Ultrasonic Study of Nitro‐Benzene/Isooctane Mixtures near the Critical Consolute Point

Abstract 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 – T c ) in the homogeneous phase region. The critical data of the system having an upper critical point are: critical temperature T c = 29.16°C; critical composition y nc = 0.525 (mass fraction of nitro‐benzene). The experiments are carried out in the temperature range 0.3 K ≤ ( T – T c ) ≤ 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 ω 0 of the temperature‐dependent characteristic relaxation frequency are available for that system. Therefore, the coefficient ω 0 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 ω 0 = 15 MHz. The agreement between theory and experiment is good and supports the universality concept of the critical acoustic behavior.

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