Mechanisms of acidic dissolution at the MgO(1 0 0) surface

A quantum chemical, atomic level simulation of the first mechanistic steps leading to dissolution at low pH of the MgO(1 0 0) surface was performed using the semi-empirical SCFMO method MSINDO. Previous experimental studies using transmission electron microscopy have shown that the initially flat surface of cubic MgO immediately restructures on immersion in dilute acid to form a patchwork of nanometer-sized square pits and protrusions. Five possible reaction sequences for the initial stages of the dissolution process were investigated using quantum-chemical calculations. From the initial planar surface an energetically feasible pathway has been determined which consists of the migration of two adjacent Mg–O pairs to form a square pit and two neighbouring protrusions with subsequent transfer of hydrated Mg2+ into solution. The calculated energy barriers for this mechanism are small and are commensurate with the observed rapid formation of pits and subsequent dissolution at the MgO surface observed at room temperature. Dissolution without the prior formation of these pits was found not to be energetically feasible.