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| Titel: |
Dielectric relaxation of α-cyclodextrin—polyiodide complexes (α-cyclodextrin)2 · LiI3 · I2 · 8H2O and (α-cyclodextrin)2 · Cd0.5 · I5 · 26H2O |
| Auteur: |
GHIKAS, THANOS C. PAPAIOANNOU, JOHN C. |
| Verschenen in: |
Molecular physics |
| Paginering: | Jaargang 100 (2002) nr. 5 pagina's 673-679 |
| Jaar: |
2002-03-10 |
| Inhoud: |
The frequency and temperature dependence of real (ε') and imaginary (ε'') parts of the dielectric constant of polycrystalline complexes (α-CD)2 · LiI3 · I2 · 8H2O and (α-CD)2 · Cd0.5 · I5 · 26H2O (α-CD = α-cyclodextrin) has been investigated over the frequency and temperature ranges of 0-100 kHz and 12-300 K. The dielectric behaviour is described well by Debye type relaxation (α-dispersion). Both systems exhibit an additional Ω dispersion at low frequencies which is attributed to ionic conductance and is much greater in the case of Li due to the greater mobility of cations Li+. The temperature dependence of ε' reveals the existence of two kinds of water molecule in the case of the (α-CD)2 · Cd0.5 · I5 · 26H2O complex; these can be classified as tiqhtly bound and easily movable water molecules that cause two steps in ε' versus T plots. In the case of the (α-CD)2 · LiI3 · I2 · 8H2O complex the water molecules are tightly bound and as a result only one step is observed in these graphs. These finding are also confirmed from the ε''max versus T plots, which exhibit the same number of steps with ε', and from calorimetric measurements. The order-disorder transition or the transformation of normal hydrogen bonds to flip-flop type has been observed as a peak in ε'' versus T plots that is more intense and narrow in the case of Li and less high but more broad in Cd. The relaxation time vanes in a α-like curve (from 120 K to 240 K) and rises rapidly for temperatures greater than 240 K, indicating the existence of a new process involving the breaking of hydrogen bonds (normal or flip-flop type). The calculated values of activation energy (0.35-0.62 kBTtrans) reveal the greater stability of the Li compared with the Cd complex. The starting value of 8.2-8.4 μs for τ is the same as observed in β-CD complexes with guest 4-t-butylbenzyl alcohol (TERB). However, the activation energies of these are greater (1.1-1.7kBTtrans), indicating greater stability for β-CD complexes. |
| Uitgever: |
Taylor & Francis |
| Bronbestand: |
Elektronische Wetenschappelijke Tijdschriften |
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