Dielectric Behavior of Water Molecules Adsorbed on Iron(III) Oxide Hydroxides

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The change in the dielectric constant of α-FeOOH, β-FeOOH, and γ-FeOOH with the amount of adsorbed water was examined at 30.0 °C in the frequency range 200 Hz–100 kHz for the purpose of clarifying the mechanism of water adsorption. The adsorption isotherms belong to the BET II type. The sample with the adsorbed water shows a particular dispersion of the dielectric constant with frequency in the low frequency range. The dispersion was ascribed to the transition between different orientational directions of the dipole moment of the adsorbed water. The change in the dielectric constant with the amount of adsorbed water for each iron(III) oxide hydroxide was interpreted in relation to the surface structure of predominant faces of crystals, in particular the number and position of hydroxyls on crystal surfaces. The following adsorption process was suggested below the monolayer adsorption coverage for each iron(III) oxide hydroxide. For α-FeOOH, water molecules are adsorbed on the (100) planes in three successive steps. For β-FeOOH, water molecules are primarily bound in the tunnels existing in the crystal along a direction parallel to the c-axis and then are adsorbed on the cracked tunnels exposing to the outer surface. For γ-FeOOH, water molecules are adsorbed on the (010) planes in two steps.
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