Abstract: The surface forces between alumina colloidal sphere and alumina flat surface in magnesium chloride
solutions were measured by an atomic force microscope. The forces between alumina surfaces were measured as a function of surface separation, salt
concentration, and pH. Experimental decay lengths for the repulsive electric double layer interaction were in good agreement with the theoretical Debye
lengths in the range of 0.0001mol/L to 0.01mol/L. The Debye length decreased with the increasing of the salt concentrations due to the compression of the
electric double layer. No attractive force could be detected in the pH range between 3.64 and 9.09, and the zeta potential was always positive, which
strongly verified the existence of the specific adsorption. The short-ranged non-DLVO forces were observed in the magnesium chloride solutions at pH 9.52.
Magnesium ions took the form of Mg(OH)⁺ and Mg₂(OH)³⁺ to form the adsorption layer, which changed into precipitation layer with the rise of pH,
the thickness was estimated as 5nm.

Key words: atomic force microscope, α-Al₂O₃, magnesium chloride, interaction force