Determining surface potential of the bitumenwater interface at nanoscale resolution using atomic force microscopy

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Atomic force microscopy (AFM) was used to measure the surface forces between a silicon nitride AFM tip and a deposited layer of Athabasca bitumen; the measurements were carried out in pure water (pH 6.0-6.5) and 1 mM KCl solution (pH 9). An AFM pyramidal-shaped tip was moved stepwise using an operator-controlled offset (10 nm per step) and the tip-bitumen colloidal forces were measured at each location. Surface charge densities at the bitumen-water interface were calculated from the measured colloidal forces using a theoretical model that combined both electrostatic and van der Waals forces for a conical tip-flat substrate system. Fitted values of the bitumen surface charge density ranged from -0.002 to - 0.004 C/m2 in water (pH 6.0-6.5), and - 0.005 to - 0.022 C/m2 in KCl solution (pH 9); the variation of local charge density along the bitumen surface appeared random. Bitumen surface potentials were also calculated from the surface charge densities using the Graham equation; the values ranged from - 90 to - 130 mV in water, and -45 to - 110 mV in KCl solution. This study suggests the presence of bitumen surface domains of different surface charge densities/surface potentials. The domains are estimated to have characteristic sizes of 20 to 40 nm or less.

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Canadian Journal of Chemical Engineering