Surface Selectivity of Calcite on Self-Assembled Monolayers

Abstract

A series of molecular dynamics simulations has been performed to analyze in detail the structure of 16-mercaptohexadecanoic acid (even) and 15-mercaptopentadecanoic acid (odd) self-assembled monolayers (SAMs) and their interface with calcium carbonate prior to crystallization. Small structural differences are noted between the odd and even SAMs that cause the headgroup arrangement of the odd SAM to change its local coordination from six nearest neighbors to a 5 + 1 arrangement. It is observed that the disordered calcium carbonate forms layers of cations and anions at the charged interface with the SAM, and this layering predisposes the structure toward the (012) calcite surface and away from the (001) calcite surface. Calculated interfacial energies of the SAM–calcium carbonate interface demonstrate that the even SAM has a far stronger interaction with the calcium carbonate than the odd due to the better alignment of the headgroups and is therefore able to stabilize the growing calcite nucleus, leading to selective growth. Both the predicted orientational order and interfacial energies are in reasonable agreement with experimental observations.