DISSOLUTION OF MAGNESIUM-OXIDE IN AQUEOUS ACID - AN ATOMIC-FORCE MICROSCOPY STUDY

The dissolution of the surfaces (100), (110), and (111) of MgO in aque ous hydrochloric acid is studied by in-situ AFM experiments in a flow cell with known hydrodynamics,which permits the modeling of the rate o f proton transport to the solid surface. Comparison with directly meas ured rates of dissolution determined via monitoring the absolute heigh t of the surface in real time shows that the dissolution of all three surfaces is a surface-controlled reaction. Examination of the surface morphology shows that the (100) plane dissolves via the growth of etch pits which are of circular or square shape depending on the acid conc entration. In contrast, the (110) surface dissolves to form a corrugat ed surface of parallel ridges whose surfaces are predominantly compose d of (100) planes. The (111) surface dissolves via triangular etch pit s of a fixed orientation where (100) planes are partly re-expressed du ring the pit growth. These observation are in concordance with previou s reports that the (100) plane is the most stable surface of MgO.