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 b roadband spectrometry (absorption and velocity measurements with coherent s ound waves) in liquids and liquid systems, covering, at present, a frequenc y 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 use ful for investigation of various chemical, biochemical, and physicochemical systems: Sound waves traversing liquids induce periodic perturbations in p ressure and temperature, which can shift equilibria, resulting in character istic sound absorption and velocity dispersion spectra. An analysis of such spectra yields valuable information about thermodynamic and kinetic parame ters of the particular system that is often difficult to obtain by other me thods. Since such periodic perturbations imposed on the system are incremen tal in nearly all cases, the system can be studied under equilibrium condit ions. All nonlinear effects (heating, nonconstant fluid compressibility, an d others) are negligible, permitting, for instance, the application of line arized rate equations. In this review, various examples of measured broadba nd spectra are presented. Related elementary processes are discussed. Among these are ionic and molecular reactions, including mechanisms of associati on and complexation, proton transfer, solvation, isomerization, interconver sion, side-group rotation, hydrogen-bonding, as well as stacking processes and micelle formation. Special attention will be given to the extensive res earch 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.