Oil sands are unconsolidated sand deposits impregnated with high-molar-mass viscous petroleum, which is an important unconventional oil resource and normally termed bitumen. To recover bitumen from oil sands, bitumen has to be liberated or detached from the sand grains. In hot-water extraction processes, surfactants play critical role in mediating interactions between bitumen and sand solids in the aqueous solution. Effect of ammonium chloride cationic surfactants with different hydrocarbon chain lengths (C8, C12, C16 and C18) on the colloidal forces between bitumen and silica surfaces were studied by atomic force microscopy (AFM). Investigation of the colloidal bitumen-silica interactions in various surfactants solutions allowed us to determine the dependence of the interaction behaviors on the surfactants chain lengths and the orientation and structure of the surfactants adsorbed on the bitumen and silica surfaces, as well as to have a deep understanding on the water-based bitumen extraction processes on a microscope.
The results showed that the hydrocarbon chain length of the cationic surfactants had a significant impact on both the long-range interactions and adhesion forces between bitumen and silica. With addition of C8 ammonium surfactant (TOAC) in the 1 mM KCl solution, the long-range repulsive force decreased to some extent and the corresponding adhesion force increased from about 3.3 mN/m to 4.6 mN/m. As the chain length of the surfactants increased to C12 (DTAC), the long-range repulsive force changed greatly to a strong attractive force and the adhesion further increased to about 6.6 mN/m. Interestingly, the long-range interaction force became repulsive again in CTAC surfactant solution, which has the chain length of C16. The adhesion force correspondingly decreased to 4.25 mN/m. Increasing the chain length to C18 (OTAC), the long-range repulsive force correspondingly enhanced and the adhesion decreased to 3.75 mN/m. It was further observed that addition of Ca2+ in various surfactants solutions would either depress or enhance the colloidal interactions according to the different chain lengths of the ammonium surfactants.
It was believed that electrostatic force and hydrophobic force between the bitumen and silica surfaces are the dominant factors of the colloidal interactions. Variation of the long-range interaction and the adhesion reflected that adsorption of the ammonium cationic surfactants molecules on the negative charged surfaces of bitumen and silica was in a manner of monolayer or bilayers according to the chain lengths of the surfactants, which was demonstrated by the Zeta potential measurements. Adsorption of the surfactants on the silica and bitumen surfaces significantly affected both the surface wettability and the surface charge characteristics. For surfactants with relative short chain lengths (C8 and C12), the adsorption of the surfactants molecules was mainly in a monolayer with the hydrocarbon chains toward in the solution, which enabled the silica and bitumen surfaces more hydrophobic and thus produced strong attractive hydrophobic force leading to the decrease of the long-range repulsive force or even a strong attractive interaction between the bitumen and silica. As the chain length of the surfactants increased to C16 and C18, the molecular order of the adsorbed surfactants increased and generated a bilayer adsorption with ammonium head group being on the most outside of the surface. Such a bilayer adsorption of the surfactants not only generated strong positive surface charges but also greatly decreased the hyrophobicity of the silica and bitumen surfaces. As a result, the silica-bitumen long-range interaction became repulsive force again and the adhesion considerably decreased as well. When the divalent metal ion of Ca2+ was added in the sufactants solution, the preferential adsorption of Ca2+ on silica and bitumen surfaces acted as a barrier to prevent the cationic surfactants from adsorbing and thus significantly affected the bitumen-silica interaction behaviors. In addition, the measured force profiles between bitumen and silica in various solutions could be well described by the extended DLVO theory considering an additional hydrophobic attractive force, which is a good evidence to support the variation of the surface wettability arising from the surfactants adsorption.
It is believed that the findings in this work deepen our understanding on the water-based bitumen extraction processes on a microscope. To have a high bitumen recovery and good froth quality, the surfactant type and concentration of the divalent metal ions in the processing slurry must be well considered.