Dielectric relaxation and the conformer equilibrium in the liquid and glassy states of β-D-fructose

To investigate the ionic and molecular dynamics in the liquid and glassy states of beta -D-fructose, its dielectric relaxation spectra (12 Hz-500 kHz) and dynamic heat capacity (3.33 mHz) have been measured from 5 K above its melting point through the vitrification range, by allowing sufficient time for attainment of the conformer (or chemical) equilibria. Effects of the change in the conformer population on thermal cycling has been further studied. The dielectric behavior of liquid beta -D-fructose is characteristically different from that of other molecular liquids in three ways: (i) the contribution to orientation polarization associated with the fast relaxation process, which persists in the glassy state. is relatively high in the liquid state of beta -D-fructose: (ii) this contribution decreases with temperature exceptionally rapidly on cooling; and (iii) the difference in the rates of the two process is exceptionally large. The dynamic heat capacity change through the vitrification region is similar to 160J/(mol K), and is spread over similar to 20 K range, and the enthalpy relaxation time is similar to 50 s at 383 K. Transformation of beta -pyranose to other conformers and other conformer transformation equilibria change on thermal cycling with the result that the overall relaxation rate increases at T > 315 K and decreases at T < 315 K. The relaxation spectrum becomes broader, the de conductivity increases and the rate of the Johari-Goldstein relaxation whose Arrhenius energy is 42.1 kJ/mol increases. (C) 2001 Elsevier Science B.V. All rights reserved.