Oscillations, Waves, and Interactions - GWDG

368 U. Kaatze and R. Behrends
with relevance to many phenomena in chemistry, physical chemistry and biochemistry,
as well as for chemical engineering and process control. In aqueous systems a
dominant factor in structure formation is the hydrophilic and hydrophobic interactions.
The pressure exerted on hydrophobic parts of the non-aqueous constituents
to reduce contact with water molecules leads to the formation of clusters, stacks,
micelles, and bilayer membranes [4,5]. A variety of liquid mixtures minimizes energy
by precritical or critical demixing [6–11]. Particularly exiting are systems forming
molecular aggregates and simultaneously exhibiting demixing behaviour. Examples
are solutions of amphiphiles which display a critical micelle concentration and also a
critical demixing point [12–17].
In this review the molecular dynamics of such mesoscopic supramolecular liquid
structures are discussed. We focus on evidence from broadband acoustical spectrometry
as ourdays experimental techniques in that field are still based on pioneering work
in Göttingen and especially also at the Dritte Physikalische Institut. We mention the
benchmark papers by Kurtze, Tamm, and Eigen on the ultrasonic spectrometry of
multivalent salt solutions revealing the stepwise dissociation of ions [18–20], by Plaß
who was among the first to reach hypersonic measurement frequencies [21,22], and by
Eggers who made the resonator method popular for liquid spectrometry [23]. Sonic
waves which couple to thermodynamic parameters and to transport properties, such
as the molecular volume and the shear viscosity, hold the potential to contribute to
an elucidation of the formation of the complex structures mentioned above. Sonic signals
probe the native systems. Therefore, no special labels or markers are necessary,
as are required in many other sophisticated methods.
One of the advantages of acoustical spectrometry is the fact that thermal equilibrium
of the sample under study is virtually kept during measurements, because
only incremental perturbations result from pressure and temperature oscillations associated
with the sonic waves. Another favourable feature is the almost universal
character of the parameters interacting with sonic signals. The universal applicability
of acoustical spetrometry, however, is connected to an often non-specific nature
of results. It is therefore imperative to vary the systems to be investigated in a
considered way, for example by variation of temperature, concentration, or solvent
composition, or to supplement measurements with data from other methods.
2 Experimental aspects
2.1 Fundamentals of acoustical spectrometry
If the small loss from heat conduction is neglected the propagation constant
γ = α + ı β (1)
of longitudinal waves in a liquid with density ϱ and shear viscosity ηs follows from
the Navier-Stokes equation [24] as
γ 2 −ωϱ
=
K + 4 . (2)
3ı ωηs