Structure of the Calcite-Water Interface
Measurements of the calcite-water interface with specular and non-specular X-ray reflectivity have led to a new understanding of the three-dimensional calcite-water interfacial structure (1,2). Key observations include:
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Calcite is terminated by a surface hydration layer having two water molecules at heights of 2.3 ± 0.1 Å and 3.45 ± 0.2 Å above the surface plane. Both are laterally ordered with respect to the lattice.
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No significant ordering of water beyond the surface hydration layer. The lower water molecule appears to complete the locally distorted oxygen coordination shell of the surface Ca ion while the second appears more weakly associated with the carbonate group, possibly through a hydrogen bond.
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Very good agreement with recent computation studies by Kerisit and Parker (3).
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Figure: (A,B) The rectangular calcite (104) surface with two CaCO3 units related by a glide plane along x, with the CO3 groups tilted by ~45°. (C) Representative specular X-ray reflectivity data. (D) Derived structural model showing substrate CaCO3 layers and hydration layers. |
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1) P. Geissbühler, P. Fenter, E. DiMasi, G. Srajer, L.B. Sorensen, N.C. Sturchio, "Three-Dimensional Structure of the Calcite-Water Interface", Surface Science, 573(2), 191-203 (2004).
2) P. Fenter, P. Geissbühler, E. DiMasi, G. Srajer, L. B. Sorensen, N. C. Sturchio, Geochimica et Cosmochimica Acta 64, 1221-1228 (2000).
3) S. Kerisit and S.C. Parker, Chemical Communications, (1), 52-53 (2004).
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