Long-range attractive forces and energy barriers in de-inking flotation: AFM studies of interactions between polyethylene and toner

The interfacial forces between a polyethylene particle and a toner substrat e in alkaline aqueous solutions were studied using an atomic force microsco pe colloidal probe technique. Measurements were taken at pH 9 in water and solutions of 5 x 10(-4) M CaCl2, 1 x 10(-4) M Na oleate, and 1 x 10(-4) M N a oleate plus 5 x 10(-4) M CaCl2 in order to mimic the conditions present d uring de-inking flotation. A polyethylene particle was used to represent th e air bubble. The observed interaction forces were described by the extende d DLVO theory. An energetic barrier caused by electrical double-layer repul sion was observed in water and Na oleate solutions but was greatly diminish ed in CaCl2 solution. A long-range attractive force was found to be present in these systems and was described using a simple exponential function. Th e long-range attractive force was virtually the same in water and CaCl2 sol ution but decreased significantly in Na oleate solution because of the redu ced hydrophobicity of the interacting surfaces caused by the adsorbed carbo xylate layer. However, in the presence of oleate and calcium ions the obser ved attraction was even stronger and of longer range than in water and CaCl 2 solutions. Moreover, no energetic barrier was observed. These results can be attributed to the presence of precipitated calcium oleates on the inter acting surfaces.