Oscillations, Waves, and Interactions - GWDG

334 R. Pottel, J. Haller and U. Kaatze
cause for the absorption characteristics. This interaction involves a stepwise substitution
of water molecules in the coordination shells of a cation-aquo complex by an
anion [9,10]
(M m+ )aq + (L l− )aq ⇀↽ (M m+ (H2O)2L l− )aq
⇀↽ (M m+ (H2O)L l− )aq (1)
⇀↽ (ML) (m−l)+
aq
In this scheme � Mm+ (H2O) 2 Ll−� denotes an outer-outer sphere complex in which
aq
the metal ion Mm+ and the ligand Ll− are separated by two layers of water molecules.
represents the outer sphere complex with one water layer be-
� M m+ (H2O) L l−�
aq
tween cation and anion, and (ML) (m−l)+
aq is the inner sphere complex, the contact ion
pair.
Both former steps in the Eigen-Tamm scheme (Eq. (1)) contribute to the sonic
excess absorption at high frequencies. In order to increase the significance of the
spectra Plaß and Kehl extended the frequency range of measurements up to 2.8
GHz [11]. Nevertheless a clear conclusion on the number of relaxation terms within
the experimental spectra was not reached and the existence of the second step
in the complex formation scheme, reflecting the equilibrium between outer-outer
sphere and outer sphere complexes, has been controversely discussed [12–16]. Combining
frequency domain (ultrasonic) techniques and time domain (pressure-jump)
techniques of the Drittes Physikalisches Institut and the Max-Planck-Institut für
Biophysikalische Chemie, Göttingen, it was possible to clearly reveal three relaxation
terms in the spectra of scandium sulfate solutions and to show thereby that
all three steps in the reaction scheme (Eq. (1)) of the Eigen-Tamm mechanism may
exist at least in 3:2 valent electrolyte solutions [17].
Complementary to the sonic spectrometry studies broadband dielectric measurement
techniques have been developed and have been employed to verify the concept of
ion complex formation. A detailed investigation into 2:2 valent electrolyte solutions
was the first to identify dipolar ion structures by relaxation terms in the dielectric
spectra of solutions [18,19]. Later 3:2 valent [20] as well as 1:1, 2:1, and 3:1 valent electrolytes
[21] were studied. Particular attention was paid to zinc(II) chloride solutions
with their intriguing complexation properties [22,23]. During the last years interest
in dielectric relaxations due to ion-pairs in solutions has been rediscovered [24–29].
The studies of ZnCl2 solutions have been supported by ultrasonic attenuation
measurements which, in addition to any evaluations in terms of the conventional
complexation scheme [30], have been alternatively dicussed employing a model of
rapidly fluctuating ion clusters [31]. Recently, the Eigen-Tamm concept of stepwise
association has been questioned even for 2:2 valent electrolytes and a mode-coupling
theory has been presented assuming fluctuations in the ion concentration to be the
cause of the high-frequency ultrasonic relaxation term of multivalent electrolytes
instead of the formation of stoichiometrically well defined outer-sphere complexes [32].
Here we briefly review evidence in favour of the multistep association mechanism
(Eq. (1)) of 2:2 and 3:2 valent electrolytes. We also discuss spectra of 1:1 and 2:1
valent salt solutions in which Coulombic interactions between cations and anions