|Publisher:||Taylor & Francis|
|Issue:||Volume 97, Number 5/September 10, 1999|
|Pages:||611 - 627|
Dielectric relaxation of beta-cyclodextrin complex with 4-t-butylbenzyl alcohol
JOHN C PAPAIOANNOU, NIKOS D PAPADIMITROPOULOS, IRENE M MAVRIDIS
The frequency and temperature dependence of the real (epsilon') and imaginary (epsilon'') parts of the dielectric constant of polycrystalline complex beta-cyclodextrin-4-t-butylbenzyl alcohol [beta-CD TERB 11.2H2O] and beta-cyclodextrin [beta-CD 9.8 H2O] and of the corresponding dried forms (beta-CD TERB 3.8H2O and beta-CD 2.4 H2O, respectively) has been investigated, in the frequency range 0-100kHz and temperature range 130-350K. The dielectric behaviour is described well by Debye-type relaxation (alpha dispersion). All systems except for the betaCD TERB 3.8H2O, exhibit an additional dispersion at low frequencies, which usually is attributed to proton transport. In the non-dried samples the temperature dependence of epsilon and epsilon max exhibits two steps, whereas in the dried samples it exhibits only the low temperature step. The low temperature step is due to the tightly bound water molecules, whereas that at higher temperatures is due to easily removable water. The temperature dependence of epsilon shows a peak which has been attributed to a transition between ordered and disordered hydroxyl beta-CD groups, and water molecules. The relaxation time varies exponentially with temperature (in the range 8-12musec), in a reverse V like curve, with maximum values located at the corresponding order-disorder transition temperatures. Activation energies of the order of 2.5kJmol-1 are calculated for the transition in every sample. The disorder in the hydrogen bonding is equivalent to a system of two dipoles with opposite directions, and the model of Frohlich can be applied to explain the order-disorder transition and the temperature dependence of the relaxation time. An apparent negative activation energy before the transition temperature can be attributed to reorientation of the hydrogen bonding around the cyclodextrin molecules, and it is related to endothermic drifts observed by calorimetric studies of beta-CD. The order-disorder transition can be probed also from the phase shift component of the current passing through the sample relative to the applied signal.