Oscillations, Waves, and Interactions - GWDG

356 R. Pottel, J. Haller and U. Kaatze
Figure 21. Ultrasonic excess absorption spectrum for a solution of 0.8 mol/l ZnCl2 in water
at 25 ◦ C. The subdivision of the spectrum into three Debye relaxation terms is indicated by
dashed lines. The full line represents the sum of these terms [30].
louin scattering study of zinc chloride hydrated melts [97]. Three terms with discrete
relaxation time are necessary to represent the experimental spectra adequately
(Fig. 21). Consistency of the dependencies of relaxation parameters upon the salt
concentration, however, requires even four Debye relaxation terms.
As a four-Debye-term model comprises too many unknown parameters the spectra
have been evaluated assuming a reduced reaction scheme
Zn 2+ + 4Cl − ⇀↽ (Zn 2+ ) ∗ + 4Cl − ⇀↽ (ZnCl) + + 3Cl −
⇀↽ ZnCl2 + 2Cl − ⇀↽ (ZnCl3) − + Cl −
⇀↽ (ZnCl4) 2−
in which (Zn 2+ ) ∗ is assumed an activated zinc ion, e.g. an ion with one hydration
molecule less than Zn 2+ . Relating the rate constants and reaction volumes of the
coupled equilibrium to one another the number of unknown parameters in the regression
analysis of the ultrasonic spectra has been reduced so that reliable rate constants
and reaction volumes followed from the fitting procedures [30].
Alternatively, the multitude of proposed ion complex structures (Fig. 19) and the
finding of precritical behaviour for concentrated zinc chloride aqueous solutions [98]
has suggested the idea of rapidly fluctuating diffuse ion clusters rather than of stoichiometrically
defined species [31]. The Romanov-Solov’ev model of noncritical concentration
fluctuations [99–101] has been implemented. But, even if an additional
Debye relaxation is taken into account, this model did not satisfactorily apply to the
spectra at all concentrations. However, a superior analytical description of the experimental
excess absorption spectra is reached with the Dissado-Hill (“DH”) relaxation
spectral function with only four unknown parameters (Fig. 22). This function has
been originally derived for a uniform representation of a diversity of dielectric spec-
(20)