Ultrasonic Relaxation Due to trans—gauche Rotational Isomerism in the Liquid Normal Paraffins

Abstract

The ultrasonic absorption has been measured in liquid normal butane, pentane, hexane, and heptane at frequencies of 5, 15, and 45 Mc/sec over the temperature range from their freezing to boiling points at atmospheric pressure. The absorption was also measured in propane and two mixtures of pentane with propane at a frequency of 45 Mc/sec for temperatures in the range −130° to −50°C. The measurements indicate internal rotation about internal C–C bonds to be the relaxation mechanism and a barrier for the conversion from a gauche to a trans configuration of 3.4±0.4 kcal/mole for n‐butane, 4.2±0.4 kcal/mole for pentane, 3.1±0.4 kcal/mole for hexane, and 3.8±0.4 kcal/mole for heptane. They also indicate a difference between the molar volume of gauche and trans isomers of butane which is less than 0.3% and a value for the ratio of the compressional to shear viscosities caused by molecular rearrangement of 1.1 for propane, 6.3 for pentane, 6.2 for hexane, and 5.7 for heptane.