. After emulsification, droplets were kept suspended by a gentle agitation. Pictures were periodically captured by means of an in situ microscope. For low asphaltenes content, droplets were found not be stable upon completion of emulsification but to grow in size upon the decrease in agitation. This coalescence process was fast yet limited: droplets quickly reached an equilibrium size. Equilibrium size was found to be independent from emulsification conditions. On the other hand, varying emulsion composition yielded to a much larger variation of equilibrium size than expected from the impact of interfacial tension on break-up. Equilibrium droplet size proved proportional to the ratio of the mass of emulsified water to the mass of molecular asphaltenes present in the organic phase. Such a dependency is reminiscent of limited coalescence within Pickering emulsions. Upon coalescence, the total interfacial area of an emulsion decreases. For irreversibly adsorbed particles, surface coverage increases up to a critical value (close to 2D maximum packing) blocking further coalescence. In the present case, irreversible adsorption of asphaltenes was favored by the large aliphatic content of the organic phase. The estimated critical mass coverage for asphaltenes (3.2–3.5 mg/m2) is close to results of direct titration found in the literature. On the other hand for high asphaltenes content (i.e. high enough to crowd water surface during emulsification), droplets do not coalesce after reduction of agitation confirming again the limited coalescence principle. Finally, the critical mass coverage was converted into a critical molecular coverage from the estimate of the molar mass of asphaltenes. This critical molecular coverage corresponds to 80–85% of a recent estimate of the surface excess coverage for asphaltenes of the same origin. Those observations tend to disqualify interfacial cross-linking as the cause for emulsion stabilization by asphaltenes. This stabilization could be due to steric jamming in dense monolayers, like for nanoparticles or proteins. Such a scenario is compatible with most of the phenomenological observations previously ascribed to slow cross-linking.
