Scaling function of critical binary mixtures: Nitrobenzene-n-hexane data revisited

Ultrasonic attenuation spectra of the nitrobenzene-n-hexane mixture of critical composition have been analysed. Data between 50 kHz and 1 GHz from different sources have been included to show that at a given temperature the spectra, in addition to the critical contribution, reveal a non-critical relaxation term. Taking this additional term into account inconsistencies in the scaling function reported in the literature are avoided. In the final analysis the scaling function of the nitrobenzene-n-hexane system follows the predictions of the Bhattacharjee-Ferrell theory with critical amplitude and relaxation rate of concentration fluctuations in nice agreement with determinations from independent methods. The low-frequency attenuation data are briefly discussed with a view to a bulk viscosity approach which yields a slightly different proportionality constant in the linear regime of the scaling function than the Bhattacharjee-Ferrell theory. Evidence in favour of the latter is obtained.