Dissolution of the calcite (104) face under specific calcite-aspartic acid interaction as revealed by in situ atomic force microscopy

In the presence of aspartic acid (Asp), the calcite (104) face shows distinct dissolution pit morphology, presumably resulting from the surface reaction between calcite and Asp. However, the specific nature of this interaction and the influence of solution hydrodynamics remain unclear. To this end, we have followed the calcite (104) surface dissolution using in situ fluid cell atomic force microscopy (AFM). The results showed that at pH 4.5 and in 100 mM Asp the surface reactions were controlled by diffusion under static conditions and that trapezoidal etch pits were formed. In contrast, elliptical etch pits were rapidly developed upon flowing due to the increased transfer of Asp to the [010] step edge and the dissolution of Asp-surface complexes away from the step edge. The occurrence of the [010], [461̄], and [4̄11] steps of trapezoidal etch pits was attributed to the stabilization of the (001), (1̄12), and (01̄1) faces by Asp through bridging between the two carboxyl groups and two adjacent Ca atoms, with the α-NH 3 + group forming a hydrogen bond with the oxygen of the H 2O from the bulk solution and the surface CO 3 groups from the (1̄12) and (01̄1) faces. The mirror images of the etch pits formed in d-Asp and l-Asp solutions resulted from the enantio-specific interaction, supporting the tripodal contact of Asp with the crystal surface. Thus, the etch pit morphology is affected by Asp concentration, mass transfer, and specific surface reaction. © 2012 American Chemical Society.