No abstract

Ultrasonic velocity and attenuation have been measured in 2,3-dimethylbutane and 2,2,4-trimethylpentane from 213 to 243 K and pressures up to 100 MPa. The temperature-and pressure-dependences of the relaxation frequency permit the calculation of the activation enthalpy and the activation volume, respectively. Furthermore, equilibrium parameters are derived from the maximum of absorption per wavelength. Large and positive activation volumes of the order of 10cm3mol-' are observed, whereas the volume change associated with the rotational isomerism is negative, indicating a preference for the gauche states at elevated pressures.

Different devices have been developed for ultrasonic measurements at atmospheric and elevated pressures up to 1 kbar. The velocities of sound for liquid toluene, 2,3‐dimethylbutane, cyclohexane, and camphene have been determined. In the case of 2,3‐dimethylbutane also preliminary absorption measurements at elevated pressures have been performed, according to which the activation volume for the hindered internal rotation is negligible. Furthermore some plastic phases have been studied. The velocities of sound (longitudinal mode) of cyclohexane and camphene increase considerably on freezing in contradiction to the findings of others. Additional measurements of the transverse modes allow a rough estimation of the elastic stiffness constants.