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

Abstract

The interfacial forces between a polyethylene particle and a toner substrate in alkaline aqueous solutions were studied using an atomic force microscope colloidal probe technique. Measurements were taken at pH 9 in water and solutions of 5 × 10-4 M CaCl2, 1 × 10-4 M Na oleate, and 1 × 10-4 M Na oleate plus 5 × 10-4 M CaCl2 in order to mimic the conditions present during de-inking flotation. A polyethylene particle was used to represent the air bubble. The observed interaction forces were described by the extended DLVO theory. An energetic barrier caused by electrical double-layer repulsion was observed in water and Na oleate solutions but was greatly diminished in CaCl2 solution. A long-range attractive force was found to be present in these systems and was described using a simple exponential function. The long-range attractive force was virtually the same in water and CaCl2 solution but decreased significantly in Na oleate solution because of the reduced hydrophobicity of the interacting surfaces caused by the adsorbed carboxylate layer. However, in the presence of oleate and calcium ions the observed attraction was even stronger and of longer range than in water and CaCl2 solutions. Moreover, no energetic barrier was observed. These results can be attributed to the presence of precipitated calcium oleates on the interacting surfaces.