AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Acceleration of Calcite Deposition Imposed by Organic Substrate of Self-Assembled Monolayers
(1) Geological Sciences, U. of Michigan, Ann Arbor, MI.
(2) Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, CA.
Underground deposition of calcium carbonate (CaCO3) withdrawing atmospheric CO2 is a crucial topic for carbon sequestration. Plenty of evidence has proved that microbes and organic matters are able to speed up carbonate deposition rate up to a couple of order of magnitudes. In the past decade, to reproduce the controls of organic molecules on carbonate mineralization, simpler self-assembled monolayers (SAMs) terminated by carboxylic groups have been heavily used as model templates for CaCO3 crystallization.
COOH-terminated 16-mercaptohexadecanoic acid (MHA) monolayer is the most representative SAM. It strongly favors nucleation of calcite on the non-natural (012) oriented 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 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.
The results reveal that (1) the MHA SAM significantly reduces the surface energy created during calcite nucleation from at least 97 mJ/m2 for homogenous nucleation in solution to about 46 mJ/m2, which confirm the strong capacity of MHA of promoting CaCO3 deposition; and (2) amorphous calcium carbonate begins to appear when the supersaturation with respect to that phase is only 0.02, suggesting that the pathway of nucleation is not determined by the free energy barrier, but rather the kinetic factors.