Static and dynamic light scattering measurements with nitrobenzene/isooctane mixtures of critical composition in combination with measurements of temperature dependence of specific heat and density are used to obtain values of parameters appearing in the Ferrell‐Bhattacharjee theory of ultrasound absorption of binary critical mixtures (σ0: characteristic temperature independent relaxation rate, ω0 = (43.8 ± 0.7) GHz; g: coupling constant, g = – (0.56 ± 0.01). Recent ultrasound absorption data of that system are reanalysed on the basis of Ferrell‐Bhattacharjee theory. The analysis of data measured at seven frequencies of ultrasound in the range 9 MHz to 45 MHz gives values of the parameters ω0 and g which are in satisfactory agreement with those obtained from light scattering and thermodynamic measurements (ω0 = (37 σ 3) GHz; lgl = 0.5).

Critical Phenomena / Liquids / Phase Transitions / ViscosityResults of measurements of the temperature dependence of shear viscosity of seven nitro-benzene/isooctane mixtures of different compositions in the critical region are reported. The analysis of the viscosity data of the mixture of the critical composition in terms of critical exponent gives a value of c/J~= 0.048 ± 0.001. It is higher by about 20% than the theoretical value based on dynamic renormalization group theory calculations. The shear viscosity data of the non-critical mixtures are in agreement with the scaling concepts of. critical phenomena. Fig. 1 Ubbelohde type viscometer used for the experiments ±2 mK) in a temperature range 25°C < T < 60°C. By rotating the viscometer it was possible to move the mixture to be studied from the lower part of the viscometer to its upper part before starting a new experiment. The viscosity of the mixtures was calculated from the experimentally determined efflux time of the fluid contained in a defined volume above the capillary (v = at-bit; v: kinematic viscosity; f/(= vie): shear viscosity; e: density; a.b: constants; t: effiux time). The viscometer was calibrated with water. The effiux time of water at 25°C had a value of about 300 s and at 45°C about 200 s. The 5 capillary '" (f! = 0.43 rnm) ai volume C 1=3.5 em l ) volume B 1=20 em l ) E u a -: 2. Experimental ViscosityThe construction of the Ubbelohde type viscometer used in this study is shown in Fig. 1. It was filled with about 20 em! of the mixture to be studied and was flame sealed thereafter. The change of composition of the mixture within the viscometer due to evaporation into its dead space could be neglected. The viscometer was mounted on a rack which allowed the viscometer to be turned by an angle of 360°and positioned again reproducibly in the vertical position. The rack with the viscometer was immersed into a carefully controlled thermostat (long time temperature stability oT = 2.1. Substances Isooctane (2,2,4-trimethylpentane (MERCK» was of Uvasol quality with a stated gas chromatographical purity of 99%. It was dried over P 4 0 IQ and fractionally distilled in a concentric tube column of 75 theoretical plates in an inert gas atmosphere (Nj-gas), Nitro-benzene ("for Kerr cells" (MERCK» was treated and purified in the same way as isooctane. The fractional destillation was carried out under reduced pressure (P = 2 mm Hg). A gas chromatographical analysis of the final product gave a purity of 99.9%.The mixtures were prepared by weight.

Ultrasonic attenuation data (frequency range 10 MHz < f < 45 MHz) of a binary isobutyric acid(COOH)/H2O mixture and a pseudo‐binary isobutyric acid(COOH)/H2O, D2O mixture of critical composition are analyzed in terms of the Ferrell‐Bhattacharjee attenuation function F(Ω). The system specific parameter ω0 necessary to calculate the temperature and frequency dependence of the universal reduced variable Ω is determined by static and dynamic light scattering experiments. It is found that F(Ω) 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 (α/f2)b,T reaches considerable values at frequencies f < 20 MHz and at temperatures away from the critical ((α/f2)b,T/(α/f2)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 f1 ≃ 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.