Three-dimensional solvation structure of ethanol on carbonate minerals

Abstract

Calcite and magnesite are important mineral constituents of the earth’s crust. In aqueous environments, these carbonates typically expose their most stable cleavage plane, the (10.4) surface. It is known that these surfaces interact with a large variety of organic molecules, which can result in surface restructuring. This process is decisive for the formation of biominerals. With the development of 3D atomic force microscopy (AFM) it is now possible to image solid–liquid interfaces with unprecedented molecular resolution. However, the majority of 3D AFM studies have been focused on the arrangement of water at carbonate surfaces. Here, we present an analysis of the assembly of ethanol – an organic molecule with a single hydroxy group – at the calcite and magnesite (10.4) surfaces by using high-resolution 3D AFM and molecular dynamics (MD) simulations. Within a single AFM data set we are able to resolve both the first laterally ordered solvation layer of ethanol on the calcite surface as well as the following solvation layers that show no lateral order. Our experimental results are in excellent agreement with MD simulations. The qualitative difference in the lateral order can be understood by the differing chemical environment: While the first layer adopts specific binding positions on the ionic carbonate surface, the second layer resides on top of the organic ethyl layer. A comparison of calcite and magnesite reveals a qualitatively similar ethanol arrangement on both carbonates, indicating the general nature of this finding.

Water Orientation at the Calcite-Water Interface

“…For comparison, the same data are reproduced for (d) the calcite-ethanol and (d) magnesite-ethanol interfaces from the literature. 21 The double red density regions for the water oxygen atoms in the first hydration layer are caused by two oxygen atoms (each from two adjacent carbonate groups) being within hydrogen bond distance. The carbonate oxygen with which the water molecule interacts is prone to vary throughout the simulations.…”

“…Another way to test the hypothesis that the small SFG signal for calcite originates from the oppositely oriented water molecules within the first and second layer is to break this specific molecular alignment by exchanging water with ethanol. The interaction of ethanol with magnesite and calcite has been studied in ref ( 21 ) using both AFM ( Figure 1 c,d) and MD ( Figure 2 c,d). We know from these MD results that ethanol on both magnesite and calcite in the first layer is oriented with the OH group pointing to the surface.…”

“…Solvation structure of ethanol at the (c) ethanol-calcite (10.4) and (d) ethanol-magnesite (10.4) interface. The ethanol data have been previously published in the literature 21.…”

Molecular assemblies on surfaces: towards physical and electronic decoupling of organic molecules

“…However, the alkane buffer layer provided the possibility to monitor the STM-induced nucleation, growth, and ripening of self-assembled monolayers in a more controlled fashion. Söngen et al [ 77 ] provide insight into the interaction of organic molecules with bulk insulators by discussing the adsorption of ethanol on both calcite and magnesite using three-dimensional AFM experiments. Although molecules adsorbed on bulk insulators are electronically decoupled, molecular self-assemblies can experience a substrate templating effect due to the presence of heterogeneous adsorption sites.…”

“…Although molecules adsorbed on bulk insulators are electronically decoupled, molecular self-assemblies can experience a substrate templating effect due to the presence of heterogeneous adsorption sites. Therefore, Söngen et al [ 77 ] found on bulk calcite and magnesite that the first ethanol layer arranges in a laterally ordered way due to ionic interactions, where ethanol adopts well-defined adsorption positions on the carbonate surface. In contrast, the following layers lack this order as they reside on ethanol layers.…”