Abstract: The surface forces between alumina colloidal sphere and alumina flat surface in sodium chloride and magnesium chloride solutions were measured by atomic force
microscope. A short-ranged, non- DLVO force was observed in 10^-4 mol/L MgCl₂ solutions keeping the pH as 3.96 and 7.92 respectively. When pH was raised to 9.5,
the short-ranged repulsive force was also observed, it hindered the two surfaces close to each other due to the attractive Wan der Walls force. We proved that
in the first condition, the short-ranged non DLVO force was originated from the specific adsorption of magnesium ions, which could be verified from Zeta potential
measurements. In another case, this short -ranged repulsive force was from the specific adsorption of several kinds of magnesium hydroxides. With the rising of
pH, the adsorbed layer became dense and lost its elasticity.

Key words: α-Al₂O₃, magnesium chloride, short-ranged repulsive force, atomic force microscope