Water Orientation at the Calcite-Water Interface

Abstract

Mineral-water interfaces play an important role in many natural as well as technological fields. Fundamental properties of these interfaces are governed by the presence of the interfacial water and its specific structure at the surface. Calcite is particularly interesting as a dominant rock-forming mineral in the earth’s crust. Here, we combine atomic force microscopy, sum-frequency generation spectroscopy, and molecular dynamics simulations to determine the position and orientation of the water molecules in the hydration layers of the calcite surface with high resolution. While atomic force microscopy provides detailed information about the position of the water molecules at the interface, sum-frequency generation spectroscopy can deduce the orientation of the water molecules. Comparison of the calcite-water interface to the interfaces of magnesite-water, magnesite-ethanol, and calcite-ethanol reveals a comprehensive picture with opposite water orientations in the first and second layer of the interface, which is corroborated by the molecular dynamics simulations.

Formation and Stability of Heterogeneous Organo–Ionic Surface Layers on Geological Carbonates

“…Vibrational bands for both hydrogen-bonded and isolated OH groups were absent for the freshly cleaved mineral surface. On wetting with ultrapure water, a major band at 3630 cm –1 (Figure S6) confirms the presence of a water layer that cannot H-bond with the well-structured surface hydration of calcite. , This nicely demonstrates the phenomenology of “ordered water that does not completely wet water” operating on the surface of a ubiquitous mineral under ambient conditions. We also observe a minor band at 3460 cm –1 , which likely arises due to small contents of adventitious carbon and natural fluorophores on surfaces of Iceland spar that H-bond with water molecules. , Thus, for asphaltenes desorbing from carbonate surfaces, the increased contents of non-hydrogen-bonded OH groups (Figure c) are better elucidated by the failure of water molecules to effectively H-bond with the surface hydration of pristine (i.e., asphaltene-free) calcite.…”

“…On wetting with ultrapure water, a major band at 3630 cm −1 (Figure S6) confirms the presence of a water layer that cannot H-bond with the well-structured surface hydration of calcite. 116,124 This nicely demonstrates the phenomenology of "ordered water that does not completely wet water" 128 operating on the surface of a ubiquitous mineral under ambient conditions. We also observe a minor band at 3460 cm −1 , which likely arises due to small contents of adventitious carbon and natural fluorophores on surfaces of Iceland spar that H-bond with water molecules.…”

“…This is supported by results of SEM imaging (Figure S5), in which LSW-treated carbonate surfaces specifically exhibit dissolution or etch pits. 127 Second, strongly ordered water molecules on carbonate surfaces 124 could enhance H-bonding inside the hydration layer and in turn reduce the possibility of H-bonding with external molecules. 128 In this scenario, the increased intensities of bands at 3630 and 3700 cm −1 on brine exposures also reflect the generation of asphaltene-free calcite surfaces but rather explained with the inability of the atmospheric water molecules to H-bond with the surface hydration layer of pristine calcite.…”

“…Signals from the surface hydration of calcite (3000−3600 cm −1 ) are absent. This is attributed to opposite orientations of the water dipoles constituting the first and second hydration layers 124 and for that reason broad O−H stretching bands from H-bonded interfacial water molecules were absent. However, we do observe vibrational bands at 3630 and 3700 cm −1 that are attributable to isolated or nonhydrogen-bonded OH groups.…”

Molecular Features of Hydration Layers: Insights from Simulation, Microscopy, and Spectroscopy

“…The signals from these two opposite orientations nearly canceled out, giving a weak signal at 3400 cm −1 . 51 The bending modes of molecular vibration were also studied by using VSFG: Moll et al observed two peaks in the VSFG signal located at 1626 and 1656 cm −1 , which were assigned to the interfacial dipolar and bulk quadrupolar responses, respectively. 24 Kim et al reported a VSFG experiment for the hydration layer on a multilayered graphene.…”