Oscillations, Waves, and Interactions - GWDG

358 R. Pottel, J. Haller and U. Kaatze
respectively. The parameter mDH (0≤ mDH ≤1) reflects correlations between different
clusters with mDH = 1 constituting the limiting situation of ideally connected
relaxation sequences in the cluster system. Such alternative considerations of the
sound absorption spectra of electrolyte solutions have been currently renewed by a
distribution function theory in which the sonic relaxations in the spectra are related
to the long-range concentration fluctuations within the liquid [32]. We mention that
the ultrasonic absorption spectra of 1:1 valent tetraalkylammonium bromide aqueous
solutions exhibit strong indications of precritical fluctuations in the local concentration
[33].
4.3 Other 2:1 valent salts
Present ultrasonic spectrometry is sufficiently sensitive to indicate the small-amplitude
relaxations due to ion complex formation in aqueous solutions of 2:1 valent salts [34,
35,50]. For a transition-metal-chloride solution an example is given in Fig. 4. A spectrum
with even smaller relaxation amplitude is shown in Fig. 23 where the ultrasonic
excess absorption for a solution of an alkaline earth metal chloride is displayed versus
frequency. All relevant excess absorption spectra measured so far can be well represented
by a single Debye-type relaxation term (Eq. (7)). Parameters as resulting
from a regression analysis of the spectra are given in Table 3.
The relaxation times τ of the solutions are strikingly similar. Obviously, the τ
values do not noticeably depend upon the cation radius and on the electronic structure
of the cation. As contact ion pair formation sensitively reflects the properties
of the cations the observed relaxation is unlikely due to a process that involves in-
Figure 23. Ultrasonic excess absorption spectrum for a 1 mol/l solution of SrCl2 in water
at 25 ◦ C [35]. In order to disclose the excess absorption not to be due to systematic errors in
the measurements, data from different specimen cells and apparatus are marked by figure
symbols. The line represents a Debye relaxation term (Eq. (7)) with the parameter values
given in Table 3.