Molecular dynamics of iso-amyl bromide by dielectric spectroscopy, and the effects of a nonpolar solvent, 2-methylpentane, on the spectral features

摘要

To gain insight into the effects of the weakening of the electrostatic interactions on molecular dynamics when polar molecules are dissolved in a nonpolar solvent, the dielectric polarization and relaxation behaviors of iso-amylbromide and its 50 mol% solution in 2-methylpentane have been studied in detail over the frequency range, 1 mHz-1 MHz, and a temperature range approaching their liquid to glass transition. Features of the (i) #alpha#-relaxation spectrum, (ii) the Johari-Goldstein relaxation process in the liquid state at low temperatures, with an asymmetric spectral shape, and (iii) the temperature dependence of the relaxation dynamics have been determined and the effects of weakening of the electrostatic interaction on these features examined. The high-frequency wing of the loss spectrum of the #alpha#-relaxation is proportional to #omega#~(-#beta#). The dynamics of its #alpha#-relaxation follows the Arrhenius equation initially at high temperatures and thereafter the Vogel-Fulcher-Tamman equation. Alternative equations for the change in the relaxation rate have been discussed. A decrease in the dipole-dipole interaction and reduction in the internal field in a solution with a nonpolar solvent leads to a remarkable change in the shape of the relaxation spectra at high frequencies such that the dielectric loss for the #alpha#-relaxation becomes proportional to #omega#~(-#alpha##beta#), with #alpha#, #beta#>1. The relaxation spectra of sio-amyl bromide dissolved in 2-methylpentane follows the H-N function and therefore behaves similar to a polymer, whereas for pure iso-amyl bromide follows the Davidson-Cole behavior.