Effects of Block Copolymer Demulsifiers on Langmuir Films of Heavy and Light Crude Oil Asphaltenes

Abstract

Three different n-heptane (n-C7) asphaltenes were extracted from three crude oils, two heavy and one light, and their film-forming properties at the air–water and heptane–water interfaces were investigated using a Langmuir trough. Surface pressure–area isotherms, compression–decompression cycles, and stability curves were carried out to get a better understanding about the asphaltene behavior at these interfaces. Although the light crude oil extract shows more contracted isotherms at the air–water interface that have been associated with a faster association dynamics, its films have been found to be more stable with more reproducible compression–decompression cycles. The influence of two demulsifiers (silicone copolymers) on the asphaltene film properties has been studied by preparing different mixtures of asphaltenes and demulsifiers, and the interaction between them at the air–water and liquid–liquid interfaces has been investigated by obtaining surface pressure–area isotherms to check the excess area (deviation from the additivity rule) for different mole fractions of demulsifier at a given surface pressure. Significant deviations from the additivity rule have been found for almost all mixed films studied, which has been attributed to the penetration and dispersion produced by demulsifiers in a real mixing with asphaltenes when they are spread at the interface. All these experiments have been completed with Brewster angle microscopy (BAM), where the in situ images at the interface show the effect of the demulsifiers on the ability of the asphaltenes to form highly aggregated surface films. Clearly, these films are already disaggregated and dispersed by a proportion of block copolymers as low as 0.1 mole fraction with total or partial disappearance of bright domains and clusters.