Journal of Inclusion Phenomena and Macrocyclic Chemistry
43 (1-2): 107-113, June 2002
Copyright © 2002 Kluwer Academic Publishers
All rights reserved
Dielectric Relaxation of the b-Cyclodextrin Complexes with Tridecanoic Acid and 1,13-Tridecanedioic Acid
John C. Papaioannou
Author for correspondence Department of Chemistry, Laboratory of Physical Chemistry, National and Kapodistrian University of Athens, P.O. BOX 64004, 157 10 Zografou, Athens, Greece
Thanos C. Ghikas
Department of Chemistry, Laboratory of Physical Chemistry, National and Kapodistrian University of Athens, P.O. BOX 64004, 157 10 Zografou, Athens, Greece
Irene M. Mavridis
Institute of Physical Chemistry, N.C.S.R. ``Demokritos'', Aghia Paraskevi, 15310, Greece
Abstract
The frequency and temperature dependence of the real ([epsi]') and imaginary ([epsi]'') parts of the dielectric permittivity of the polycrystalline complex b-cyclodextrin-tridecanoic acid in two hydration forms (with 16.2 and 10.7 water molecules) and b-cyclodextrin-1,13-tridecanedioic acid with 16.4 and 10.5 water molecules have been investigated, in the frequency range 0.1–100 kHz and temperature range 120–310 K. The dielectric behavior is described well by Debye-type relaxation a dispersion. All systems except for the complex of partially dehydrated monocarboxylic acid, exhibit an additional W-dispersion, at low frequencies (f < 1000 Hz). Only one-step was found in the [epsi]' vs. T plots of both complexes in the two hydration forms, a fact indicating that the water molecules cannot be divided into strongly bound and easily movable molecules. The [epsi]''vs. T plots, at a fixed frequency (200 Hz), show the characteristic peak attributed to a transition between ordered and disordered b-CD hydroxyl groups and water molecules. The transition temperature was 202.7 K for all systems examined except for the complex b-CD-tridecanoic acid.16.2 H2O (214.5 K). This means that the order to disorder transformation process was unaffected by the dehydration process in the case of the dicarboxylic acid complex, whereas in the case of the monocarboxylic acid, it was unexpectedly facilitated. The relaxation time varies with temperature, in a L like curve (in the range 8–14 ms), with maximum values located at the corresponding order-disorder transition temperatures. The activation energies of the fully hydrated complexes have absolute values of ~5 kJ/mol in the range 1.98–3.82 KBT transition which are higher than the corresponding values of ~:2 kJ/mol of the dehydrated complexes. A thermal hysteresis observed in all complexes is a result of the order-disorder transformation.
Keywords beta-cyclodextrin dimer, dielectric permittivity, dielectric properties, tridecanoic acid, 1,13-tridecanedioic acid
Article ID: 405974