Dielectric relaxation of β-cyclodextrin–polyiodide complexes (β-cyclodextrin)2·LiI7·8H2O and (β-cyclodextrin)2·KI7·8H2O

Abstract

The frequency and temperature dependence of real (ε′) and imaginary (ε′′) parts of the dielectric constant of polycrystalline complexes (β-CD)2·LiI7·8H2O and (β-CD)2·KI7·8H2O (β-CD = β-cyclodextrin) have been investigated over the frequency and temperature ranges of 0–100 kHz and 120–300 K. The temperature dependence of ε′, ε′′ and phase shift ϕ showing two steps, two peaks and two minima respectively, reveals the existence of two kinds of water molecule, the tightly bound and the easily movable water molecules, in both complexes. The first peak of ε′′(T) or the first minimum of ϕ(T) present the transformation of flip-flop hydrogen bonds to the normal state. The second ε′′(T) peak or ϕ(T) minimum correspond to the easily movable water molecules or to a partial transformation of tightly bound to easily movable water molecules. Both samples for T>275 K show semiconductive behaviour with energy gaps 0.72 eV for the (β-CD)2·LiI7·8H2O and 0.58 eV for the (β-CD)2·KI7·8H2O complex. The conductivity at temperatures T>220 K is greater for the Li complex and at T < 220 K both complexes have similar conductivity values. The relaxation time varies in a Λ-like curve (from 180 K to 260 K) and rises rapidly for temperatures greater than 260 K, indicating the process of ionic movements. The activation energies around the transition temperature 0.40–0.50 k B T trans for the (β-CD)2·LiI7·8H2O and 0.78–1.00 k B T trans for the (β-CD)2·KI7·8H2O reveal the greater stability of the β-K complex against that of the β-Li complex.