Kaolinite surface charges developed in cyclohexane suspension with dissolved Span 80 or bitumen: Electrodeposition and adsorption/ desorption studies

In this study, we investigated the surface charges of kaolinite particles suspended in cyclohexane with or without the addition of Span 80 or bitumen. Kaolinite particles did not carry a net charge when dispersed in cyclohexane. They were found to acquire positive charges after adding Span 80 and negative charges after adding bitumen. Furthermore, when Span 80 was removed from the cyclohexane solution, the kaolinite surface charges disappeared, but when bitumen was removed, the kaolinite particles remained negatively charged. Our investigations showed that the bitumen adsorbed on the kaolinite surface caused the negative surface charges of kaolinite in cyclohexane. This was different from the widely accepted conventional charging mechanism which states that solid particles in a nonpolar organic liquid acquire charges due to the formation of reverse micelles from added surfactants, such as Span 80, and the reverse micelles house and stabilize free opposite charges to maintain the net particle surface charge. On the other hand, bitumen-coated kaolinite could carry net charges in cyclohexane due to the selective dissolution of charge-carrying species induced by an external electric field. Our electrochemical quartz crystal microbalance with dissipation (EQCM-D) measurements showed that about 3.7 wt % of the adsorbed bitumen was desorbed from kaolinite surface under a 5 V external electric field. Elemental analysis showed that the carbon content on the surface of bitumen-coated kaolinite particles decreased from 4.43 wt% for suspended kaolinite particles to 4.21 wt% for kaolinite particles deposited on the electrodes. Both measurements revealed shedding of bitumen from the bitumen coating layer on kaolinite surface.