Spatially resolved organic coating on clay minerals in bitumen froth revealed by atomic force microscopy adhesion mapping

Mineral solids in oil sands are often coated with organic matter. These organically-modified mineral solids hinder bitumen aeration and stabilize water-in-oil emulsions, leading to low bitumen recovery and poor bitumen product quality. The study of these organic coating on the fine solids has been elusive due to the nanometer length scale and the unsuitability of sampling in high vacuum sample chambers. In the present work, we report the first attempt of using PeakForce Quantitative Nanomechanical Mapping (QNM) Atomic Force Microscopy (AFM) to study the organic matter adsorption on fine mineral solids extracted from bitumen froth. Taking advantage of the simultaneous nano-scale resolution topographic imaging and adhesion force mapping enabled by PeakForce QNM-AFM, the mineral and organic components in the bitumen froth fine solids were clearly distinguished due to their variations in shapes and mechanical properties. The inhomogeneous spatial distribution of adhesion force on surfaces of platy particles indicated the existence of organic patches on clay minerals. The area percentage of surface organic matter coverage on the clay basal faces was calculated to be 17 ± 6%, and the average thickness of the organic coating was estimated to be 1.4 nm based on the adhesion force maps. The organic matter associated with the mineral solids, which cannot be washed off by toluene, was different from asphaltenes in terms of mechanical properties. More specifically, the irreversibly-adsorbed organic matter on clay minerals was found to be softer than the asphaltene fraction of oil sands bitumen.