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Thickness and structure of the water film deposited from vapour on calcite surfaces |
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| Authors: | J Bohr RA Wogelius PM Morris |
| Institution: | a Nano-Science Center, Department of Chemistry, University of Copenhagen, Denmark b Department of Physics, Technical University of Denmark, Denmark c School of Earth, Atmospheric, and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, University of Manchester, UK |
| Abstract: | Synchrotron X-ray reflectivity (SXR) was used to measure the thickness of the water film that adsorbs on a { } cleavage surface of calcite (CaCO3) in a sample chamber where relative humidity could be controlled within the range from <4% to 90%. Gases used to carry water vapour were initially either 100% N2 or 100% CO2. The product water film was remarkably constant in thickness at 15.5 Å (±1 Å) and independent of humidity. When N2 was used as the carrier gas, this film displayed a gap in its electron density at between 0.6 and 2 Å distance from the calcite surface, depending on humidity. This implies that a change in the arrangement of water molecules occurs in direct proximity to the surface. This electron density discontinuity was measurably further from the calcite surface, at 3.4 Å, when CO2 was used as the carrier gas. Except for this thin low density region proximate to the calcite surface, the density of the adsorbed water layer was 0.9 g cm−3, therefore suggesting a significant degree of ordering. Atomic force microscopy (AFM) images were completed in conjunction with the SXR measurements on similarly prepared calcite cleavage surfaces. AFM showed that terraces may be atomically flat over 1 μm or more. SXR corroborated this observation, with results showing that carefully cleaved surfaces had a starting root mean square (r.m.s.) roughness of ∼1.2 Å. Diffuse scatter measurements constrained the correlation lengths of these surfaces to be at least 18,000 Å. For comparison with the cleaved samples, a surface was also prepared by chemo-mechanical Syton polishing. This surface gave an r.m.s roughness by SXR that was an order of magnitude higher, equal to 12.1 Å. In this case, diffuse scatter resolved a correlation length of 950 Å, and revealed a fractal dimension that was higher than for the cleaved surface. On Syton polished samples, the water film determined by SXR was about twice as thick as for freshly cleaved surfaces, with a density of 1.0 g cm−3, equal to that of bulk water. However, surface roughness was too large to allow resolution of any gap in the electron density within the water layer proximate to the solid surface. Our AFM observations also confirm previous reports of calcite surface recrystallization. The electron density of the solid surface determined by SXR is indistinguishable from that of calcite, indicating that any material recrystallized within the adsorbed water film is compositionally indistinguishable from the calcite substrate. |
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