Interactions between Organophosphonate-Bearing Solutions and (10(1)over-bar4) Calcite Surfaces: An Atomic Force Microscopy and First-Principles Molecular Dynamics Study

Ruiz-Agudo E, Di Tommaso D, Putnis CV, de Leeuw NH, Putnis A

Abstract

The dissolution of (10 (1) over bar4) calcite surfaces was investigated in the presence of 1-hydroxy ethylidene-1,1-diphosphonic acid) (HEDP) (0-10 mM) at pH = 8 using in situ atomic force microscopy (A FM). The presence of the organophosphonate resulted in a change in the appearance of the dissolution features from the typical rhombohedral to elongated, tear shapes. Additionally, dissolution rates were drastically reduced, although they progressively increased with increasing additive concentration. Stabilization of polar steps and effects of HEM) on the structure and dynamics of the hydration shell of Ca2+ may explain such observations. First principles molecular dynamics simulations have been used to study such aspects. The results suggest that the presence of HEDP can increase the frequency of water exchange in the hydration shell of calcium and consequently affect its reactivity in solution. For [HEDP] > 5 mM, we observed the nucleation and growth of Ca(CH3C(OH)(PO3H)(2)center dot 2H(2)O on calcite surfaces. The reaction between solid calcite and HEDP solutions seems to be controlled by the composition of a boundary layer at the carbonate-fluid interface. Dissolution of the carbonate causes this fluid boundary layer to become supersaturated with respect to the phosphonate phase, which then precipitates. The presence of this overgrowth reduces the calcite dissolution rate, thus representing a new treatment aimed at reducing solution-induced weathering of building stone via the formation of a protective nanofilm.