Surface forces between spherical ZnS particles in aqueous electrolyte

An atomic force microscope has been used to measure the forces of interaction between ZnS spheres in aqueous electrolyte. The ZnS surface, characterized by scanning electron microscopy, tapping mode atomic force microscopy, and gas adsorption, was found to be physically heterogeneous. The measured interaction behavior is not well described by conventional theory where interacting surfaces are assumed to be physically ideal. In particular the van der Waals attraction could not be measured at pH(iep) and elevated ionic strength, where the electrical repulsion between surfaces is minimal. In addition no short range repulsion that could be attributed to hydration was observed. If the range of a surface force is small compared to the roughness of the surface, then it may have little effect on particle interaction. In contrast only forces of long range compared to roughness, electrical and hydrophobic in this case, are directly measurable. The hydrophobic attraction which becomes evident at low pH was found to be dependent upon electrolyte concentration.