Asphaltene fraction of crude oil is commonly considered to be responsible for formation of highly undesirable, stable water-in-crude oil emulsions and rag layers. We developed a new procedure for sub-fractionation of asphaltenes based on their interfacial activity. The most interfacially-active asphaltene (IAA) sub-fraction was extracted as an interfacial material from emulsified water droplets. IAA sub-fraction represents less than 2 wt.% of whole asphaltenes (WA) but its removal had profound effect on interfacial and thin emulsion film properties. It was found that IAA sub-fraction is a main contributor to emulsions stabilization and formation of rigid water/oil interfaces. IAA adsorbed irreversibly at water/oil interface and formed interfacial layers of low compressibility. Thin emulsion films of toluene stabilized by IAA were thicker and exhibited severe ageing effects in contrast to the films stabilized with remaining asphaltenes fractions which were thin and less rigid without any noticeable ageing effects.

Drainage kinetics, thickness, and stability of water-in-oil thin liquid emulsion films obtained from asphaltenes, heavy oil (bitumen), and deasphalted heavy oil (maltenes) diluted in toluene are studied. The results show that asphaltenes stabilize thin organic liquid films at much lower concentrations than maltenes and bitumen. The drainage of thin organic liquid films containing asphaltenes is significantly slower than the drainage of the films containing maltenes and bitumen. The films stabilized by asphaltenes are much thicker (40-90 nm) than those stabilized by maltenes (∼10 nm). Such significant variation in the film properties points to different stabilization mechanisms of thin organic liquid films. Apparent aging effects, including gradual increase of film thickness, rigidity of oil/water interface, and formation of submicrometer size aggregates, were observed for thin organic liquid films containing asphaltenes. No aging effects were observed for films containing maltenes and bitumen in toluene. The increasing stability and lower drainage dynamics of asphaltene-containing thin liquid films are attributed to specific ability of asphaltenes to self-assemble and form 3D network in the film. The characteristic length of stable films is well beyond the size of single asphaltene molecules, nanoaggregates, or even clusters of nanoaggregates reported in the literature. Buildup of such 3D structure modifies the rheological properties of the liquid film to be non-Newtonian with yield stress (gel like). Formation of such network structure appears to be responsible for the slower drainage of thin asphaltenes in toluene liquid films. The yield stress of liquid film as small as ∼10(-2) Pa is sufficient to stop the drainage before the film reaches the critical thickness at which film rupture occurs.

The demulsification mechanism of asphaltene stabilized water-in-toluene emulsions by an ethylene-oxide/propylene oxide (EO/PO) based polymeric demulsifier was studied.Demulsification efficiency was determined by bottle tests and correlated to the physicochemical properties of asphaltene interfacial films after demulsifier addition. From bottle tests and droplet coalescence experiments, the demulsifier showed an optimal performance at 2.3 ppm (mass basis) in toluene. At high concentrations, the demulsification performance deteriorated due to the intrinsic stabilizing capacity of the demulsifier, which was attributed to steric repulsion between water droplets. Addition of demulsifier was shown to soften the asphaltene film under (i.e. reduce the viscoelastic moduli of asphaltene films) both shear and compressional interfacial deformations. Study of the micro and macro structure, and the chemical composition of asphaltene film at the toluene-water interface after demulsifier addition demonstrated gradual penetration of the demulsifier into the asphaltene film. Demulsifier penetration in the asphaltene film changed the asphaltene interfacial mobility and morphology, as probed with Brewster Angle and Atomic Force Microscopy.

After successful isolation of the most interfacially active subfraction of asphaltenes (IAA) reported in part one of this series of publications, comprehensive chemical analyses including ES-MS, elemental analysis, FTIR and NMR were used to determine how the molecular fingerprint features of IAA are different from those of the remaining asphaltenes (RA).Compared with RA, the IAA molecules were shown to have higher molecular weight and higher contents of heteroatoms (e.g., three times higher oxygen content). The analysis on the elemental content and FTIR spectroscopy suggested that IAA contained a higher content of high polarity sulfoxide groups which were not present in the RA. The results of ES-MS, NMR, FTIR and elemental analysis were used to construct average molecular representations of IAA and RA molecules. These structures were used in molecular dynamic (MD) simulation to study interfacial and aggregation behaviors of the proposed representative molecules. MD simulation study showed little affinity of representative RA molecules to the oil/water interface while the representative IAA molecules had a much higher interfacial activity, which corresponds to the extraction method. The aggregation of IAA molecules in the bulk oil phase and their adsorption at oil/water interface were not directly related to the ring system but rather to the associations between or including sulfoxide groups. The IAA molecules self-assembled in solvent, forming supramolecular structures and a porous network at the oil/water interface as suggested in our previous work. The results obtained in this study provide a better understanding of the role of asphaltenes in stabilizing petroleum emulsions.

Physical properties of interfacial layers formed at the water-xylene interface by the adsorption of a polyaromatic organic compound, N-(1-hexylheptyl)-N'-(5-carbonylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide (in brief C5Pe), were studied systematically. The deprotonation of the carboxylic group of C5Pe at alkaline pH made it highly interfacially active, significantly reducing the water-xylene interfacial tension. Thin liquid film experiments showed a continuous build-up of heterogeneous C5Pe interfacial layers at water-xylene interfaces, which contributed to the formation of stable W/O emulsions. Continual accumulation and rearrangement of C5Pe aggregates at the water-xylene interface to form a thick layer was confirmed by in-situ Brewster angle microscopy (BAM) and atomic force microscopy (AFM). The rheology measurement of the interfacial layer by double wall ring interfacial rheometry under oscillatory shear showed that the interfacial layers formed from high C5Pe concentrations were substantially elastic and rigid. The presence of elastically dominant interfacial layers of C5Pe led to the formation of stable water-inxylene emulsions.Keywords: polyaromatic surface active compound, water-in-oil emulsions; thin liquid films; insitu Brewster angle imaging; interfacial rheology