SURFACE FORCES BETWEEN SPHERICAL ZNS PARTICLES IN AQUEOUS-ELECTROLYTE

An atomic force microscope has been used to measure the forces of inte raction between ZnS spheres in aqueous electrolyte. The ZnS surface, c haracterized by scanning electron microscopy, tapping mode atomic forc e microscopy, and gas adsorption, was found to be physically heterogen eous. The measured interaction behavior is not well described by conve ntional theory where interacting surfaces are assumed to be physically ideal. In particular the van der Waals attraction could not be measur ed at pH(iep) and elevated ionic strength, where the electrical repuls ion 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 fo rces of long range compared to roughness, electrical and hydrophobic i n this case, are directly measurable. The hydrophobic attraction which becomes evident at low pH was found to be dependent upon electrolyte concentration.