Ultrasonic Viscoelastic Properties of Associated Liquids

Abstract

Measurements of ultrasonic propagation of longitudinal and shear waves over a wide frequency and temperature range were made in an homologous series of associated liquids, butanediol 1,3, 2‐methyl pentanediol 2,4 and hexanetriol 1,2,6. In each case, the absorption data demonstrated the presence of shear and structural or volume viscosities which are of the same order of magnitude and have the same temperature dependence. In order to account for the shear and compressional data, it was necessary to assume that a distribution of relaxation times excited. It was found that a different distribution was necessary for the compressional data than that used in accounting for the shear relaxation data. Comparison of the average relaxation time of structural and shear processes in the associated liquids shows that they are very close in value departing by a maximum of a factor of 4. In addition, it was found that the shear modulus was about 20 to 30% of the high‐frequency compressional modulus. The ratio of shear compressional modulus in these liquids was very close to the values found in typical solids, even though the magnitudes of the moduli of the liquids was about a factor of 10 smaller than found in typical solids. The temperature dependence of the shear modulus and the relaxation part of the compressional modulus was found to be the same. The moduli linearly increase with decreasing temperature in a manner which is not accounted for by the Eyring‐Hirai theory. It was found that the Tobolsky‐Leaderman Ferry reduction formula, which is based on the assumption that the moduli are proportional to temperature, does not hold for these liquids and probably not for any high frequency visco‐elastic data not associated with an ``entropy'' modulus. The data in the associated liquids were reduced by using the proper temperature dependence of the moduli. In considering the data of high frequencies, the absorption could not be accounted for by the same distribution which was used to fit the velocity data. At frequencies well above the dispersion region it was found that an attenuation set was independent of frequency. This appears to be characteristic of these liquids at very high frequencies and viscosities. At this time, there seems to be no acceptable mechanism to explain this type of loss. The latter authors have suggested that the hysteresis effect is not related to the viscous flow mechanism causing absorption at the lower frequencies and viscosities.