| The recovery of bitumen from oil sands depends critically on the coalescence of the dispersed oil drops. When suspended in water, the surfaces of the bitumen drops are known to be negatively charged. According to classical DLVO theory, the resulting double layer repulsion should be sufficiently strong to prevent any coalescence of the droplets. From experience, however, it is known that the oil droplets do coalesce, albeit randomly. Further, the probability of coalescence was seen to increase sharply with bigger drop size. The underlying mechanism of coalescence appears therefore not to be deterministic; it may, however, be understood through a probabilistic approach. In this study, we propose a model of surface charge heterogeneities which postulates that the bitumen-water interface is randomly charged, with the zeta potential being uniform only within surface domains (or patches). The classical DLVO theory, according to this model, remains sound---but only locally for the interaction between two charged patches. Applying this new approach to correlate the observed probabilities of coalescence, we concluded that the patchy domains had a characteristic size of about 0.6 micrometre.;Finally, atomic force microscopy (AFM) was used to directly probe surface charge variations at the bitumen surface. By fitting experimental force-separation profiles to the DLVO theory, variations of zeta potential along the bitumen-water interface were observed. The AFM study confirmed the existence of surface charge heterogeneities; it also pointed to a surface domain size which was consistent with the above probability-based result (i.e. domain size of a fraction of a micron).;The probabilistic model was extended to the aeration of bitumen drops, the coalescence of bitumen drops at elevated temperatures, and the coalescence of pure oil droplets in water. It was observed that all of these phenomena were stochastic in nature. Moreover, our surface charge heterogeneity model was able to successfully predict the observed probabilities in all cases. |
