The dark and radiation-induced microwave conductivity of frozen aqueous gels

Charge migration has been studied in frozen aqueous gels of the polysaccharide κ-carrageenan by measuring the microwave conductivity after pulsed irradiation with high-energy electrons. It is concluded that the conductivity, which is observed above a critical water weight fraction of 27%, is due to the migration of highly mobile excess electrons in the ice-like regions of the sample, as previously proposed for hydrated DNA, gelatin and collagen. At low temperatures and in samples with only slightly more than the ''critical'' water content the electrons were found to decay non-exponentially over many orders of magnitude in time. Results are also presented on the dark conductivity of the samples in the X-band region and for the water contents and temperatures applicable to the pulse-radiolysis experiments. The dark conductivity shows discontinuities at the same water content as the critical value for the radiation-induced conductivity, further illustrating the transition in the nature of the aqueous medium surrounding the biopolymer. The results are compared with those obtained for other biopolymers. A general conclusion is that charge carriers formed within the biopolymer-bound water regions are immobile or, if mobile, become localised within a timescale much shorter than a nanosecond, thus excluding long-range migration. However, the fully hydrated polymer does not appear to behave as an effective trapping site for the mobile electrons formed in adjacent ice regions. (author)