ULTRASONIC-ATTENUATION OF AN ISOBUTYRIC ACID WATER MIXTURE OF CRITICAL COMPOSITION

Ultrasonic attenuation data (frequency range 10 MHz < closed-integral < 45 MHz) of a binary isobutyric acid (COOH)/H2O mixture and a pseudobinary isobutyric acid (COOH)/H2O, D2O mixture of critical composition are analyzed in terms of the Ferrell-Bhattacharjee attenuation function F(OMEGA). The system specific parameter omega(o) necessary to calculate the temperature and frequency dependence of the universal reduced variable OMEGA is determined by static and dynamic light scattering experiments. It is found that F(OMEGA) fails to describe the ultrasonic attenuation in both systems. The data do not scale as expected theoretically and indicate a temperature and frequency dependent background attenuation caused by chemical processes. The background attenuation (alpha/integral-2)b,T reaches considerable values at frequencies integral < 20 MHz and at temperatures away from the critical ((alpha/integral-2)b,T/(alpha/integral-2)T of the order of 0.1). The chemical background attenuation reflects parts of the broad spectrum of low frequency relaxation processes (frequency at the principle relaxation time integral-1 congruent-to 2 MHz) found by Kaatze et al. (J. Chem. Phys. 93, 4955 (1989)) in a broadband ultrasonic attenuation study in a critical isobutyric acid (COOH)/H2O mixture.