Rayleigh Scattering: Orientational Relaxation in Liquids

Abstract

A systematic investigation of orientational relaxation in liquids has been initiated employing the technique of Rayleigh scattering. The experimental results are interpreted based on an approximate theory which is developed and justified. Scattering spectra from 15 different liquids, including the homologous alkyl bromides and polyethylene glycols as well as glycerol, 1,3‐butanediol, and n‐octyl alcohol, were obtained as a function of temperature using a 6328‐Å Ne–He laser source and a Fabry–Perot interferometer. All spectra were found to have approximately Lorentzian form symmetric about the source frequency, which implies molecular reorientation is a relaxational process. Also, the width of these Lorentzian lines (which is reciprocally related to the orientational relaxation time) increased with increasing temperature in such a way that an effective orientational activation enthalpy was determined. In both the alkyl bromide and polyethylene glycol series, increased molecular size resulted in increased relaxation time, which is explained using both activation‐enthalpy and free‐volume approaches. The measured orientational relaxation times which reflect motion of the induced dipole moments are compared with dielectric relaxation times associated with the permanent moments and mechanical relaxation times determined ultrasonically. This comparison provides additional information about the detailed nature of the reorientation mechanism. For example since orientational relaxation is shown to be governed by rotation of ellipsoidal‐shaped molecules about their minor axes, it is concluded that mechanical motions are more nearly about major axes. In addition, it is possible to determine whether reorientation is a diffusion, jump, or an intermediate process. Experimental results indicate molecules in n‐octyl alcohol and probably in the alkyl bromides reorient by a diffusional process and that glycerol molecules reorient by rather large jumps, while 1,3‐butanediol is an intermediate case. These results are in excellent agreement with predictions based upon the McCuffie–Litovitz explanation for the distribution of dielectric relaxation times.