J. Adhesion Sci. Technol., Vol. 14, No. 14, pp. 1829–1843 (2000)  VSP 2000. Long-range attractive forces and energy barriers in de-inking flotation: AFM studies of interactions between polyethylene and toner J. DRELICH 1,∗ , J. NALASKOWSKI 2 , A. GOSIEWSKA 1 , E. BEACH 1 and J. D. MILLER 2 1 Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931, USA 2 Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112, USA Received in final form 17 June 2000 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. Mea- surements were taken at pH 9 in water and solutions of 5 × 10 −4 M CaCl 2 ,1 × 10 −4 M Na oleate, and 1 × 10 −4 M Na oleate plus 5 × 10 −4 M CaCl 2 in order to mimic the conditions present during de-inking flotation. A polyethylene particle was used to represent the air bubble. The observed inter- action 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 CaCl 2 solution. A long-range attractive force was found to be present in these systems and was de- scribed using a simple exponential function. The long-range attractive force was virtually the same in water and CaCl 2 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 CaCl 2 solutions. Moreover, no energetic barrier was observed. These results can be attributed to the presence of precipitated calcium oleates on the interacting surfaces. Keywords: Atomic force microscopy; de-inking; flotation; hydrophobic forces; interfacial forces; surface chemistry; wastepaper. 1. INTRODUCTION Paper recycling requires the collection of wastepaper and the cleaning of cellulose fibers from ink and other additives introduced during paper production and printing ∗ To whom correspondence should be addressed. Phone: (906)487-2932; Fax: (906)487-2934; E-mail: jwdrelic@mtu.edu