Dielectric Relaxation of the β -Cyclodextrin Complexes with Tridecanoic Acid and 1,13-Tridecanedioic Acid

Abstract

The frequency and temperature dependence of the real (∈') and imaginary (∈'') parts of the dielectric permittivity of the polycrystalline complexβ-cyclodextrin-tridecanoic acid in two hydration forms (with 16.2 and 10.7 water molecules) and β-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 α dispersion. All systems except for the complex of partially dehydrated monocarboxylic acid, exhibit an additional Ω-dispersion, at low frequencies (f < 1000 Hz). Only one-step was found in the ∈' vs. Tplots of both complexes in the two hydration forms, a fact indicating that the watermolecules cannot be divided into strongly bound and easily movable molecules. The∈''vs. T plots, at a fixed frequency (200 Hz), show the characteristic peakattributed to a transition between ordered and disordered β-CD hydroxyl groupsand water molecules. The transition temperature was 202.7 K for all systems examinedexcept for the complex β-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 Λ like curve (in the range 8–14 μs), 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.