Ultrasonic broadband spectrometry of liquids: A research tool in pure and applied chemistry and chemical physics

This review is a survey of the many scientific applications of ultrasonic broadband spectrometry (absorption and velocity measurements with coherent sound waves) in liquids and liquid systems, covering, at present, a frequency range from nearly 10 kHz to 10 GHz. Ultrasonic spectrometry has proved to be an almost universal research tool in many laboratories, one that is useful for investigation of various chemical, biochemical, and physicochemical systems: Sound waves traversing liquids induce periodic perturbations in pressure and temperature, which can shift equilibria, resulting in characteristic sound absorption and velocity dispersion spectra. An analysis of such spectra yields valuable information about thermodynamic and kinetic parameters of the particular system that is often difficult to obtain by other methods. Since such periodic perturbations imposed on the system are incremental in nearly all cases, the system can be studied under equilibrium conditions. All nonlinear effects (heating, nonconstant fluid compressibility, and others) are negligible, permitting, for instance, the application of linearized rate equations. In this review, various examples of measured broadband spectra are presented. Related elementary processes are discussed. Among these are ionic and molecular reactions, including mechanisms of association and complexation, proton transfer, solvation, isomerization, interconversion, side-group rotation, hydrogen-bonding, as well as stacking processes and micelle formation. Special attention will be given to the extensive research on chemical relaxation. Fundamental early and recent publications are cited and discussed. Many references are included with particular emphasis on less well known research and publications from countries of the former USSR. This review aims at a demonstration of the widespread applications of modem ultrasonic techniques in many fields of liquid-state research.