Recebido em 29/10/09; aceito em 8/4/10; publicado na web em 24/8/10Water-in-crude oil emulsions are formed during petroleum production and asphaltenes play an important role in their stabilization. Demulsifiers are added to destabilize such emulsions,however the demulsification mechanism is not completely known. In this paper, the performances of commercial poly(ethylene oxide-b-propylene oxide) demulsifiers were studied using synthetic water-in-oil emulsions and model-systems (asphaltenes in organic solvent). No change in the asphaltene aggregate size induced by the demulsifier was observed. The demulsification performance decreased as the asphaltene aggregate size increased, so it can be suggested that the demulsification mechanism is correlated to the voids between the aggregates adsorbed on the water droplets surface.

During the petroleum dehydration process, it is necessary to use chemical demulsifiers to break the water-oil emulsions that are formed during oil extraction. The majority of the products used are formulations containing nonionic surfactants based on poly(ethylene oxidepropylene oxide) (PEO-PPO) block copolymers with different ethylene oxide/propylene oxide molar ratios. In this work, PEO-PPO block copolymers of different structures were used. The physical-chemical properties of aqueous PEO-PPO solutions and their mixtures were analyzed, along with their effectiveness as water-oil emulsion destabilizing agents. The results showed that all the PEO-PPO copolymers could reduce the interfacial tension between water and oil. Nevertheless, the most efficient water-oil emulsion demulsification was achieved by the PEO-PPO branched copolymer, whose structure presented hydrophilic segments [poly(ethylene oxide) and OH] side by side at the free end of the molecule. This copolymer also exhibited the highest water solubility. Such behavior could be attributed to its structure, which promoted better interaction with the water droplets dispersed in the water-oil emulsion. The performance of the surfactant mixtures appeared to be related to their capacity to reduce the water-oil interfacial tension. The addition of a surfactant at a concentration of roughly 30% without demulsifying action does not compromise the action of a well-performing surfactant.