Absorption of Ultrasonic Waves in Organic Liquids (I) Liquids with Positive Temperature Coefficients of Sound Absorption

Ultrasonic absorption coefficients have been determined for 12 organic liquids by the pulse technique in dependence of temperature (0°〜40℃). The frequency employed was 8.25 Mc/sec, 14.5 Mc/sec or 20.0 Mc/sec. The "frequency-independent" absorption α/v^2 (α: the absorption coefficient. v: frequency) at 20℃ are as follows (in units of 10^<-17> sec^2/cm): benzene 838 carbon tetrachloride 510, chloroform 406, cyclohexane 503, ethylene chloride 184, chlorobenzene 141, nitrobenzene 86, hexane 80, heptane 55, toluene 94, xylene 81, benzylchloride (13℃) 75. All these liquids proved to have positive temperature coefficient of ultrasonic absorption in the temperature range of 0°〜40℃, the absorption increasing about 0.5〜0.6% per degree Toluene (1.06%/℃) and benzene (0.84%/℃) showed greater temperature coefficients of absorption and carbon tetrachloride and cyclohexane (ca. 0.27%/℃) showed lower values of temperature dependence than the other liquids. The chief mechanism of ultrasonic absorption in these liquids is attributed to the thermal relaxation of molecular vibrations, though the absorption due to viscosity amounts to 20% in some of the liquids. The relaxation times of vibrational energy have been computed for three of the liquids, giving β=3.64×10^<-10> sec for benzene, 1.37×10^<-10> sec for carbon tetrachloride and 1.51×10^<-10> sec for chloroform at 20℃. The activation energy for the deactivation of molecular vibration are of the order of several hundred cal/mol at most for benzene, carbon tetrachloride and chloroform. The exact values of the relaxation time were not to be calculated for other liquids because of the lack of the molecular vibrational data, but they are of the order of 10^<-11> see. The activation energy is presumably smaller than several hundred cal/mol.