Structural effects on the ultrasonic properties in the normal–supercooled liquid transition region of diphenyl ether and o -terphenyl

Ultrasonic absorption and velocity were measured in diphenyl ether and o-terphenyl over a temperature range extending from the liquid to the supercooled region. Experimental results indicate a different degree of association in the two liquids with different volume viscosity mechanisms. While a coexistence of structural and thermal relaxation processes appears to operate in the polar diphenyl, only structural effects can be used to interpret the observed behavior in the terphenyl. Calculated ratios of structural to shear viscosity (?2 for diphenyl and ≈2.7 for terphenyl) agree with previous determinations in analogous systems. In both liquids, at a few degrees above the freezing point, the sound velocity shows a positive departure from the linear behavior observed at higher temperature. This effect is attributed to the onset of some structural change occurring in the molecular aggregates present in this region.