The Absorption of Ultrasonic Waves in a Number of Pure Liquids over the Frequency Range 100 to 200 Mc/s

Extended frequency range measurements for determining the Kneser-type acoustic relaxation time

In the present paper, the authors discuss studies carried out for many years dealing particularly with two compounds: benzene and carbon disulphide and compare them with the results obtained by numerous acoustics researchers. These compounds are typical liquids in which acoustic Kneser-type relaxation occurs, caused by an irreversible vibrational and translational (VT) transition. Since magnitudes describing the relaxation process were diverse in many papers, we have undertaken an attempt to clarify these differences and to indicate how to avoid errors resulting from instrumental imperfections and the disregard of the considerable measurement errors when investigating velocity dispersion in the hypersonic (GHz) range. The results of these researches changed the interpretation of previous papers.

Ultrasonic relaxation and rotational isomerism in liquid sym -tetrabromoethane and tri- n -butylamine

Measurements have been made of the absorption of ultrasonic waves at frequencies in the range 10 to 200 Mc/s in sym -tetrabromoethane from 0 to 60°C and in tri- n -butylamine from – 60 to 60°C. For both liquids the absorption is determined in part by a relaxing process which is associated with the equilibrium between rotational isomers. The results for sym -tetrabromoethane are compared with infra-red data obtained by Kagarise (1956), and the potential energy barriers hindering rotation between the gauche and trans forms of the molecule are determined from the measurements. The energy parameters associated with a two-state model for tri- n -butylamine are evaluated and compared with previous results for triethylamine. This leads to a molecular interpretation of the isomeric configurations responsible for the observed relaxation.

Ultrasonic relaxation in the study of rotational isomers

The ultrasonic absorption in a series of unsaturated aldehyde and ketone liquids has been measured over the frequency range 5 to 200 Mc/s. The observed relaxational behaviour is attributed to perturbation by the sound wave of an equilibrium between rotational isomers. In a number of cases the measurements are sufficiently extensive to permit the evaluation of the potential energy barrier restricting internal rotation and of the energy difference between the equilibrium configurations of the molecule in question. In all cases the results can be explained in terms of a single relaxation time. Relaxational behaviour has also been observed in some saturated aldehydes: in such a liquid the relaxation time, τ, is less than in a corresponding unsaturated compound since in the latter conjugation across a single C— C bond restricts the freedom of rotation about that bond. The relaxational behaviour of solutions of crotonaldehyde in a non-polar solvent ( n -hexane) and in a polar solvent (acetone) has been investigated and the characteristic frequency has been found to be approximately independent of concentration. Further, the maximum absorption per wavelength is linearly dependent on the molar concentration. These results confirm the intra-molecular nature of the reaction: the potential barrier to rotation is pre­dominantly determined by forces internal to the molecule and is relatively independent of the force fields due to surrounding solvent molecules. Incipient relaxation has been observed in butadiene; both this and the relaxations found in a number of vinyl ethers are attributed to the existence of rotational isomers.

Ultrasonic relaxation and the vibrational specific heat of liquid sulphur dioxide

Measurements have been made of the absorption of ultrasonic waves in liquid sulphur dioxide over the range of frequency from 3 to 45 Mc/s and at temperatures of 0, 25 and 50°C. A pronounced relaxation is found, centred about a frequency of approximately 23 Mc/s; this is associated with the time delay in deactivation of two of the three vibrational modes. The mode which does not participate is that of lowest wave-number which is presumed to be responsible for a relaxation at higher frequencies (of the order of 1500 Mc/s). The results are discussed with reference to dispersion measurements in the gas phase.

Ultrasonic relaxation processes in liquid triethylamine

Measurements have been made of the velocity of propagation and the absorption of ultrasonic waves in liquid triethylamine over the temperature range 25 to 70° C and at frequencies of approximately 23, 66, 107, 148 and 192 Mc/s. The absorption results demonstrate the existence of a relaxation process which is attributed to the perturbation by the sound wave of an equilibrium between rotational isomers. The activation energy barrier and the difference in energy between the states are evaluated from an analysis of the results. The calculated dispersion of velocity is less than 1%. Values of the specific heat of triethvlamine are required in the analysis and are given in the paper for the temperature range 25 to 60°C. The results are discussed in the light of other measurements concerned with rotational isomers.