Direct Observation of the Influence of Additives on Calcite Hydration by Frequency Modulation Atomic Force Microscopy

The effects of inorganic and organic additives on the hydration structure on the crystal surface have been discussed in X-ray reflectivity studies and in molecular dynamics simulations. We now demonstrate their effects on the hydration structure by conducting in situ observations of the hydration structure at a growing calcite surface by frequency modulation atomic force microscopy (FM-AFM). We show the atomic scale change of the hydration structure on the calcite surface in a supersaturated solution of CaCO3 by the addition of magnesium ions and a hydrophilic polypeptide. The FM-AFM images of the hydration structure revealed that magnesium ions increase the number of hydration layers on the terrace of the calcite surface from two to four layers. On the other hand, the hydrophilic polypeptide was ineffective for the hydration of the calcite surface. When both the magnesium ions and the hydrophilic polypeptide were added to the CaCO3 solution, the number of hydration layers increased and the magnitude of the oscillation hydration force as well as the long-range electrostatic force became larger than in the case when they were individually added. This is a noteworthy effect on the hydration structure on the calcite surface by cooperation of the magnesium ions and the polypeptide.