Oscillations, Waves, and Interactions - GWDG

Oscillations, Waves and Interactions, pp. 367–404
edited by T. Kurz, U. Parlitz, and U. Kaatze
Universitätsverlag Göttingen (2007) ISBN 978–3–938616–96–3
urn:nbn:de:gbv:7-verlag-1-14-1
Liquids: Formation of complexes
and complex dynamics
Udo Kaatze 1 and Ralph Behrends 2
1 Drittes Physikalisches Institut, Georg-August-Universität Göttingen
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
2 Fakultät für Physik, Georg-August-Universität Göttingen
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Abstract. Acoustical relaxation spectra, measured in the frequency range between roughly
10 4 and 5·10 9 Hz, are discussed in view of the formation of mesoscopic molecular structures,
like small complexes, stacks, and micelles, as well as of microheterogeneous liquid structures,
as characteristic for noncritical and critical concentration fluctuations in binary systems. A
variety of results is presented to show the capability of the method. An extended version of
the model of micelle formation/decay kinetics is given that accounts for the special features
of surfactant solutions near their critical micelle concentration. Also a unifying model
of noncritical concentration fluctuations, that includes all previous theories, is shown to
favourably apply the experimental findings. Evidence is presented indicating the need for
a comprehensive theoretical treatment of systems revealing both, critical micelle formation
and critical demixing properties.
1 Introduction
Liquids owe their fascinating and diverse features to molecular interaction energies
on the order of the thermal energy. Thermal motions thus prevent liquids from
establishing long-range order. Additionally, short-range order fluctuates rapidly. Let
us consider water, the omnipresent chemical on our planet, as an example. Water
is among the associating liquids. As the water molecule is capable of forming four
hydrogen bonds, liquid water establishes a percolating three-dimensional hydrogen
bond network. Due to thermal agitation a single hydrogen bond fluctuates with
correlation time on the order of 0.1 to 1 ps [1]. Even the dielectric relaxation time of
water, which reflects the period required for the reorientation of the dipolar molecules
through a significant angle [2], is as small as 10 ps at room temperature [3]. Hence
liquids are characterized by the rapid fluctuations of their short-range order. In order
to understand liquid properties we need to investigate their molecular motions.
In addition to the establishment of the hydrogen bond network, mesoscopic molcular
structures may be formed in aqueous solutions and in mixtures of water with
other constituents. The knowledge of such structures and of their formation and decay
processes is most important for our understanding of self-organization in liquids