Oscillations, Waves, and Interactions - GWDG

Oscillations, Waves and Interactions, pp. 333–366
edited by T. Kurz, U. Parlitz, and U. Kaatze
Universitätsverlag Göttingen (2007) ISBN 978–3–938616–96–3
urn:nbn:de:gbv:7-verlag-1-13-5
Multistep association of cations and anions.
The Eigen-Tamm mechanism some decades later
Reinhard Pottel, Julian Haller, and Udo Kaatze
Drittes Physikalisches Institut, Georg-August-Universität Göttingen
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Abstract. Broadband ultrasonic absorption spectra and complex dielectric spectra for
aqueous solutions of electrolytes are reported and are discussed in terms of cation-anion association
schemes. Ultrasonic spectra for solutions of 3:2 valent salts clearly reveal the complete
Eigen-Tamm multistep association mechanism, including inner-sphere, outer-sphere,
and outer-outer-sphere complexes. Reduced association schemes follow for solutions of 2:2
valent and 2:1 valent salts, the latter just revealing the equilibrium between the complex of
encounter and the outer-sphere complex. Dielectric and ultrasonic spectra are evaluated in
terms of single steps in the intriguingly complex association scheme of ZnCl2 aqueous solutions.
The latter spectra are alternatively discussed assuming a fluctuating cluster model.
1 Introduction
The incomplete dissociation of multivalent salts in solutions is of considerable significance
not just for the theory of electrolytes but also for biochemistry and wide
fields of chemical engineering. The exploration of the molecular dynamics of electrolyte
solutions and of the kinetics of ion complex formation has been an enduring
topic of research at the Dritte Physikalische Institut from the very first. Interest
was originally inspired by the technical problem of measuring distances in sea water
by means of acoustical signals [1]. It was found that sea water may absorb sound
more strongly than distilled water and that, in addition, the absorption depends in
an unexpected manner upon the frequency of the sound field. Already in the early
fifties of the last century Tamm and Kurtze developed techniques, enabling sound
absorption measurements over the remarkably broad frequency range from 5 kHz to
300 MHz, and demonstrated the relaxation characteristics in the frequency dependent
sonic absorption coefficient of 2:2 valent electrolyte solutions [2–7]. Consideration of
sonic spectra also for solutions of 2:1 electrolytes lead to the conclusion that neither
a simple inter-ionic interaction, without involvement of water, nor an interaction of
cations or anions, respectively, just with water could be the reason for the ultrasonic
excess absorption spectra of the aqueous systems [8]. As other effects, such as hydrolysis
and ion cloud interactions were not consistent with the experimental findings,
an interaction between cations, anions, and water molecules has been proposed the