Ultrasonic Absorption Measurements Near the Critical Point of a Reactive Binary Mixture

Abstract Measurements of sound attenuation and velocity in the homogeneous phase near the critical mixing point of isobutyric acid and water have been carried out. The data are obtained by a pulse transmission technique at seven frequencies between 9 and 45 MHz in a temperature range between 26.29°C and 40 °C. The experimental results are analysed in terms of a theory of critical ultrasonic absorption proposed by Bhattacharjee and Ferrell which contains no adjustable parameters. All the values of these parameters are known for the investigated system from independent experiments. The frequency dependence of the absorption coefficient of ultrasound at the critical temperature is in good agreement with this theory which takes into account only sound absorption by local concentration fluctuations. The absorption data obtained at temperatures away from the critical temperature do not scale when plotted as function of a reduced frequency given by the theory. This clearly indicates an additional mechanism causing absorption of ultrasound. This additional sound absorption can be represented by a single temperature dependent relaxation process. It is assumed that this process is due to an unspecified chemical reaction. The temperature dependence of the relaxation time follows a power law of the form τ r ∝︁ ( T ‐ T c ) −a with 1 ≤ a ≤ 1.25. This result seems to support the idea of Gitterman. Procaccia and Hentschel that chemical reactions slow down near the critical point.

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