DISRUPTION OF THE HYDROGEN-BONDING NETWORK AT THE SURFACE OF I-H ICE NEAR-SURFACE PREMELTING

By exploiting the potential of highly brilliant Synchrotron X-radiatio n, and taking advantage of oxygen-forbidden reflections we have been a ble to measure surface-sensitive X-ray diffraction from pure hydrogen Bragg reflections at prism surfaces of I-h ice. In contrast to the bul k we observe, at temperatures well below the onset for surface melting (T-s = -12.8 degrees C), broad Lorentzian lines disclosing that the n ear-surface hydrogen bonding network has lost its long-range coherence prior to surface melting. A quantitative analysis of the results show s that a Bjerrum domain structure with a typical domain size of xi(B) = 20-30 Angstrom is present in a mesoscopic surface layer of a thickne ss greater than 250 Angstrom. Within this surface layer the Bjerrum de fect density is [n(B)] = (5.7 +/- 2.5) x 10(25) m(-3). These findings are discussed in view of theoretical models of the surface structure o f ice which are based on the dipolar nature of the H2O molecule, and w e find from this evidence that the Bjerrum domain structure is compati ble with a diffuse screening layer which follows a charged surface. Fr om 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.