Flow Birefringence Measurements in Low‐Viscosity Liquids with the Help of Boundary Layer Shear Waves; and Orientational Relaxation in n‐Alkyl Carboxylic Acids (Normal Fatty Acids)

Abstract

AbstractA new measuring method for determining flow birefringence constants by generating shear velocity gradients with the help of Rayleigh boundary layer shear waves (21.3 kHz) will be described. This method has an advantage over previous measuring systems with Couette apparatuses in that disturbing heating influences in the birefringent layer can be avoided by pulsed excitation and faulty depolarizations can be eliminated by narrow‐band filtering.With this technique flow birefringence was investigated in n‐heptanoic, n‐octanoic, n‐nonanoic acid, o‐dichlorobenzene and cinnamaldehyde. The temperature dependency of flow birefringence in the carboxylic acids will be discussed on the basis of the theory of Peterlin and Stuart which leads to a simple relation between flow birefringence and orientational relaxation in low molecular liquids. The results will be compared with flow birefringence measurements in n‐paraffins and with investigations of orientational Rayleigh scattering in n‐alkyl bromides. The comparison leads to a confirmation of Peterlin's and Stuart's theoretical starting point. In addition it will be shown that molecular rearrangement processes in n‐alkyl carboxylic acids beginning with propionic acid are carried out predominantly by dimers, whereby the Arrhenius activation enthalpies of these processes are dependent on the molecular chain length in a way comparable to other simple n‐alkyl families.