Disruption of the hydrogen-bonding network at the surface of Ih ice near surface premelting

By exploiting the potential of highly brilliant Synchrotron X-radiation, and taking advantage of oxygen-forbidden reflections we have been able to measure surface-sensitive X-ray diffraction from pure hydrogen Bragg reflections at prism surfaces of Ih ice. In contrast to the bulk we observe, at temperatures well below the onset for surface melting (TB = −12.8°C), broad Lorentzian lines disclosing that the near-surface hydrogen bonding network has lost its long-range coherence prior to surface melting. A quantitative analysis of the results shows that a Bjerrum domain structure with a typical domain size of ξB= 20−30Å is present in a mesoscopic surface layer of a thickness greater than 250 Å. Within this surface layer the Bjerrum defect density is 〈nB〉 = (5.7 ± 2.5) × 1025m−3. These findings are discussed in view of theoretical models of the surface structure of ice which are based on the dipolar nature of the H2O molecule, and we find from this evidence that the Bjerrum domain structure is compatible with a diffuse screening layer which follows a charged surface. From straightforward screening arguments and the observed Bjerrum defect density, we can then estimate the surface charge density, and we find a remarkable agreement with recent friction experiments