Molecular Ordering of Ethanol at the Calcite Surface

Abstract

To produce biominerals, such as shells, bones, and teeth, living beings create organic compounds that control the growth of the solid phase. Investigating the atomic scale behavior of individual functional groups at the mineral-fluid interface provides fundamental information that is useful for constructing accurate predictive models for natural systems. Previous investigations of the activity of coccolith-associated polysaccharides (CAP) on calcite, using atomic force microscopy (AFM) [Henriksen, K., Young, J. R., Bown, P. R., and Stipp, S. L. S. Palentology 2004, 43 (Part 3), 725-743] and molecular dynamics (MD) modeling [Yang, M., Stipp, S. L. S., and Harding, J. H. Cryst. Growth Des. 2008, 8 (11), 4066-4074], have suggested that OH functional groups control polysaccharide attachment. The purpose of this work was to characterize, using X-ray reflectivity (XR) combined with molecular dynamics (MD) simulations, the structuring on calcite of a layer of the simplest carbon chain molecule that contains an OH group, ethanol (CH(3)-CH(2)-OH). We found evidence that EtOH forms a highly ordered structure at the calcite surface, where the first layer molecules bond with calcite. The ethanol molecules stand up perpendicularly at the interface or nearly so. As a consequence, the fatty, CH(3) ends form a new surface, about 6 Å from the termination of the bulk calcite, and beyond that, there is a thin gap where ethanol density is low. Following is a more disordered layer that is two to three ethanol molecules thick, about 14 Å, where density more resembles that of bulk liquid ethanol. The good agreement between theory and experiment gives confidence that a theoretical approach can offer information about behavior in more complex systems.

Probing the energetics of organic–nanoparticle interactions of ethanol on calcite

“…In this work, to explore the nature of organic ligand-NP interactions, ethanol vapor was used as the adsorbate, whereas a series of nanophase calcite particles was selected to represent a nanoscale inorganic material. The results show a complex pattern of adsorption energetics which lends support to the structural model proposed by computational and microscopic methods (20)(21)(22).…”

“…Strong thermochemical evidence linking the energetics and existence of a predicted spatially thin gap with low ethanol density (22) between the first adsorbed ethanol monolayer and the subsequent layer has been identified.…”

“…They also pointed out that once immersed in bulk ethanol, the calcite surface may impact the molecular arrangement of bulk liquid phase for a significant distance. Furthermore, using coupled X-ray reflectivity and MD, Pasarín et al (22) described a more detailed surface configuration with a highly ordered ethanol monolayer about 6 Å in height forming at the ethanol-calcite interface, and a subsequent thin gap of about 1 Å indicated by an extremely low density of ethanol. This gap is then followed by a region of more disordered layers about 14 Å, mostly with their hydrophobic ends pointing toward the gap.…”

“…Previous studies by molecular dynamics (MD), density functional theory (DFT), and atomic force microscopy (AFM) suggest that, once adsorbed, ethanol molecules self-assemble to a highly ordered monolayer on the calcite surface (20,21). A low ethanol density, spatially thin gap between the first and second layer of adsorbed ethanol is also predicted to exist (22). However, so far, no direct experimental evidence describes the energetics of such a unique surface configuration.…”

Calcium phosphate coatings for marble conservation: Influence of ethanol and isopropanol addition to the precipitation medium on the coating microstructure and performance

“…ii) When a water-ethanol solution is in contact with calcite (independently of the presence of phosphate ions in the solution), ethanol adsorbs to the calcite surface, orienting itself with the hydrophilic OH -group pointing towards the ionic calcite surface and the hydrophobic CH3 group pointing away from the surface towards the bulk solution [41]. This might be counterproductive for the goal of forming a coating over calcite, because adsorbed ethanol could actually provide a barrier to crystal growth perpendicular to calcite surface, since a hydrophobic layer is formed after the first monolayer of ethanol molecules adsorbs to the surface [42].…”