The dielectric properties of humid cellulose

The electrical characteristics of cellophane have been measured at variable moisture contents and temperatures in the frequency range 10(-3)-10(5) Hz. The data show very strong dependence on moisture content, which is typical of hydrophilic solids, and exhibit constant phase angles over wide frequency ranges. Complex admittance is proportional to (jomega)(-(beta-1)) where omega is the frequency and beta approximate to 0.95. We use a model of hopping charge transport to explain the frequency and moisture dependence of the measured spectra. At low frequencies conduction occurs predominantly by phonon-assisted proton hopping between absorbed water molecules, with beta dependent on the energetic disorder of the protonic states. The moisture dependence of the conductivity a is shown to arise from its exponential dependence on the critical percolation distance R-C and to have the form exp{-BN-1/3}, where N is the water concentration and B is a constant. This result explains the universally observed logarithmic dependence on N of the low frequency conductivity in hydrophilic solids. The model also explains the characteristic low frequency dispersion observed in solid ionic conductors. In hydrophilic solids in-phase charge transport due to rotating water molecule dipoles increases with frequency and eventually becomes the dominant contribution to the total conductivity. (C) 2004 Elsevier B.V. All rights reserved.