Comparative Study of Stabilizing Agents for Water-in-Oil Emulsions

Abstract

The mechanism and ability to stabilize water-in-oil (w/o) emulsions have been compared for different emulsifiers, including asphaltenes extracted from a crude oil sample, hydrophobic silica particles, and a model acidic surfactant, N-(1-hexylheptyl)-N′-(5-carbonylicpentyl)perylene-3,4,9,10-tetracarboxylic bisimide (C5Pe). The results indicated that C5Pe describes the behavior of asphaltenes relatively well, although the interfacial activity was substantially higher than for the real asphaltenes at high pH. The silica particles produced rather different results. The particles did not appear to desorb from the oil–water interface, leading to a time-independent situation without any visible signs of coalescence. Additionally, the transition from unstable to stable emulsions happened at a “critical” particle concentration. For asphaltenes and C5Pe, the influence of other parameters has been investigated, such as solvent composition and pH. In both cases, the interfacial activity increased when a poor solvent was added, because the new oil phase made it more energetically favorable for the molecules to assemble at the interface. C5Pe became a better emulsion stabilizer at high pH, because of dissociation of the carboxylic acid groups and enhanced interfacial activity of the molecule. The asphaltenes had the lowest ability to stabilize around pH 6, while their ability increased for both higher and lower pH values because of ionization of acids and bases. Finally, the asphaltene concentration in the residual oil phase was measured by ultraviolet spectroscopy, to compare to results obtained by other authors. All of the results suggested that only a subfraction of the asphaltenes is responsible for emulsion stabilization.