Slowing down in chemical reactions. The isobutyric acid/water system in the critical region

Ultrasonic attenuation spectra (30 kHz less than or equal to nu less than o r equal to 400 MHz) of the isobutyric acid/water mixture of critical compos ition and also of the acid itself (50 kHz less than or equal to nu less tha n or equal to 1100 MHz) are discussed at different temperatures. Quasielast ic light scattering data from photon correlation spectrometry of the critic al system are evaluated to yield the amplitude D-0 of the mutual diffusion coefficient in the homogeneous phase. Using literature values for the ampli tude of the fluctuation correlation length, the background and critical par t of the heat capacity, and the linear coefficient in the pressure dependen ce of the critical temperature T-C, the sonic attenuation spectrum as predi cted by the Bhattacharjee-Ferrell model has been calculated for the critica l mixture at T-C. Following again this theoretical model, the contribution due to concentration fluctuations at the temperatures of measurement and al so the high-frequency asymptotic background contribution has been subtracte d from the experimental spectra. The resulting excess attenuation spectra o f the isobutyric acid/water mixture reveal two relaxation processes, both c haracterized by a discrete relaxation time. These Debye-type relaxations ar e discussed in terms of the monomer/linear dimer and linear dimer/cyclic di mer equilibria of the carboxylic acid. The relaxation times of the mixture of critical composition exhibit slowing characteristics in the chemical rea ctions near the critical temperature (T - T-C < 5 K) which cannot be explai ned by the critical behavior of the viscosity. Rather there seems to be an intrinsic effect that slows down near the critical point.