Absorption of Sound Near the Critical Point of the Nitrobenzene—Iso-octane System

Abstract

Measurements by the pulse technique were made of the absorption of sound in the nitrobenzene—iso‐octane system (T_c=30.2°C) over the full composition range, at temperatures of 30.5°C, 34.8°, and 44.8° for five frequencies between 4.5 and 16.5 Mc/sec. At the lowest frequency and at 0.3°C above the critical temperature, the absorption is anomalously large by a factor of 47; 14.6°C above T_c, the absorption is still in excess of classical predictions by a factor of 11. The data cannot be represented by a model involving only a single relaxation frequency. The Fixman theory is in reasonable, but not exact, agreement with the data with regard to functional conformity in temperature and frequency; in particular, the predicted frequency dependence, α/f²∝f^−5/4, at the critical state is verified within experimental error. The Fixman theory and the data preclude anything but a simple pole at the critical temperature for the reciprocal square of κ, the exponential decay constant in the long‐range correlation function. However, if it is assumed that $${\kappa }^2{\text{=6l}}^{\text{−2}}{\mathrm{(T/T}}_c{\text{−1)[1+a}}_1{\mathrm{(T/T}}_c\text{−1)],}$$ the theory and experiment are in agreement over the full range of temperature and frequency, with a value of a₁=0.0296.