Interaction forces between hydrophobic coal particles and air bubbles during flotation are of significant academic and practical importance. When ionic surfactants are adsorbed by interacting surfaces, the flotation process can change due to changes in interfacial forces. An atomic force microscope (AFM) colloidal probe technique was used to measure the interaction forces between a coal surface and a polystyrene sphere in the presence of dodecylamine hydrochloride and sodium dodecylsulfate solutions. A significant dependence of these interaction forces on surfactant concentration was observed. In addition, zeta potential, bubble attachment time and contact angle measurements were performed, which together with AFM force measurements, give a more detailed description of this interfacial system involving two hydrophobic surfaces.