The temperature dependence of the dielectric properties of κ- and ι-carrageenan aqueous solutions with various counterion species, K, Ca, and Na, were investigated over the frequency range 10-2-106 Hz and the temperature range 2.0-80.0 °C. In the case of κ-carrageenan solutions, just below the coil-to-helix transition temperature, TCH, the dc conductivity sharply decreases within a few degrees of temperature. In contrast, the sharp decrease was not observed for ι-carrageenan solutions. The dielectric relaxation process with the relaxation time ∼100 μs, which can be assigned to the counterion fluctuation in the parallel direction to the helical axis, arises below TCH. Just below TCH, the relaxation strength increases sharply with decreasing temperature for both the carrageenan type with all the counterion species exhibiting the coil-to-helix transition, reaching ∼103 at the temperatures far below TCH. These findings indicate that the counterions are tightly bound to helical molecules due to an increase of the charge density along the helical axis during the coil-helix transition. The relaxation time reflecting the fluctuation distance increases sharply in the initial stage of gelation and gradually reaches a constant value. We think that the formation of the high charge density region, which is connected with the aggregated region of helices, is determined by which process, a growth of the length of the helices or the aggregation of the helices, precedes in the initial stage of gelation.
ASJC Scopus subject areas
- Materials Chemistry