Structural Behavior of Isolated Asphaltene Molecules at the Oil–Water Interface

Asphaltenes are the heaviest component of crude oil, causing the formation of a stable oil–water emulsion. Even though asphaltenes are known to behave as an emulsifying agent for emulsion formation, their arrangement at the oil–water interface is poorly understood. We investigated the effect of asphaltene structure (island type vs archipelago type) and heteroatom type (Oxygen-O, Nitrogen-N, and Sulfur-S) on their structural behavior in the oil–water system. Out of six asphaltenes studied here, only three asphaltenes remain at the oil–water interface while others are soluble in the oil phase. Molecular orientation of asphaltene at the interface, position, and angle of asphaltene with the interface has also been determined. We observed that the N-based island type asphaltene is parallel, while the O-based island type asphaltene and N-based archipelago type are perpendicular to the interface. These asphaltene molecules are anchored at the interface by the heteroatom. The S-based asphaltenes (both island and archipelago type) and O-based archipelago type asphaltenes are soluble in the oil phase due to their inability to form a hydrogen bond with water and steric crowding near the heteroatom. This study will help in understanding the role of asphaltenes in oil–water emulsion formation based on its structure and how to avoid it.