Optical Tweezers-Based Measurements of Colloidal Forces between Asphaltene Thin Films: Effect of Ultrasonication.

Oil production and processing often involve the treatment of water-in-oil emulsions stabilized by asphaltenes. The asphaltenes adsorb irreversibly at the water-oil interface and, by self-association at the interface, form a viscoelastic film that stabilizes the emulsions mechanically and sterically. Hydrophobic forces associated with these films may also contribute to the emulsion stability. A key step in treating these emulsions is to weaken the asphaltene film at the interface, and one way to do so is with ultrasonic treatment. The effect of ultrasonic waves on the interactions between asphaltene films was investigated at a silica-water interface using optical tweezers. Silica microparticles were aged in asphaltene solutions to form asphaltene coatings on their surfaces. The particles were dispersed in water, and interparticle force measurements were performed with optical tweezers to capture the steric force and hydrophobic force contributions. The asphaltene coating thickness and hydrophobic coefficient (a factor resembling the strength of the hydrophobic interaction) were obtained from fitting these forces. The effect of ultrasonication on the thickness of the asphaltene films on the surfaces of the particles was investigated. No change in the hydrophobic coefficient was observed upon changing the interfacial asphaltene concentration. The asphaltene film thickness increased with the concentration of the asphaltene solution and aging time. After treatment of the dispersion with ultrasonic waves for different durations (between 5 and 40 min), a significant reduction in the coating thickness was observed. This reduction was confirmed by thermogravimetric analysis (TGA) measurements. It is hypothesized that cavitation at the interface removed part of the surface layer of asphaltenes from the coated particles. Based on these findings, we proved that a low-power ultrasound field can effectively break asphaltene-stabilized water-in-oil emulsions.

Effect of surfactants on interfacial films and stability of water-in-oil emulsions stabilized by asphaltenes.

The effect of additives on asphaltene interfacial films and emulsion stability was analyzed through the change in film properties. Surface pressure isotherms were measured at 23°C for model interfaces between aqueous surfactant solutions and asphaltenes dissolved in toluene and heptane-toluene mixtures. Compressibility, crumpling film ratio and surface pressure were determined from the surface pressure isotherms. The stability of water-in-oil emulsions was determined for the same systems based on the proportion of unresolved emulsified water after repeated treatment involving heating at 60°C and centrifugation. Experimental variables included concentration of asphaltenes (5 and 10 kg/m(3)), concentration and type of surfactant (Aerosol OT, nonylphenol ethoxylates, polypropylene oxide block-copolymer, dodecylbenzene sulfonic acids, dodecylbenzene sulfonic acid-polymer blend, diisopropyl naphthalene sulfonic acid, and sodium naphthenate) and aging time (from 10 min to 4 h). Additives were found to have two opposing effects on film properties and emulsion stability: (1) decreasing or eliminating the crumpling ratio which destabilized emulsions and (2) decreasing interfacial tension which enhanced emulsion stability. A stability parameter was defined to include both the crumpling ratio and interfacial tension and provided a consistent correlation for the percent residual emulsified water.

Effect of interfacial rheology on model emulsion coalescence II. Emulsion coalescence.

In Part I, surface pressure isotherms were measured for model interfaces between a dispersed water phase and a continuous phase of asphaltenes, toluene, and heptane. Here, the coalescence rate of model emulsions prepared from the same components is determined from measured drop size distributions at 23 degrees C. A correlation is found between the initial coalescence rate and the interfacial compressibility. It is shown that the change in coalescence rate as the emulsion ages and coalesces can be predicted from surface pressure isotherm data also obtained at 23 degrees C. The stability of the emulsions was further assessed in terms of free water resolved after a treatment of heating at 60 degrees C and centrifugation. The emulsions were aged up to 24 h prior to treatment. The free water resolution appears to correlate to the "capacity for coalescence" of the interfacial film; that is, to the product of the initial film compressibility and (1-CR), where CR is the film ratio at which the film crumples.

Effect of interfacial rheology on model emulsion coalescence I. Interfacial rheology.

Interfacial elasticity and "dynamic" surface pressure isotherms were measured for interfaces between a dispersed water phase and a continuous phase of asphaltenes, toluene, and heptane. The interfacial modulus is a function of asphaltene concentration and in all cases reached a maximum at an asphaltene concentration of approximately 1 kg/m(3). The modulus increased significantly as the interface aged and slightly as the heptane content increased to a practical limit of 50 vol%. The modulus was approximately the same at 23 and 60 degrees C. The modulus correlated with the inverse of the initial compressibility determined from surface pressure isotherms. The surface pressure isotherms also indicated that a phase transition occurred as the interface was compressed leading to the formation of low compressibility films. Crumpling was observed upon further compression. The phase transition shifted to a higher film ratio with an increase in heptane content and interface age. Asphaltene concentration and temperature (23 and 60 degrees C) has little effect on the surface pressure isotherms. The surface pressure and elasticity measurements are consistent with the gradual formation of a cross-linked asphaltene network on the interface.