Depolarized light scattering studies of liquids: 1,5- and 2,4-pentanediol

Abstract

The temperature dependence of the shear viscosity of the isometric pair 1,5‐pentanediol and 2,4‐pentanediol exhibits some remarkable differences in the liquid phase. The purpose of this study is to rationalize these differences in terms of molecular structure of the two diols. In order to achieve this, we performed low frequency depolarized light scattering measurements and auxiliary viscosity measurements of the bulk liquids and their solutions over a wide temperature (230–350 K) and concentration range (0.2< χ <1). The integrated scattering intensity is shown to be mainly induced by H‐bonded intermolecular structures and indicates marked structural differences between the two liquids obviously due to the capacity of 2,4‐pentanediol to form bulky ring structures. The relaxation times obtained from the spectral linewidth measurements indicate that the local microviscosity to which molecular segments are subject during rotational motions is markedly smaller than the macroscopic shear viscosity especially at lower temperatures in 2,4‐pentanediol, whereas this difference is negligible in 1,5‐pentanediol. Even small concentrations of a solvent tend to reduce this effect drastically.