The absorption and velocity of ultrasound are measured in a critical mixture of nitro‐benzene and isooctane (2,2,4‐trimethyl‐pentane) as function of the temperature difference (T–Tc) in the homogeneous phase region. The critical data of the system having an upper critical point are: critical temperature Tc = 29.16°C; critical composition ync = 0.525 (mass fraction of nitro‐benzene). The experiments are carried out in the temperature range 0.3 K ≤ (T–Tc) ≤ 21.8 K at seven frequencies between 9 MHz and 45 MHz. Four mixtures with off‐critical compositions and the pure components are also studied. — The sound absorption of the critical mixture is analysed in terms of the Bhattacharjee‐Ferrell theory. At the critical temperature the frequency dependence of ultrasound absorption is in agreement with the predictions of this theory. The data obtained at temperatures away from the critical temperature collapse on a single scaling curve when plotted as function of a reduced frequency given by the theory. Not all of the input data for the calculation of the temperature‐independent coefficient ω0 of the temperature‐dependent characteristic relaxation frequency are available for that system. Therefore, the coefficient ω0 is used as free parameter in a fitting procedure of the theoretical scaling function to the experimental scaling function. The best fit is obtained with ω0 = 15 MHz. The agreement between theory and experiment is good and supports the universality concept of the critical acoustic behavior.

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.