Dielectric relaxation of beta-cyclodextrin complex with 4-t-butylbenzyl alcohol

The frequency and temperature dependence of the real (epsilon') and imagina ry (epsilon ") parts of the dielectric constant of polycrystalline complex beta-cyclodextrin-4-t-butylbenzyl alcohol [beta-CD.TERB.11.2H(2)O] and beta -cyclodextrin [beta-CD 9.8H(2)O] and of the corresponding dried forms (beta -CD. TERB.3.8H(2)O and beta-CD.2.4H(2)O, respectively) has been investigate d, in the frequency range 0-100kHz and temperature range 130-350K. The diel ectric behaviour is described well by Debye-type relaxation (alpha dispersi on). All systems except for the beta-CD.TERB.3.8H(2)O, exhibit an additiona l Omega dispersion at low frequencies, which usually is attributed to proto n transport. In the non-dried samples the temperature dependence of epsilon ' and epsilon(max)(") exhibits two steps, whereas in the dried samples it e xhibits only the low temperature step. The low temperature step is due to t he tightly bound water molecules, whereas that at higher temperatures is du e to easily removable water. The temperature dependence of epsilon " shows a peak which has been attributed to a transition between ordered and disord ered hydroxyl beta-CD groups, and water molecules. The relaxation time vari es exponentially with temperature (in the range 8-12 mu sec), in a reverse V like curve, with maximum values located at the corresponding: order-disor der transition temperatures. Activation energies of the order of similar to 2.5 kJ mol(-1) are calculated for the transition in every sample. The diso rder 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 th e order-disorder transition and the temperature dependence of the relaxatio n time. An apparent negative activation energy before the transition temper ature can be attributed to reorientation of the hydrogen bonding around the cyclodextrin molecules, and it is related to endothermic drifts observed b y calorimetric studies of beta-CD. The order-disorder transition can be pro bed also from the phase shift component of the current passing through the sample relative to the applied signal.