Ultrasonic Relaxation in Aqueous Solutions of Tetraalkyl Ammonium Salts

Abstract

Ultrasonic-absorption measurements have been made with send-receive pulse techniques in aqueous solutions of tetrabutyl ammonium bromide as a function of frequency (up to 500 MHz), concentration, pressure (up to 2 kbar), and temperature. A double-Carstensen cell system in a high-pressure vessel has been used for the determination of the pressure dependence at and below 50 MHz. The results give evidence of two relaxation frequencies, one above 300 MHz for concentrations of 0.6 M or greater at 25°C and 1 atm, and the second below 50 MHz. The molecular absorption cross section increases abnormally fast with increasing concentration above 1 M, particularly at lower frequencies, suggesting that a large number of solvent molecules are involved with the solute in the low-frequency relaxation process. Various mechanisms are considered and predictions based thereon compared with experiment. The high-frequency relaxation is attributed to a second-order diffusion controlled-association process. [Research partially supported by the Office of Saline Water and the Office of Naval Research.]