Molecular modulation of calcite dissolution by organic acids

Dissolution of the calcite (104) surface in aqueous solution in the presence of 10 organic acids has been studied using fluid-cell atomic force microscopy (AFM) in vitro. Etch pit morphology varies as species conformation changes. [421̄] steps appeared in the presence of each of Gly, l-Glu, l-Lys, malonate, and succinate. The overall shape of etch pits became hexagonal in Gly, malonate, and succinate, while a pseudotriangular shape in l-Glu solution and a sectorial shape in l-Lys solution were observed, primarily as a result of molecular chirality. Unexpectedly, [010] instead of [421̄] steps emerged in l-Asp solution, giving a trapezoidal pit shape. Despite the differences in molecular structure of 6-aminohexanoate, acetate, oxalate, and glutarate, these molecules did not show any influence on pit morphology, revealing that solid/fluid recognition must depend on the geometry of additives, especially the distance between functional groups. We show that both the ammonium and the carboxylate groups are active in surface binding and that the organic acids tend to bind through more than one functional group to the calcite face. Our AFM results confirm the crucial role of geometrical matching between calcite and modifiers and show that step edge reactivity, stereochemical correspondence, electrostatic attraction, and molecular chirality play a secondary role in surface modification. This conclusion will give guidelines for synthesizing bioinspired materials with specific shape. © 2011 American Chemical Society.