Oscillations, Waves, and Interactions - GWDG

Multistep association of cations and anions 335
are considerably reduced [30,31,33–35]. The latter include systems with calcium as
cation, which have been investigated because of the far-reaching biochemical implications
of that ion.
2 Experimental methods
2.1 Fundamental aspects
Sonic as well as dielectric spectrometry utilize naturally present molecular marks, the
molar volume and the electrical dipole moment, respectively, to monitor the microdynamics
and fast elementary kinetics of liquids (Fig. 1). Experimental techniques are
currently available in the frequency range from 10 3 Hz to 10 10 Hz for sonic absorption
measurements [36,37] and in the even broader range from 10 −6 Hz to 10 12 Hz for the
dielectric spectrometry [38]. Basically both methods aim at the study of the sample
at thermal equilibrium. In order to reach sufficient accuracy in the measurements,
however, in practice the sample is exposed to a small amplitude disturbing sonic
or electromagnetic field, respectively. Methods in use consist in the observation of
the response of the sample to either step pulses or harmonically alternating signals.
The former method, globally named time-domain spectrometry uses pressure jumps,
temperature jumps, or electrical field jumps to slightly disturb the system and to follow
its relaxation into thermal equilibrium by continuous measurement of a suitable
sample property, such as density, electrical conductivity, or dielectric polarization.
Due to molecular interactions this property is unable to instantaneously obey the exterior
force and therefore retardedly reaches its new equilibrium, typically following
an exponential. From a fundamental point of view time domain techniques entail an
unfavourable concentration of energy of the exciting signal in a short period of time.
For this reason, frequency domain techniques, in which the sample response to a
harmonically alternating acoustical or electromagnetic field is observed, are popular,
particularly for high frequency measurements.
Because of the phase lag between the density and the pressure in the sonic wave
and between the polarization and electrical field in the dielectric measurements, energy
of the applied wave is absorbed in the liquid. The amplitude of a plane wave,
Figure 1. Ensemble of water molecules illustrating fluctuations in the direction of the
electric dipole moment (left) and in the molar volume (right).