AFM Investigation for the Initial Growth Processes of Calcium Carbonate on Hydrophilic and Hydrophobic Substrate

Heterogeneous nucleation and crystal growth of calcium carbonate on the mica and the highly oriented pyrolytic graphite (HOPG) substrate have been investigated by means of an atomic force microscope (AFM) with a contact mode procedure. AFM observation could lead to initial crystallization processes that were amorphous and crystalline, with an intermediate state between them. Amorphous calcium carbonate deposited on surfaces could be observed as a transient precursor phase of crystal. The shape of the amorphous calcium carbonate on the substrates is indicative of final crystal shape. Classical heterogeneous nucleation theory has been considered in order to describe the surface crystallization on the mica and the HOPG. We then measured the contact angle between the amorphous calcium carbonate and the substrates. The formation energy required for the heterogeneous nucleation on the HOPG surface was 40 times higher than that on the mica surface. To explain the difference in the mica and the HOPG surface, we have discussed a possible model for the interfacial structure between the solution and the substrate.