Dielectric behaviour of adsorbed water films on the ?-Fe2O3 surface

Abstract

The electrical capacitance at 70 c/s to 300 kc/s of physically adsorbed water on α-Fe₂O₃ is constant within the monolayer volume but rises abruptly at low frequencies with the onset of a second layer. This behaviour suggests that the first layer of physically adsorbed water is immobile, but that succeeding layers are mobile. Characteristic relaxation frequencies obtained from Cole-Cole arc plots increase smoothly from 10 c/s to 10 kc/s at B.E.T. coverages of three or higher. Three intermediate stages in the build-up of the adsorbed water film are suggested : an immobile layer of physically adsorbed water doubly hydrogen bonded to the underlying hydroxyl layer, a more loosely held second layer of water on the average singly hydrogen bonded, and finally ordered ice-like layers. Dielectric activation energies calculated from the temperature dependence of the characteristic frequency give further evidence for a structured adsorbate. At 2½ layers, the activation energy is 16 kcal/mol, a value near that characteristic of dipole rotation in bulk ice. With increased coverage, the activation energy increases toward 25 kcal/mol, suggestive of increased interaction between adsorbate dipoles.