Interfacial Energy of Calcite-MHA/MUA Self-assembled Monolayers during Nucleation (Qiaona Hu)

 

Geologic calcium carbonate (CaCO3) mineralization is a crucial topic for underground CO2 storage. Plenty of evidence has proved that the types of polymorphs, the morphology, and, thus, crystallographic orientations of CaCO3 are controlled by organic matter. In the past decade, self-assembled monolayers (SAMs) of alkanethiols (relatively simple organic molecules as analogues for more complex organic templates) have been heavily used as model templates for crystallization to reproduce the controls of organic molecules on biomineralization of CaCO3.

COOH-terminated 16-mercaptohexadecanoic acid (MHA) monolayer is one of the most representative SAMs. It strongly favors nucleation of calcite on the non-natural (012) faces (Aizenberg et al., 1999; Whitesides et al., 1994). However, the kinetic and thermodynamic controls imposed by MHA are poorly understood. The reduction of formation energy of calcite by MHA has not been addressed. In this study, we cultivate calcite at MHA surfaces that are assembled on Au (111) faces and quantify the interfacial energy associated with template-directed calcite on MHA by measuring the dependence of the nucleation rate on supersaturation.

To evaluate the role of the odd–even effect of the alkyl chain and of different metal substrates on interfacial energies, experiments analyzing the interfacial energies of calcite with 11-mercaptoundecanoic acid (MUA) on Au are conducted, as well as with MUA on Ag and with MHA on Ag.

The Influence of Ammonia on CaCO3 Polymorph Formation (Qiaona Hu)


Our studies show the ability of ammonium/ammonia to promote the formation of vaterite over calcite. Based on the results of this study, changes in growth morphologies cannot only be attributed to organic additives or templates. Inorganic components in the growth solution, e.g., ammonia when supplied by the NH3 diffusion method and growth substrates from the reaction container or suspended materials may be equally powerful in promoting certain growth polymorphs. Thus, experiments have to be carefully designed to understand (and, if necessary, eliminate) the roles of ammonium and the substrate, especially if these are not growth modifiers to be studied. Otherwise, it may be impossible to tell whether the studied factors, such as organic additives, are the real causes that select vaterite over calcite.


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