Ultrasonic relaxation in liquid C5-C10, C12 and C14 n-alkanes

Abstract

Ultrasonic absorption has been measured in liquid n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-dodecane and n-tetradecane, over the temperature range from –40°C or the melting point to 25°C and frequency range 15 to 230 MHz. A relaxation observed in the four higher members of this series has been discussed in terms of rotational isomeric processes. The important feature of the results is that the frequency of the acoustically active conformation change decreases with increasing chain length. The variation is similar to that expected for the first normal mode of motion of a chain of damped torsional oscillators. The ratio of compressional to shear viscosities in these systems decreases with increasing chain length, again indicating that a rotational isomeric change of finite volume change is involved in both shear and compressional relaxation.