Direct measurement of forces between silica surfaces in aqueous CaCl₂ solutions using an atomic force microscope

Forces between a silica surface and a small silica particle, in aqueous CaCl₂ solutions, have been measured using an atomic force microscope (AFM). This technique allows the measurement of forces between a surface and virtually any colloid particle of interest. In conditions where the surface coverage of Ca²⁺ was very low (low pH and low cation concentration), a short-range repulsion was measured, which appeared to be due to hydration of the surfaces. When Ca²⁺ ions were adsorbed onto the surface (high pH or high cation concentration), the force measured was always attractive and could be reasonably explained using DLVO theory.